How do I make a textbox that only accepts numbers

Question

I have a windows forms app with a textbox control that I want to only accept integer values  In the past I ve done this kind of validation by overloading the KeyPress event and just removing characters which didn t fit the specification  I ve looked at the MaskedTextBox control but I d like a more general solution that could work with perhaps a regular expression  or depend on the values of other controls   Ideally this would behave such that pressing a non numeric character would either produce no result or immediately provide the user with feedback about the invalid character

User · Answer

i like terse code      private void xmm textbox KeyPress object sender  KeyPressEventArgs e            double x          e Handled    double TryParse   TextBox sender  Text  out x

User · Answer

Two options    Use a NumericUpDown instead  NumericUpDown does the filtering for you  which is nice  Of course it also gives your users the ability to hit the up and down arrows on the keyboard to increment and decrement the current value  Handle the appropriate keyboard events to prevent anything but numeric input  I ve had success with this two event handlers on a standard TextBox   private void textBox1 KeyPress object sender  KeyPressEventArgs e        if   char IsControl e KeyChar   amp  amp   char IsDigit e KeyChar   amp  amp           e KeyChar                            e Handled   true                only allow one decimal point     if   e KeyChar          amp  amp    sender as TextBox  Text IndexOf       gt  -1                 e Handled   true             You can remove the check for      and the subsequent check for more than one      if your TextBox shouldn t allow decimal places  You could also add a check for  -  if your TextBox should allow negative values   If you want to limit the user for number of digit  use  textBox1 MaxLength   2     this will allow the user to enter only 2 digits

User · Answer

Take a look at Input handling in WinForm  I have posted my solution which uses the ProcessCmdKey and OnKeyPress events on the textbox  The comments show you how to use a Regex to verify the keypress and block allow appropriately

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This is exactly what the Validated Validating events were designed for   Here s the MSDN article on the topic  http   msdn microsoft com en-us library system windows forms control validating aspx  The TL DR version  check the  Text property in the Validating event and set e Cancel True when the data is invalid   When you set e Cancel True  the user can t leave the field  but you will need to give them some kind of feedback that something s wrong  I change the box s background color to light red to indicate a problem  Make sure to set it back to SystemColors Window when Validating is called with a good value

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Take a look at Input handling in WinForm  I have posted my solution which uses the ProcessCmdKey and OnKeyPress events on the textbox  The comments show you how to use a Regex to verify the keypress and block allow appropriately

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Using the approach described in Fabio Iotti s answer I have created a more generic solution   public abstract class ValidatedTextBox   TextBox       private string m lastText   string Empty      protected abstract bool IsValid string text       protected sealed override void OnTextChanged EventArgs e            if   IsValid Text                 var pos   SelectionStart - Text Length   m lastText Length              Text   m lastText              SelectionStart   Math Max 0  pos                     m lastText   Text          base OnTextChanged e              ValidatedTextBox   which contains all nontrivial validation behavior  All that s left to do is inherit from this class and override  IsValid  method with whatever validation logic is required  For example  using this class  it is possible to create  RegexedTextBox  which will accept only strings which match specific regular expression   public abstract class RegexedTextBox   ValidatedTextBox       private readonly Regex m regex      protected RegexedTextBox string regExpString            m regex   new Regex regExpString             protected override bool IsValid string text            return m regex IsMatch Text             After that  inheriting from the  RegexedTextBox  class  we can easily create  PositiveNumberTextBox  and  PositiveFloatingPointNumberTextBox  controls   public sealed class PositiveNumberTextBox   RegexedTextBox       public PositiveNumberTextBox     base     d            public sealed class PositiveFloatingPointNumberTextBox   RegexedTextBox       public PositiveFloatingPointNumberTextBox             base      d      CultureInfo CurrentCulture NumberFormat NumberDecimalSeparator        d

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Here is a simple standalone Winforms custom control  derived from the standard TextBox  that allows only System Int32 input  it could be easily adapted for other types such as System Int64  etc    It supports copy paste operations and negative numbers   public class Int32TextBox   TextBox       protected override void OnKeyPress KeyPressEventArgs e                base OnKeyPress e            NumberFormatInfo fi   CultureInfo CurrentCulture NumberFormat           string c   e KeyChar ToString            if  char IsDigit c  0               return           if   SelectionStart    0   amp  amp   c Equals fi NegativeSign                return              copy paste         if     int e KeyChar    22       int e KeyChar    3                amp  amp    ModifierKeys  amp  Keys Control     Keys Control               return           if  e KeyChar      b               return           e Handled   true             protected override void WndProc ref System Windows Forms Message m                const int WM PASTE   0x0302          if  m Msg    WM PASTE                        string text   Clipboard GetText                if  string IsNullOrEmpty text                   return               if   text IndexOf       gt   0   amp  amp   SelectionStart    0                   return               int i              if   int TryParse text  out i      change this for other integer types                 return               if   i  lt  0   amp  amp   SelectionStart    0                   return                    base WndProc ref m           Update 2017  My first answer has some issues    you can type something that s longer than an integer of a given type  for example 2147483648 is greater than Int32 MaxValue   more generally  there s no real validation of the result of what has been typed  it only handles int32  you ll have to write specific TextBox derivated control for each type  Int64  etc     So I came up with another version that s more generic  that still supports copy paste    and - sign  etc   public class ValidatingTextBox   TextBox       private string  validText      private int  selectionStart      private int  selectionEnd      private bool  dontProcessMessages       public event EventHandler lt TextValidatingEventArgs gt  TextValidating       protected virtual void OnTextValidating object sender  TextValidatingEventArgs e    gt  TextValidating  Invoke sender  e        protected override void WndProc ref Message m                base WndProc ref m           if   dontProcessMessages              return           const int WM KEYDOWN   0x100          const int WM ENTERIDLE   0x121          const int VK DELETE   0x2e           bool delete   m Msg    WM KEYDOWN  amp  amp   int m WParam    VK DELETE          if   m Msg    WM KEYDOWN  amp  amp   delete     m Msg    WM ENTERIDLE                        DontProcessMessage      gt                                 validText   Text                   selectionStart   SelectionStart                   selectionEnd   SelectionLength                                     const int WM CHAR   0x102          const int WM PASTE   0x302          if  m Msg    WM CHAR    m Msg    WM PASTE    delete                        string newText   null              DontProcessMessage      gt                                newText   Text                               var e   new TextValidatingEventArgs newText               OnTextValidating this  e               if  e Cancel                                DontProcessMessage      gt                                        Text    validText                      SelectionStart    selectionStart                      SelectionLength    selectionEnd                                                         private void DontProcessMessage Action action                 dontProcessMessages   true          try                       action                      finally                        dontProcessMessages   false                     public class TextValidatingEventArgs   CancelEventArgs       public TextValidatingEventArgs string newText    gt  NewText   newText      public string NewText   get        For Int32  you can either derive from it  like this   public class Int32TextBox   ValidatingTextBox       protected override void OnTextValidating object sender  TextValidatingEventArgs e                e Cancel    int TryParse e NewText  out int i             or w o derivation  use the new TextValidating event like this   var vtb   new ValidatingTextBox        vtb TextValidating     sender  e    gt  e Cancel    int TryParse e NewText  out int i     but what s nice is it works with any string  and any validation routine

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In our webpage with the definition of textbox we can add an onkeypress event for accepting only numbers  It will not show any message but it will prevent you from wrong input  It worked for me  user could not enter anything except number    lt asp TextBox runat  server  ID  txtFrom       onkeypress  if isNaN String fromCharCode event keyCode    return false   gt

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I ve been working on a collection of components to complete missing stuff in WinForms  here it is  Advanced Forms  In particular this is the class for a Regex TextBox       lt summary gt Represents a Windows text box control that only allows input that matches a regular expression  lt  summary gt  public class RegexTextBox   TextBox        NonSerialized      string lastText            lt summary gt A regular expression governing the input allowed in this text field  lt  summary gt       Browsable false   EditorBrowsable EditorBrowsableState Never        DesignerSerializationVisibility DesignerSerializationVisibility Hidden       public virtual Regex Regex   get  set              lt summary gt A regular expression governing the input allowed in this text field  lt  summary gt       DefaultValue null        Category  Behavior         Description  Sets the regular expression governing the input allowed for this control         public virtual string RegexString           get               return Regex    null   string Empty   Regex ToString                      set               if  string IsNullOrEmpty value                   Regex   null              else                 Regex   new Regex value                        protected override void OnTextChanged EventArgs e            if  Regex    null  amp  amp   Regex IsMatch Text                 int pos   SelectionStart - Text Length    lastText    string Empty  Length              Text   lastText              SelectionStart   Math Max 0  pos                      lastText   Text           base OnTextChanged e             Simply adding something like myNumbericTextBox RegexString        d       should suffice

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In button click you can check text of textbox by for loop   char   c   txtGetCustomerId Text ToCharArray    bool IsDigi   true   for  int i   0  i  lt  c Length  i                  if  c i   lt   0     c i   gt   9           IsDigi   false            if  IsDigi                 do something

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Two options    Use a NumericUpDown instead  NumericUpDown does the filtering for you  which is nice  Of course it also gives your users the ability to hit the up and down arrows on the keyboard to increment and decrement the current value  Handle the appropriate keyboard events to prevent anything but numeric input  I ve had success with this two event handlers on a standard TextBox   private void textBox1 KeyPress object sender  KeyPressEventArgs e        if   char IsControl e KeyChar   amp  amp   char IsDigit e KeyChar   amp  amp           e KeyChar                            e Handled   true                only allow one decimal point     if   e KeyChar          amp  amp    sender as TextBox  Text IndexOf       gt  -1                 e Handled   true             You can remove the check for      and the subsequent check for more than one      if your TextBox shouldn t allow decimal places  You could also add a check for  -  if your TextBox should allow negative values   If you want to limit the user for number of digit  use  textBox1 MaxLength   2     this will allow the user to enter only 2 digits

User · Answer

Try a MaskedTextBox  It takes a simple mask format so you can limit the input to numbers or dates or whatever

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Use a RegularExpressionValidator   lt asp TextBox ID  quot TextBoxNum quot  runat  quot server quot  gt  lt  asp TextBox gt  lt br   gt   lt asp RegularExpressionValidator ID  quot RegularExpressionValidator1 quot                          ControlToValidate  quot TextBoxNum quot  runat  quot server quot                          ErrorMessage  quot Only numbers allowed quot                          ValidationExpression  quot  d  quot  gt   lt  asp RegularExpressionValidator gt

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You can use the TextChanged event  private void textBox BiggerThan TextChanged object sender  EventArgs e        long a      if    long TryParse textBox BiggerThan Text  out a                    If not int clear textbox text or Undo   last operation         textBox LessThan Clear

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you could use TextChanged  Keypress event  use a regex to filter on numbers and take some action

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3 solution  1     Add to the textbox s KeyPress event   using Regex for number only textBox  private void txtBox KeyPress object sender  KeyPressEventArgs e    if   System Text RegularExpressions Regex IsMatch e KeyChar ToString       d     e Handled   true      2  an another solution from msdn     Boolean flag used to determine when a character other than a number is entered  private bool nonNumberEntered   false     Handle the KeyDown event to determine the type of character entered into the     control  private void textBox1 KeyDown object sender  KeyEventArgs e       Initialize the flag to false  nonNumberEntered   false     Determine whether the keystroke is a number from the top of the keyboard  if  e KeyCode  lt  Keys D0    e KeyCode  gt  Keys D9           Determine whether the keystroke is a number from the keypad      if  e KeyCode  lt  Keys NumPad0    e KeyCode  gt  Keys NumPad9                   Determine whether the keystroke is a backspace          if  e KeyCode    Keys Back                           A non-numerical keystroke was pressed                 Set the flag to true and evaluate in KeyPress event              nonNumberEntered   true                         private void textBox1 KeyPress object sender  KeyPressEventArgs e        if  nonNumberEntered    true               MessageBox Show  Please enter number only               e Handled   true            source  http   msdn microsoft com en-us library system windows forms control keypress v VS 90  aspx  3   using the MaskedTextBox  http   msdn microsoft com en-us library system windows forms maskedtextbox aspx

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I have made something for this on CodePlex   It works by intercepting the TextChanged event  If the result is a good number it will be stored  If it is something wrong  the last good value will be restored  The source is a bit too large to publish here  but here is a link to the class that handles the core of this logic

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In button click you can check text of textbox by for loop   char   c   txtGetCustomerId Text ToCharArray    bool IsDigi   true   for  int i   0  i  lt  c Length  i                  if  c i   lt   0     c i   gt   9           IsDigi   false            if  IsDigi                 do something

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FAIL-SAFE and simple  recursive  method  which can be used with multiple textboxes   It blocks the wrong keyboard typed characters and also pasted values etc  It only accepts integer numbers  and the maximum number length is the maximum length of a string type  which is int  really long    public void Check If Int On TextChanged object sender  EventArgs e          This method checks that each inputed character is a number  Any non-numeric       characters are removed from the text     TextBox textbox    TextBox sender         If the text is empty  return    if  textbox Text Length    0    return           Check the new Text value if it s only numbers    byte parsedValue     if   byte TryParse textbox Text  textbox Text Length - 1   ToString    out parsedValue                 Remove the last character as it wasn t a number       textbox Text   textbox Text Remove  textbox Text Length - 1              Move the cursor to the end of text       textbox SelectionStart   textbox Text Length

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I am assuming from context and the tags you used that you are writing a  NET C  app  In this case  you can subscribe to the text changed event  and validate each key stroke   private void textBox1 TextChanged object sender  EventArgs e        if  System Text RegularExpressions Regex IsMatch textBox1 Text     0-9                   MessageBox Show  Please enter only numbers             textBox1 Text   textBox1 Text Remove textBox1 Text Length - 1

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Here is a simple solution that works for me   public static bool numResult      public static bool checkTextisNumber string numberVal                try                       if  numberVal Equals                                     numResult   true                            else if  numberVal Equals                                    numResult   true                            else                               decimal number3   0                  bool canConvert   decimal TryParse numberVal  out number3                   if  canConvert    true                                        numResult   true                                    else                     numResult   false                                   catch  System Exception ex                        numResult   false                    return numResult            string correctNum      private void tBox NumTester TextChanged object sender  TextChangedEventArgs e                  if checkTextisNumber tBox NumTester Text                         correctNum   tBox NumTester Text                    else                       tBox NumTester Text   correctNum

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I ve been working on a collection of components to complete missing stuff in WinForms  here it is  Advanced Forms  In particular this is the class for a Regex TextBox       lt summary gt Represents a Windows text box control that only allows input that matches a regular expression  lt  summary gt  public class RegexTextBox   TextBox        NonSerialized      string lastText            lt summary gt A regular expression governing the input allowed in this text field  lt  summary gt       Browsable false   EditorBrowsable EditorBrowsableState Never        DesignerSerializationVisibility DesignerSerializationVisibility Hidden       public virtual Regex Regex   get  set              lt summary gt A regular expression governing the input allowed in this text field  lt  summary gt       DefaultValue null        Category  Behavior         Description  Sets the regular expression governing the input allowed for this control         public virtual string RegexString           get               return Regex    null   string Empty   Regex ToString                      set               if  string IsNullOrEmpty value                   Regex   null              else                 Regex   new Regex value                        protected override void OnTextChanged EventArgs e            if  Regex    null  amp  amp   Regex IsMatch Text                 int pos   SelectionStart - Text Length    lastText    string Empty  Length              Text   lastText              SelectionStart   Math Max 0  pos                      lastText   Text           base OnTextChanged e             Simply adding something like myNumbericTextBox RegexString        d       should suffice

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It seems like many of the current answers to this question are manually parsing the input text  If you re looking for a specific built-in numeric type  e g  int or double   why not just delegate the work to that type s TryParse method  For example   public class IntTextBox   TextBox       string PreviousText           int BackingResult       public IntTextBox                 TextChanged    IntTextBox TextChanged             public bool HasResult   get  private set         public int Result               get                       return HasResult   BackingResult   default int                        void IntTextBox TextChanged object sender  EventArgs e                HasResult   int TryParse Text  out BackingResult            if  HasResult    string IsNullOrEmpty Text                            Commit             PreviousText   Text                    else                          Revert             var changeOffset   Text Length - PreviousText Length              var previousSelectionStart                   Math Max 0  SelectionStart - changeOffset                Text   PreviousText              SelectionStart   previousSelectionStart                      If you want something more generic but still compatible with Visual Studio s Designer   public class ParsableTextBox   TextBox       TryParser BackingTryParse      string PreviousText           object BackingResult       public ParsableTextBox             this null                   public ParsableTextBox TryParser tryParse                TryParse   tryParse           TextChanged    ParsableTextBox TextChanged             public delegate bool TryParser string text  out object result        public TryParser TryParse               set                       Enabled     ReadOnly   value    null                BackingTryParse   value                       public bool HasResult   get  private set         public object Result               get                       return GetResult lt object gt                          public T GetResult lt T gt                  return HasResult    T BackingResult   default T              void ParsableTextBox TextChanged object sender  EventArgs e                if  BackingTryParse    null                        HasResult   BackingTryParse Text  out BackingResult                      if  HasResult    string IsNullOrEmpty Text                            Commit             PreviousText   Text                    else                          Revert             var changeOffset   Text Length - PreviousText Length              var previousSelectionStart                   Math Max 0  SelectionStart - changeOffset                Text   PreviousText              SelectionStart   previousSelectionStart                      And finally  if you want something fully generic and don t care about Designer support   public class ParsableTextBox lt T gt    TextBox       TryParser BackingTryParse      string PreviousText      T BackingResult       public ParsableTextBox             this null                   public ParsableTextBox TryParser tryParse                TryParse   tryParse           TextChanged    ParsableTextBox TextChanged             public delegate bool TryParser string text  out T result        public TryParser TryParse               set                       Enabled     ReadOnly   value    null                BackingTryParse   value                       public bool HasResult   get  private set         public T Result               get                       return HasResult   BackingResult   default T                        void ParsableTextBox TextChanged object sender  EventArgs e                if  BackingTryParse    null                        HasResult   BackingTryParse Text  out BackingResult                      if  HasResult    string IsNullOrEmpty Text                            Commit             PreviousText   Text                    else                          Revert             var changeOffset   Text Length - PreviousText Length              var previousSelectionStart                   Math Max 0  SelectionStart - changeOffset                Text   PreviousText              SelectionStart   previousSelectionStart

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I was also looking for the best way to check only numbers in textbox and problem with keypress was it does not support copy paste by right click or clipboard so came up with  this code which validates the when cursor leaves the text field and also it checks for empty field   adapted version of newguy   private void txtFirstValue MouseLeave object sender  EventArgs e        int num      bool isNum   int TryParse txtFirstValue Text Trim    out num        if   isNum  amp  amp  txtFirstValue Text    String Empty                MessageBox Show  The First Value You Entered Is Not a Number  Please Try Again    Invalid Value Detected   MessageBoxButtons OK  MessageBoxIcon Error           txtFirstValue Clear

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Use a RegularExpressionValidator   lt asp TextBox ID  quot TextBoxNum quot  runat  quot server quot  gt  lt  asp TextBox gt  lt br   gt   lt asp RegularExpressionValidator ID  quot RegularExpressionValidator1 quot                          ControlToValidate  quot TextBoxNum quot  runat  quot server quot                          ErrorMessage  quot Only numbers allowed quot                          ValidationExpression  quot  d  quot  gt   lt  asp RegularExpressionValidator gt

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I had created a Re-Usable Textbox Extension class for all sorts of validations and thought about sharing it   All you need to do is raise a TextChange event and then call the Validate method  It looks like this    private void tbxAmount TextChanged object sender  EventArgs e         tbxAmount Validate TextValidator ValidationType Amount       Here is the extension class   public static class TextValidator       public enum ValidationType               Amount          Integer                 lt summary gt          Validate a textbox on text change           lt  summary gt           lt param name  tbx  gt  lt  param gt           lt param name  validationType  gt  lt  param gt      public static void Validate this TextBox tbx  ValidationType validationType                PerformValidation tbx  validationType           tbx Select tbx Text Length  0               private static void PerformValidation this TextBox tbx  ValidationType validationType                char   enteredString   tbx Text ToCharArray            switch  validationType                        case ValidationType Amount                  tbx Text   AmountValidation enteredString                   break               case ValidationType Integer                  tbx Text   IntegerValidation enteredString                   break               default                  break                     tbx SelectionStart   tbx Text Length               private static string AmountValidation char   enteredString                string actualString   string Empty          int count   0          foreach  char c in enteredString AsEnumerable                          if  count  gt   1  amp  amp  c                       actualString Replace c        actualString Trim                  else                               if  Char IsDigit c                                         actualString   actualString   c                                     if  c                                               actualString   actualString   c  count                                       else                                       actualString Replace c                            actualString Trim                                                      return actualString              private static string IntegerValidation char   enteredString                string actualString   string Empty          foreach  char c in enteredString AsEnumerable                          if  Char IsDigit c                                 actualString   actualString   c                            else                               actualString Replace c                        actualString Trim                                    return actualString            You can find the full code here

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This one works with copy and paste  drag and drop  key down  prevents overflow and is pretty simple  public partial class IntegerBox   TextBox        public IntegerBox                 InitializeComponent            this Text   0 ToString               protected override void OnPaint PaintEventArgs pe                base OnPaint pe              private String originalValue   0 ToString         private void Integerbox KeyPress object sender  KeyPressEventArgs e                originalValue   this Text             private void Integerbox TextChanged object sender  EventArgs e                try                       if String IsNullOrWhiteSpace this Text                                 this Text   0 ToString                              this Text   Convert ToInt64 this Text Trim    ToString                      catch  System OverflowException                        MessageBox Show  Value entered is to large max value      Int64 MaxValue ToString     Error   MessageBoxButtons OK  MessageBoxIcon Error               this Text   originalValue                    catch  System FormatException                                        this Text   originalValue                    catch  System Exception ex                        this Text   originalValue              MessageBox Show ex Message   Error   MessageBoxButtons OK   MessageBoxIcon Error

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private void txt3 KeyPress object sender  KeyPressEventArgs e        for  int h   58  h  lt   127  h                  if  e KeyChar    h                58 to 127 is alphabets tat will be         blocked                       e Handled   true                      for int k 32 k lt  47 k                  if  e KeyChar    k                 32 to 47 are special characters tat will                                             be blocked             e Handled   true                      try this is very simple

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Working solution for WPF  amp  a simple TextChangedEventArgs   private void TextBox TextChanged object sender  TextChangedEventArgs e        var TextBox    sender as TextBox           if not a numeric value  remove news characters     if  Regex IsMatch TextBox Text     0-9                   foreach  TextChange Change in e Changes                        TextBox Text   TextBox Text Remove Change Offset  Change AddedLength                TextBox CaretIndex   Change Offset

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Both integers and floats need to be accepted  including the negative numbers   private void textBox1 KeyPress object sender  KeyPressEventArgs e           Text     string text     Control  sender  Text          Is Negative Number      if  e KeyChar     -   amp  amp  text Length    0                e Handled   false          return                Is Float Number      if  e KeyChar         amp  amp  text Length  gt  0  amp  amp   text Contains                     e Handled   false          return                Is Digit      e Handled     char IsDigit e KeyChar   amp  amp   char IsControl e KeyChar

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Sorry to wake the dead  but I thought someone might find this useful for future reference   Here is how I handle it   It handles floating point numbers  but can easily be modified for integers   Basically you can only press 0 - 9 and    You can only have one 0 before the    All other characters are ignored and the cursor position maintained       private bool  myTextBoxChanging   false       private void myTextBox TextChanged object sender  EventArgs e                validateText myTextBox              private void validateText TextBox box                   stop multiple changes          if   myTextBoxChanging              return           myTextBoxChanging   true           string text   box Text          if  text                    return          string validText               bool hasPeriod   false          int pos   box SelectionStart          for  int i   0  i  lt  text Length  i                           bool badChar   false              char s   text i               if  s                                       if  hasPeriod                      badChar   true                  else                     hasPeriod   true                            else if  s  lt   0     s  gt   9                   badChar   true               if   badChar                  validText    s              else                               if  i  lt   pos                      pos--                                      trim starting 00s         while  validText Length  gt   2  amp  amp  validText 0      0                         if  validText 1                                        validText   validText Substring 1                   if  pos  lt  2                      pos--                            else                 break                     if  pos  gt  validText Length              pos   validText Length          box Text   validText          box SelectionStart   pos           myTextBoxChanging   false          Here is a quickly modified int version       private void validateText TextBox box                   stop multiple changes          if   myTextBoxChanging              return           myTextBoxChanging   true           string text   box Text          if  text                    return          string validText               int pos   box SelectionStart          for  int i   0  i  lt  text Length  i                           char s   text i               if  s  lt   0     s  gt   9                                 if  i  lt   pos                      pos--                            else                 validText    s                        trim starting 00s          while  validText Length  gt   2  amp  amp  validText StartsWith  00                            validText   validText Substring 1                if  pos  lt  2                   pos--                       if  pos  gt  validText Length              pos   validText Length          box Text   validText          box SelectionStart   pos           myTextBoxChanging   false

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Here is a simple solution that works for me   public static bool numResult      public static bool checkTextisNumber string numberVal                try                       if  numberVal Equals                                     numResult   true                            else if  numberVal Equals                                    numResult   true                            else                               decimal number3   0                  bool canConvert   decimal TryParse numberVal  out number3                   if  canConvert    true                                        numResult   true                                    else                     numResult   false                                   catch  System Exception ex                        numResult   false                    return numResult            string correctNum      private void tBox NumTester TextChanged object sender  TextChangedEventArgs e                  if checkTextisNumber tBox NumTester Text                         correctNum   tBox NumTester Text                    else                       tBox NumTester Text   correctNum

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Simpler answer    textBox TextChanged    delegate System Object o  System EventArgs e        TextBox  tbox   o as TextBox       tbox Text   new string  tbox Text Where c   gt   char IsDigit c       c          ToArray

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This is my aproach    using linq  easy to modify filter  copy paste proof code keeps caret position when you press a forbidden character accepts left zeroes and any size numbers  private void numeroCuenta TextChanged object sender  EventArgs e        string org   numeroCuenta Text      string formated   string Concat org Where c   gt   c  gt    0   amp  amp  c  lt    9          if  formated    org                int s   numeroCuenta SelectionStart          if  s  gt  0  amp  amp  formated Length  gt  s  amp  amp  org s - 1     formated s - 1   s--          numeroCuenta Text   formated          numeroCuenta SelectionStart   s

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I am assuming from context and the tags you used that you are writing a  NET C  app  In this case  you can subscribe to the text changed event  and validate each key stroke   private void textBox1 TextChanged object sender  EventArgs e        if  System Text RegularExpressions Regex IsMatch textBox1 Text     0-9                   MessageBox Show  Please enter only numbers             textBox1 Text   textBox1 Text Remove textBox1 Text Length - 1

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Hi you can do something like this in the textchanged event of the textbox    here is a demo      private void textBox1 TextChanged object sender  EventArgs e                string actualdata   string Empty          char   entereddata   textBox1 Text ToCharArray            foreach  char aChar in entereddata AsEnumerable                          if  Char IsDigit aChar                                 actualdata   actualdata   aChar                     MessageBox Show aChar ToString                               else                               MessageBox Show aChar     is not numeric                    actualdata Replace aChar                        actualdata Trim                                    textBox1 Text   actualdata

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And just because it s always more fun to do stuff in one line      private void textBox1 KeyPress object sender  KeyPressEventArgs e                e Handled    char IsDigit e KeyChar   amp  amp   char IsControl e KeyChar           NOTE  This DOES NOT prevent a user from Copy   Paste into this textbox  It s not a fail safe way to sanitize your data

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Both integers and floats need to be accepted  including the negative numbers   private void textBox1 KeyPress object sender  KeyPressEventArgs e           Text     string text     Control  sender  Text          Is Negative Number      if  e KeyChar     -   amp  amp  text Length    0                e Handled   false          return                Is Float Number      if  e KeyChar         amp  amp  text Length  gt  0  amp  amp   text Contains                     e Handled   false          return                Is Digit      e Handled     char IsDigit e KeyChar   amp  amp   char IsControl e KeyChar

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you can simply prevent adding non-numerical chars by this simple code   if  long TryParse TextBox Text out long isparsable                          your code to handle numbers                   else                       TextBox Text  Only Numbers Allowed               TextBox Focus                TextBox SelectAll

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Here are more than 30 answers and a lot of answers are helpful  But I want to share a generalized form for the System Windows Forms TextBox and System Windows Controls TextBox  There is not available KeyPress event in System Windows Controls TextBox  This answer is for those people who want to implement with the same logic for System Windows Forms TextBox and System Windows Controls TextBox  This is NumberTextBox code  Use commented line instead of the previous line for System Windows Controls TextBox   public class NumberTextBox   System Windows Forms TextBox   public class NumberTextBox   System Windows Controls TextBox       private double  maxValue      private double  minValue      private bool  flag      private string  previousValue       public NumberTextBox                 this TextAlign   HorizontalAlignment Right            TextAlignment   TextAlignment Right          KeyDown    TextBox KeyDown          TextChanged    TextBox TextChanged           minValue   double MinValue           maxValue   double MaxValue             private void TextBox KeyDown object sender  KeyEventArgs e                 previousValue   this Text           flag   this SelectedText Length  gt  0             private void TextBox TextChanged object sender  EventArgs e        private void TextBox TextChanged object sender  TextChangedEventArgs e                var text   this Text          if  text Length  lt  1  return          var cursorPosition   SelectionStart    0   SelectionStart   SelectionStart - 1          var insertedChar   text cursorPosition           if  IsInvalidInput insertedChar  cursorPosition  text                         HandleText text  cursorPosition                     ValidateRange text  cursorPosition              private bool IsInvalidInput char insertedChar  int cursorPosition  string text                return  char IsDigit insertedChar   amp  amp  insertedChar         amp  amp  insertedChar     -                    insertedChar     -   amp  amp  cursorPosition    0                   text Count x   gt  x          gt  1                   text Count x   gt  x     -    gt  1             private void HandleText string text  int cursorPosition                this Text    flag    previousValue   text Remove cursorPosition  1           this SelectionStart   cursorPosition          this SelectionLength   0             private void ValidateRange string text  int cursorPosition                try                       if  text     quot   quot      minValue  lt  0  amp  amp  text     quot - quot   return              var doubleValue   Convert ToDouble text               if  doubleValue  gt   maxValue    doubleValue  lt   minValue                                HandleText text  cursorPosition                                   catch  Exception                        HandleText text  cursorPosition                        protected void SetProperties double minValue   double MinValue  double maxValue   double MaxValue                 minValue   minValue           maxValue   maxValue                   PositiveNumberTextBox code  public class PositiveNumberTextBox   NumberTextBox       public PositiveNumberTextBox                 SetProperties 0            FractionNumberTextBox code  public class FractionNumberTextBox   NumberTextBox       public FractionNumberTextBox                 SetProperties 0  0 999999

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you could use TextChanged  Keypress event  use a regex to filter on numbers and take some action

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Two options    Use a NumericUpDown instead  NumericUpDown does the filtering for you  which is nice  Of course it also gives your users the ability to hit the up and down arrows on the keyboard to increment and decrement the current value  Handle the appropriate keyboard events to prevent anything but numeric input  I ve had success with this two event handlers on a standard TextBox   private void textBox1 KeyPress object sender  KeyPressEventArgs e        if   char IsControl e KeyChar   amp  amp   char IsDigit e KeyChar   amp  amp           e KeyChar                            e Handled   true                only allow one decimal point     if   e KeyChar          amp  amp    sender as TextBox  Text IndexOf       gt  -1                 e Handled   true             You can remove the check for      and the subsequent check for more than one      if your TextBox shouldn t allow decimal places  You could also add a check for  -  if your TextBox should allow negative values   If you want to limit the user for number of digit  use  textBox1 MaxLength   2     this will allow the user to enter only 2 digits

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you could use TextChanged  Keypress event  use a regex to filter on numbers and take some action

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Two options    Use a NumericUpDown instead  NumericUpDown does the filtering for you  which is nice  Of course it also gives your users the ability to hit the up and down arrows on the keyboard to increment and decrement the current value  Handle the appropriate keyboard events to prevent anything but numeric input  I ve had success with this two event handlers on a standard TextBox   private void textBox1 KeyPress object sender  KeyPressEventArgs e        if   char IsControl e KeyChar   amp  amp   char IsDigit e KeyChar   amp  amp           e KeyChar                            e Handled   true                only allow one decimal point     if   e KeyChar          amp  amp    sender as TextBox  Text IndexOf       gt  -1                 e Handled   true             You can remove the check for      and the subsequent check for more than one      if your TextBox shouldn t allow decimal places  You could also add a check for  -  if your TextBox should allow negative values   If you want to limit the user for number of digit  use  textBox1 MaxLength   2     this will allow the user to enter only 2 digits

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Just use a NumericUpDown control and set those ugly up down buttons visibility to false   numericUpDown1 Controls 0  Visible   false    NumericUpDown is actually a collection of controls containing a  spin box   up down buttons   a text box and some code to validate and wange-jangle it all together     Marking   YourNumericUpDown Controls 0  visible   false    will hide the buttons while keeping the underlying code active     While not being an obvious solution it is simple and effective   Controls 1  would hide the text box portion if you wanted to do that instead

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Just use a NumericUpDown control and set those ugly up down buttons visibility to false   numericUpDown1 Controls 0  Visible   false    NumericUpDown is actually a collection of controls containing a  spin box   up down buttons   a text box and some code to validate and wange-jangle it all together     Marking   YourNumericUpDown Controls 0  visible   false    will hide the buttons while keeping the underlying code active     While not being an obvious solution it is simple and effective   Controls 1  would hide the text box portion if you wanted to do that instead

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Sorry to wake the dead  but I thought someone might find this useful for future reference   Here is how I handle it   It handles floating point numbers  but can easily be modified for integers   Basically you can only press 0 - 9 and    You can only have one 0 before the    All other characters are ignored and the cursor position maintained       private bool  myTextBoxChanging   false       private void myTextBox TextChanged object sender  EventArgs e                validateText myTextBox              private void validateText TextBox box                   stop multiple changes          if   myTextBoxChanging              return           myTextBoxChanging   true           string text   box Text          if  text                    return          string validText               bool hasPeriod   false          int pos   box SelectionStart          for  int i   0  i  lt  text Length  i                           bool badChar   false              char s   text i               if  s                                       if  hasPeriod                      badChar   true                  else                     hasPeriod   true                            else if  s  lt   0     s  gt   9                   badChar   true               if   badChar                  validText    s              else                               if  i  lt   pos                      pos--                                      trim starting 00s         while  validText Length  gt   2  amp  amp  validText 0      0                         if  validText 1                                        validText   validText Substring 1                   if  pos  lt  2                      pos--                            else                 break                     if  pos  gt  validText Length              pos   validText Length          box Text   validText          box SelectionStart   pos           myTextBoxChanging   false          Here is a quickly modified int version       private void validateText TextBox box                   stop multiple changes          if   myTextBoxChanging              return           myTextBoxChanging   true           string text   box Text          if  text                    return          string validText               int pos   box SelectionStart          for  int i   0  i  lt  text Length  i                           char s   text i               if  s  lt   0     s  gt   9                                 if  i  lt   pos                      pos--                            else                 validText    s                        trim starting 00s          while  validText Length  gt   2  amp  amp  validText StartsWith  00                            validText   validText Substring 1                if  pos  lt  2                   pos--                       if  pos  gt  validText Length              pos   validText Length          box Text   validText          box SelectionStart   pos           myTextBoxChanging   false

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This one works with copy and paste  drag and drop  key down  prevents overflow and is pretty simple  public partial class IntegerBox   TextBox        public IntegerBox                 InitializeComponent            this Text   0 ToString               protected override void OnPaint PaintEventArgs pe                base OnPaint pe              private String originalValue   0 ToString         private void Integerbox KeyPress object sender  KeyPressEventArgs e                originalValue   this Text             private void Integerbox TextChanged object sender  EventArgs e                try                       if String IsNullOrWhiteSpace this Text                                 this Text   0 ToString                              this Text   Convert ToInt64 this Text Trim    ToString                      catch  System OverflowException                        MessageBox Show  Value entered is to large max value      Int64 MaxValue ToString     Error   MessageBoxButtons OK  MessageBoxIcon Error               this Text   originalValue                    catch  System FormatException                                        this Text   originalValue                    catch  System Exception ex                        this Text   originalValue              MessageBox Show ex Message   Error   MessageBoxButtons OK   MessageBoxIcon Error

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This might be useful  It allows  real  numeric values  including proper decimal points and preceding plus or minus signs  Call it from within the related KeyPress event          private bool IsOKForDecimalTextBox char theCharacter  TextBox theTextBox                   Only allow control characters  digits  plus and minus signs             Only allow ONE plus sign             Only allow ONE minus sign             Only allow the plus or minus sign as the FIRST character             Only allow ONE decimal point             Do NOT allow decimal point or digits BEFORE any plus or minus sign           if                char IsControl theCharacter               amp  amp   char IsDigit theCharacter               amp  amp   theCharacter                      amp  amp   theCharacter     -                amp  amp   theCharacter                                            Then it is NOT a character we want allowed in the text box              return false                          Only allow one decimal point          if  theCharacter                     amp  amp  theTextBox Text IndexOf       gt  -1                           Then there is already a decimal point in the text box              return false                        Only allow one minus sign          if  theCharacter     -               amp  amp  theTextBox Text IndexOf  -    gt  -1                           Then there is already a minus sign in the text box              return false                        Only allow one plus sign          if  theCharacter                     amp  amp  theTextBox Text IndexOf       gt  -1                           Then there is already a plus sign in the text box              return false                        Only allow one plus sign OR minus sign  but not both          if                                  theCharacter     -                       theCharacter                                    amp  amp                                  theTextBox Text IndexOf  -    gt  -1                                      theTextBox Text IndexOf       gt  -1                                                       Then the user is trying to enter a plus or minus sign and                there is ALREADY a plus or minus sign in the text box              return false                        Only allow a minus or plus sign at the first character position          if                                  theCharacter     -                       theCharacter                                    amp  amp  theTextBox SelectionStart    0                                        Then the user is trying to enter a minus or plus sign at some position                 OTHER than the first character position in the text box              return false                        Only allow digits and decimal point AFTER any existing plus or minus sign         if                                             Is digit or decimal point                     char IsDigit theCharacter                                              theCharacter                                            amp  amp                                           A plus or minus sign EXISTS                      theTextBox Text IndexOf  -    gt  -1                                              theTextBox Text IndexOf       gt  -1                                     amp  amp                         Attempting to put the character at the beginning of the field                      theTextBox SelectionStart    0                                        Then the user is trying to enter a digit or decimal point in front of a minus or plus sign              return false                        Otherwise the character is perfectly fine for a decimal value and the character            may indeed be placed at the current insertion position          return true

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I have made something for this on CodePlex   It works by intercepting the TextChanged event  If the result is a good number it will be stored  If it is something wrong  the last good value will be restored  The source is a bit too large to publish here  but here is a link to the class that handles the core of this logic

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I have made something for this on CodePlex   It works by intercepting the TextChanged event  If the result is a good number it will be stored  If it is something wrong  the last good value will be restored  The source is a bit too large to publish here  but here is a link to the class that handles the core of this logic

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I have made something for this on CodePlex   It works by intercepting the TextChanged event  If the result is a good number it will be stored  If it is something wrong  the last good value will be restored  The source is a bit too large to publish here  but here is a link to the class that handles the core of this logic

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Here is a simple standalone Winforms custom control  derived from the standard TextBox  that allows only System Int32 input  it could be easily adapted for other types such as System Int64  etc    It supports copy paste operations and negative numbers   public class Int32TextBox   TextBox       protected override void OnKeyPress KeyPressEventArgs e                base OnKeyPress e            NumberFormatInfo fi   CultureInfo CurrentCulture NumberFormat           string c   e KeyChar ToString            if  char IsDigit c  0               return           if   SelectionStart    0   amp  amp   c Equals fi NegativeSign                return              copy paste         if     int e KeyChar    22       int e KeyChar    3                amp  amp    ModifierKeys  amp  Keys Control     Keys Control               return           if  e KeyChar      b               return           e Handled   true             protected override void WndProc ref System Windows Forms Message m                const int WM PASTE   0x0302          if  m Msg    WM PASTE                        string text   Clipboard GetText                if  string IsNullOrEmpty text                   return               if   text IndexOf       gt   0   amp  amp   SelectionStart    0                   return               int i              if   int TryParse text  out i      change this for other integer types                 return               if   i  lt  0   amp  amp   SelectionStart    0                   return                    base WndProc ref m           Update 2017  My first answer has some issues    you can type something that s longer than an integer of a given type  for example 2147483648 is greater than Int32 MaxValue   more generally  there s no real validation of the result of what has been typed  it only handles int32  you ll have to write specific TextBox derivated control for each type  Int64  etc     So I came up with another version that s more generic  that still supports copy paste    and - sign  etc   public class ValidatingTextBox   TextBox       private string  validText      private int  selectionStart      private int  selectionEnd      private bool  dontProcessMessages       public event EventHandler lt TextValidatingEventArgs gt  TextValidating       protected virtual void OnTextValidating object sender  TextValidatingEventArgs e    gt  TextValidating  Invoke sender  e        protected override void WndProc ref Message m                base WndProc ref m           if   dontProcessMessages              return           const int WM KEYDOWN   0x100          const int WM ENTERIDLE   0x121          const int VK DELETE   0x2e           bool delete   m Msg    WM KEYDOWN  amp  amp   int m WParam    VK DELETE          if   m Msg    WM KEYDOWN  amp  amp   delete     m Msg    WM ENTERIDLE                        DontProcessMessage      gt                                 validText   Text                   selectionStart   SelectionStart                   selectionEnd   SelectionLength                                     const int WM CHAR   0x102          const int WM PASTE   0x302          if  m Msg    WM CHAR    m Msg    WM PASTE    delete                        string newText   null              DontProcessMessage      gt                                newText   Text                               var e   new TextValidatingEventArgs newText               OnTextValidating this  e               if  e Cancel                                DontProcessMessage      gt                                        Text    validText                      SelectionStart    selectionStart                      SelectionLength    selectionEnd                                                         private void DontProcessMessage Action action                 dontProcessMessages   true          try                       action                      finally                        dontProcessMessages   false                     public class TextValidatingEventArgs   CancelEventArgs       public TextValidatingEventArgs string newText    gt  NewText   newText      public string NewText   get        For Int32  you can either derive from it  like this   public class Int32TextBox   ValidatingTextBox       protected override void OnTextValidating object sender  TextValidatingEventArgs e                e Cancel    int TryParse e NewText  out int i             or w o derivation  use the new TextValidating event like this   var vtb   new ValidatingTextBox        vtb TextValidating     sender  e    gt  e Cancel    int TryParse e NewText  out int i     but what s nice is it works with any string  and any validation routine

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Simpler answer    textBox TextChanged    delegate System Object o  System EventArgs e        TextBox  tbox   o as TextBox       tbox Text   new string  tbox Text Where c   gt   char IsDigit c       c          ToArray

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FAIL-SAFE and simple  recursive  method  which can be used with multiple textboxes   It blocks the wrong keyboard typed characters and also pasted values etc  It only accepts integer numbers  and the maximum number length is the maximum length of a string type  which is int  really long    public void Check If Int On TextChanged object sender  EventArgs e          This method checks that each inputed character is a number  Any non-numeric       characters are removed from the text     TextBox textbox    TextBox sender         If the text is empty  return    if  textbox Text Length    0    return           Check the new Text value if it s only numbers    byte parsedValue     if   byte TryParse textbox Text  textbox Text Length - 1   ToString    out parsedValue                 Remove the last character as it wasn t a number       textbox Text   textbox Text Remove  textbox Text Length - 1              Move the cursor to the end of text       textbox SelectionStart   textbox Text Length

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private void txt3 KeyPress object sender  KeyPressEventArgs e        for  int h   58  h  lt   127  h                  if  e KeyChar    h                58 to 127 is alphabets tat will be         blocked                       e Handled   true                      for int k 32 k lt  47 k                  if  e KeyChar    k                 32 to 47 are special characters tat will                                             be blocked             e Handled   true                      try this is very simple

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i like terse code      private void xmm textbox KeyPress object sender  KeyPressEventArgs e            double x          e Handled    double TryParse   TextBox sender  Text  out x

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And just because it s always more fun to do stuff in one line      private void textBox1 KeyPress object sender  KeyPressEventArgs e                e Handled    char IsDigit e KeyChar   amp  amp   char IsControl e KeyChar           NOTE  This DOES NOT prevent a user from Copy   Paste into this textbox  It s not a fail safe way to sanitize your data

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The simplest and crudest way is to use a combobox instead of textbox with ReadOnly  But of course its only useful if you are expecting the user to use a decent and reasonable set of whole numbers  not try to touch infinity

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int Number  bool isNumber  isNumber   int32 TryPase textbox1 text  out Number    if   isNumber          code if not an integer     else        code if an integer

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Working solution for WPF  amp  a simple TextChangedEventArgs   private void TextBox TextChanged object sender  TextChangedEventArgs e        var TextBox    sender as TextBox           if not a numeric value  remove news characters     if  Regex IsMatch TextBox Text     0-9                   foreach  TextChange Change in e Changes                        TextBox Text   TextBox Text Remove Change Offset  Change AddedLength                TextBox CaretIndex   Change Offset

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you could use TextChanged  Keypress event  use a regex to filter on numbers and take some action

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I would handle it in the KeyDown event    void TextBox KeyDown object sender  KeyEventArgs e                        char c   Convert ToChar e PlatformKeyCode               if   char IsDigit c                                 e Handled   true

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I am assuming from context and the tags you used that you are writing a  NET C  app  In this case  you can subscribe to the text changed event  and validate each key stroke   private void textBox1 TextChanged object sender  EventArgs e        if  System Text RegularExpressions Regex IsMatch textBox1 Text     0-9                   MessageBox Show  Please enter only numbers             textBox1 Text   textBox1 Text Remove textBox1 Text Length - 1

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Try a MaskedTextBox  It takes a simple mask format so you can limit the input to numbers or dates or whatever

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Do not forget that a user can paste an invalid text in a TextBox    If you want to restrict that  follow the below code   private void ultraTextEditor1 TextChanged object sender  EventArgs e        string append         foreach  char c in ultraTextEditor1 Text                if    Char IsNumber c    amp  amp   c    Convert ToChar Keys Back                                 else                       append    c                       ultraTextEditor1 Text   append

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The simplest and crudest way is to use a combobox instead of textbox with ReadOnly  But of course its only useful if you are expecting the user to use a decent and reasonable set of whole numbers  not try to touch infinity

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And just because it s always more fun to do stuff in one line      private void textBox1 KeyPress object sender  KeyPressEventArgs e                e Handled    char IsDigit e KeyChar   amp  amp   char IsControl e KeyChar           NOTE  This DOES NOT prevent a user from Copy   Paste into this textbox  It s not a fail safe way to sanitize your data

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Hi you can do something like this in the textchanged event of the textbox    here is a demo      private void textBox1 TextChanged object sender  EventArgs e                string actualdata   string Empty          char   entereddata   textBox1 Text ToCharArray            foreach  char aChar in entereddata AsEnumerable                          if  Char IsDigit aChar                                 actualdata   actualdata   aChar                     MessageBox Show aChar ToString                               else                               MessageBox Show aChar     is not numeric                    actualdata Replace aChar                        actualdata Trim                                    textBox1 Text   actualdata

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This is my aproach    using linq  easy to modify filter  copy paste proof code keeps caret position when you press a forbidden character accepts left zeroes and any size numbers  private void numeroCuenta TextChanged object sender  EventArgs e        string org   numeroCuenta Text      string formated   string Concat org Where c   gt   c  gt    0   amp  amp  c  lt    9          if  formated    org                int s   numeroCuenta SelectionStart          if  s  gt  0  amp  amp  formated Length  gt  s  amp  amp  org s - 1     formated s - 1   s--          numeroCuenta Text   formated          numeroCuenta SelectionStart   s

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And just because it s always more fun to do stuff in one line      private void textBox1 KeyPress object sender  KeyPressEventArgs e                e Handled    char IsDigit e KeyChar   amp  amp   char IsControl e KeyChar           NOTE  This DOES NOT prevent a user from Copy   Paste into this textbox  It s not a fail safe way to sanitize your data

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simply use this code in textbox    private void textBox1 TextChanged object sender  EventArgs e         double parsedValue       if   double TryParse textBox1 Text  out parsedValue                 textBox1 Text

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Take a look at Input handling in WinForm  I have posted my solution which uses the ProcessCmdKey and OnKeyPress events on the textbox  The comments show you how to use a Regex to verify the keypress and block allow appropriately

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Try a MaskedTextBox  It takes a simple mask format so you can limit the input to numbers or dates or whatever

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you can simply prevent adding non-numerical chars by this simple code   if  long TryParse TextBox Text out long isparsable                          your code to handle numbers                   else                       TextBox Text  Only Numbers Allowed               TextBox Focus                TextBox SelectAll

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I would handle it in the KeyDown event    void TextBox KeyDown object sender  KeyEventArgs e                        char c   Convert ToChar e PlatformKeyCode               if   char IsDigit c                                 e Handled   true

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This is exactly what the Validated Validating events were designed for   Here s the MSDN article on the topic  http   msdn microsoft com en-us library system windows forms control validating aspx  The TL DR version  check the  Text property in the Validating event and set e Cancel True when the data is invalid   When you set e Cancel True  the user can t leave the field  but you will need to give them some kind of feedback that something s wrong  I change the box s background color to light red to indicate a problem  Make sure to set it back to SystemColors Window when Validating is called with a good value

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simply use this code in textbox    private void textBox1 TextChanged object sender  EventArgs e         double parsedValue       if   double TryParse textBox1 Text  out parsedValue                 textBox1 Text

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I am assuming from context and the tags you used that you are writing a  NET C  app  In this case  you can subscribe to the text changed event  and validate each key stroke   private void textBox1 TextChanged object sender  EventArgs e        if  System Text RegularExpressions Regex IsMatch textBox1 Text     0-9                   MessageBox Show  Please enter only numbers             textBox1 Text   textBox1 Text Remove textBox1 Text Length - 1

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You can use the TextChanged event  private void textBox BiggerThan TextChanged object sender  EventArgs e        long a      if    long TryParse textBox BiggerThan Text  out a                    If not int clear textbox text or Undo   last operation         textBox LessThan Clear

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I was also looking for the best way to check only numbers in textbox and problem with keypress was it does not support copy paste by right click or clipboard so came up with  this code which validates the when cursor leaves the text field and also it checks for empty field   adapted version of newguy   private void txtFirstValue MouseLeave object sender  EventArgs e        int num      bool isNum   int TryParse txtFirstValue Text Trim    out num        if   isNum  amp  amp  txtFirstValue Text    String Empty                MessageBox Show  The First Value You Entered Is Not a Number  Please Try Again    Invalid Value Detected   MessageBoxButtons OK  MessageBoxIcon Error           txtFirstValue Clear

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int Number  bool isNumber  isNumber   int32 TryPase textbox1 text  out Number    if   isNumber          code if not an integer     else        code if an integer

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In our webpage with the definition of textbox we can add an onkeypress event for accepting only numbers  It will not show any message but it will prevent you from wrong input  It worked for me  user could not enter anything except number    lt asp TextBox runat  server  ID  txtFrom       onkeypress  if isNaN String fromCharCode event keyCode    return false   gt

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This might be useful  It allows  real  numeric values  including proper decimal points and preceding plus or minus signs  Call it from within the related KeyPress event          private bool IsOKForDecimalTextBox char theCharacter  TextBox theTextBox                   Only allow control characters  digits  plus and minus signs             Only allow ONE plus sign             Only allow ONE minus sign             Only allow the plus or minus sign as the FIRST character             Only allow ONE decimal point             Do NOT allow decimal point or digits BEFORE any plus or minus sign           if                char IsControl theCharacter               amp  amp   char IsDigit theCharacter               amp  amp   theCharacter                      amp  amp   theCharacter     -                amp  amp   theCharacter                                            Then it is NOT a character we want allowed in the text box              return false                          Only allow one decimal point          if  theCharacter                     amp  amp  theTextBox Text IndexOf       gt  -1                           Then there is already a decimal point in the text box              return false                        Only allow one minus sign          if  theCharacter     -               amp  amp  theTextBox Text IndexOf  -    gt  -1                           Then there is already a minus sign in the text box              return false                        Only allow one plus sign          if  theCharacter                     amp  amp  theTextBox Text IndexOf       gt  -1                           Then there is already a plus sign in the text box              return false                        Only allow one plus sign OR minus sign  but not both          if                                  theCharacter     -                       theCharacter                                    amp  amp                                  theTextBox Text IndexOf  -    gt  -1                                      theTextBox Text IndexOf       gt  -1                                                       Then the user is trying to enter a plus or minus sign and                there is ALREADY a plus or minus sign in the text box              return false                        Only allow a minus or plus sign at the first character position          if                                  theCharacter     -                       theCharacter                                    amp  amp  theTextBox SelectionStart    0                                        Then the user is trying to enter a minus or plus sign at some position                 OTHER than the first character position in the text box              return false                        Only allow digits and decimal point AFTER any existing plus or minus sign         if                                             Is digit or decimal point                     char IsDigit theCharacter                                              theCharacter                                            amp  amp                                           A plus or minus sign EXISTS                      theTextBox Text IndexOf  -    gt  -1                                              theTextBox Text IndexOf       gt  -1                                     amp  amp                         Attempting to put the character at the beginning of the field                      theTextBox SelectionStart    0                                        Then the user is trying to enter a digit or decimal point in front of a minus or plus sign              return false                        Otherwise the character is perfectly fine for a decimal value and the character            may indeed be placed at the current insertion position          return true

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I had created a Re-Usable Textbox Extension class for all sorts of validations and thought about sharing it   All you need to do is raise a TextChange event and then call the Validate method  It looks like this    private void tbxAmount TextChanged object sender  EventArgs e         tbxAmount Validate TextValidator ValidationType Amount       Here is the extension class   public static class TextValidator       public enum ValidationType               Amount          Integer                 lt summary gt          Validate a textbox on text change           lt  summary gt           lt param name  tbx  gt  lt  param gt           lt param name  validationType  gt  lt  param gt      public static void Validate this TextBox tbx  ValidationType validationType                PerformValidation tbx  validationType           tbx Select tbx Text Length  0               private static void PerformValidation this TextBox tbx  ValidationType validationType                char   enteredString   tbx Text ToCharArray            switch  validationType                        case ValidationType Amount                  tbx Text   AmountValidation enteredString                   break               case ValidationType Integer                  tbx Text   IntegerValidation enteredString                   break               default                  break                     tbx SelectionStart   tbx Text Length               private static string AmountValidation char   enteredString                string actualString   string Empty          int count   0          foreach  char c in enteredString AsEnumerable                          if  count  gt   1  amp  amp  c                       actualString Replace c        actualString Trim                  else                               if  Char IsDigit c                                         actualString   actualString   c                                     if  c                                               actualString   actualString   c  count                                       else                                       actualString Replace c                            actualString Trim                                                      return actualString              private static string IntegerValidation char   enteredString                string actualString   string Empty          foreach  char c in enteredString AsEnumerable                          if  Char IsDigit c                                 actualString   actualString   c                            else                               actualString Replace c                        actualString Trim                                    return actualString            You can find the full code here

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Do not forget that a user can paste an invalid text in a TextBox    If you want to restrict that  follow the below code   private void ultraTextEditor1 TextChanged object sender  EventArgs e        string append         foreach  char c in ultraTextEditor1 Text                if    Char IsNumber c    amp  amp   c    Convert ToChar Keys Back                                 else                       append    c                       ultraTextEditor1 Text   append

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Try a MaskedTextBox  It takes a simple mask format so you can limit the input to numbers or dates or whatever

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Using the approach described in Fabio Iotti s answer I have created a more generic solution   public abstract class ValidatedTextBox   TextBox       private string m lastText   string Empty      protected abstract bool IsValid string text       protected sealed override void OnTextChanged EventArgs e            if   IsValid Text                 var pos   SelectionStart - Text Length   m lastText Length              Text   m lastText              SelectionStart   Math Max 0  pos                     m lastText   Text          base OnTextChanged e              ValidatedTextBox   which contains all nontrivial validation behavior  All that s left to do is inherit from this class and override  IsValid  method with whatever validation logic is required  For example  using this class  it is possible to create  RegexedTextBox  which will accept only strings which match specific regular expression   public abstract class RegexedTextBox   ValidatedTextBox       private readonly Regex m regex      protected RegexedTextBox string regExpString            m regex   new Regex regExpString             protected override bool IsValid string text            return m regex IsMatch Text             After that  inheriting from the  RegexedTextBox  class  we can easily create  PositiveNumberTextBox  and  PositiveFloatingPointNumberTextBox  controls   public sealed class PositiveNumberTextBox   RegexedTextBox       public PositiveNumberTextBox     base     d            public sealed class PositiveFloatingPointNumberTextBox   RegexedTextBox       public PositiveFloatingPointNumberTextBox             base      d      CultureInfo CurrentCulture NumberFormat NumberDecimalSeparator        d

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It seems like many of the current answers to this question are manually parsing the input text  If you re looking for a specific built-in numeric type  e g  int or double   why not just delegate the work to that type s TryParse method  For example   public class IntTextBox   TextBox       string PreviousText           int BackingResult       public IntTextBox                 TextChanged    IntTextBox TextChanged             public bool HasResult   get  private set         public int Result               get                       return HasResult   BackingResult   default int                        void IntTextBox TextChanged object sender  EventArgs e                HasResult   int TryParse Text  out BackingResult            if  HasResult    string IsNullOrEmpty Text                            Commit             PreviousText   Text                    else                          Revert             var changeOffset   Text Length - PreviousText Length              var previousSelectionStart                   Math Max 0  SelectionStart - changeOffset                Text   PreviousText              SelectionStart   previousSelectionStart                      If you want something more generic but still compatible with Visual Studio s Designer   public class ParsableTextBox   TextBox       TryParser BackingTryParse      string PreviousText           object BackingResult       public ParsableTextBox             this null                   public ParsableTextBox TryParser tryParse                TryParse   tryParse           TextChanged    ParsableTextBox TextChanged             public delegate bool TryParser string text  out object result        public TryParser TryParse               set                       Enabled     ReadOnly   value    null                BackingTryParse   value                       public bool HasResult   get  private set         public object Result               get                       return GetResult lt object gt                          public T GetResult lt T gt                  return HasResult    T BackingResult   default T              void ParsableTextBox TextChanged object sender  EventArgs e                if  BackingTryParse    null                        HasResult   BackingTryParse Text  out BackingResult                      if  HasResult    string IsNullOrEmpty Text                            Commit             PreviousText   Text                    else                          Revert             var changeOffset   Text Length - PreviousText Length              var previousSelectionStart                   Math Max 0  SelectionStart - changeOffset                Text   PreviousText              SelectionStart   previousSelectionStart                      And finally  if you want something fully generic and don t care about Designer support   public class ParsableTextBox lt T gt    TextBox       TryParser BackingTryParse      string PreviousText      T BackingResult       public ParsableTextBox             this null                   public ParsableTextBox TryParser tryParse                TryParse   tryParse           TextChanged    ParsableTextBox TextChanged             public delegate bool TryParser string text  out T result        public TryParser TryParse               set                       Enabled     ReadOnly   value    null                BackingTryParse   value                       public bool HasResult   get  private set         public T Result               get                       return HasResult   BackingResult   default T                        void ParsableTextBox TextChanged object sender  EventArgs e                if  BackingTryParse    null                        HasResult   BackingTryParse Text  out BackingResult                      if  HasResult    string IsNullOrEmpty Text                            Commit             PreviousText   Text                    else                          Revert             var changeOffset   Text Length - PreviousText Length              var previousSelectionStart                   Math Max 0  SelectionStart - changeOffset                Text   PreviousText              SelectionStart   previousSelectionStart

User · Answer

3 solution  1     Add to the textbox s KeyPress event   using Regex for number only textBox  private void txtBox KeyPress object sender  KeyPressEventArgs e    if   System Text RegularExpressions Regex IsMatch e KeyChar ToString       d     e Handled   true      2  an another solution from msdn     Boolean flag used to determine when a character other than a number is entered  private bool nonNumberEntered   false     Handle the KeyDown event to determine the type of character entered into the     control  private void textBox1 KeyDown object sender  KeyEventArgs e       Initialize the flag to false  nonNumberEntered   false     Determine whether the keystroke is a number from the top of the keyboard  if  e KeyCode  lt  Keys D0    e KeyCode  gt  Keys D9           Determine whether the keystroke is a number from the keypad      if  e KeyCode  lt  Keys NumPad0    e KeyCode  gt  Keys NumPad9                   Determine whether the keystroke is a backspace          if  e KeyCode    Keys Back                           A non-numerical keystroke was pressed                 Set the flag to true and evaluate in KeyPress event              nonNumberEntered   true                         private void textBox1 KeyPress object sender  KeyPressEventArgs e        if  nonNumberEntered    true               MessageBox Show  Please enter number only               e Handled   true            source  http   msdn microsoft com en-us library system windows forms control keypress v VS 90  aspx  3   using the MaskedTextBox  http   msdn microsoft com en-us library system windows forms maskedtextbox aspx

User · Answer

Take a look at Input handling in WinForm  I have posted my solution which uses the ProcessCmdKey and OnKeyPress events on the textbox  The comments show you how to use a Regex to verify the keypress and block allow appropriately

User · Answer

Here are more than 30 answers and a lot of answers are helpful  But I want to share a generalized form for the System Windows Forms TextBox and System Windows Controls TextBox  There is not available KeyPress event in System Windows Controls TextBox  This answer is for those people who want to implement with the same logic for System Windows Forms TextBox and System Windows Controls TextBox  This is NumberTextBox code  Use commented line instead of the previous line for System Windows Controls TextBox   public class NumberTextBox   System Windows Forms TextBox   public class NumberTextBox   System Windows Controls TextBox       private double  maxValue      private double  minValue      private bool  flag      private string  previousValue       public NumberTextBox                 this TextAlign   HorizontalAlignment Right            TextAlignment   TextAlignment Right          KeyDown    TextBox KeyDown          TextChanged    TextBox TextChanged           minValue   double MinValue           maxValue   double MaxValue             private void TextBox KeyDown object sender  KeyEventArgs e                 previousValue   this Text           flag   this SelectedText Length  gt  0             private void TextBox TextChanged object sender  EventArgs e        private void TextBox TextChanged object sender  TextChangedEventArgs e                var text   this Text          if  text Length  lt  1  return          var cursorPosition   SelectionStart    0   SelectionStart   SelectionStart - 1          var insertedChar   text cursorPosition           if  IsInvalidInput insertedChar  cursorPosition  text                         HandleText text  cursorPosition                     ValidateRange text  cursorPosition              private bool IsInvalidInput char insertedChar  int cursorPosition  string text                return  char IsDigit insertedChar   amp  amp  insertedChar         amp  amp  insertedChar     -                    insertedChar     -   amp  amp  cursorPosition    0                   text Count x   gt  x          gt  1                   text Count x   gt  x     -    gt  1             private void HandleText string text  int cursorPosition                this Text    flag    previousValue   text Remove cursorPosition  1           this SelectionStart   cursorPosition          this SelectionLength   0             private void ValidateRange string text  int cursorPosition                try                       if  text     quot   quot      minValue  lt  0  amp  amp  text     quot - quot   return              var doubleValue   Convert ToDouble text               if  doubleValue  gt   maxValue    doubleValue  lt   minValue                                HandleText text  cursorPosition                                   catch  Exception                        HandleText text  cursorPosition                        protected void SetProperties double minValue   double MinValue  double maxValue   double MaxValue                 minValue   minValue           maxValue   maxValue                   PositiveNumberTextBox code  public class PositiveNumberTextBox   NumberTextBox       public PositiveNumberTextBox                 SetProperties 0            FractionNumberTextBox code  public class FractionNumberTextBox   NumberTextBox       public FractionNumberTextBox                 SetProperties 0  0 999999

[c#] How do I make a textbox that only accepts numbers?

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