Only allow specific characters in textbox

Question

How can I only allow certain characters in a Visual C  textbox  Users should be able to input the following characters into a text box  and everything else should be blocked  0-9     -               I ve used Google to look up this problem  but the only solutions I m getting are allowing only alphabetic characters  only numerical or disallowing certain characters  What I want is not disallowing certain characters  I want to disallow everything by default except the characters that I put in the code

User · Accepted Answer

As mentioned in a comment  and another answer as I typed  you need to register an event handler to catch the keydown or keypress event on a text box  This is because TextChanged is only fired when the TextBox loses focus  The below regex lets you match those characters you want to allow  Regex regex   new Regex    0-9  -             MatchCollection matches   regex Matches textValue     and this does the opposite and catches characters that aren t allowed  Regex regex   new Regex     0-9    -                 MatchCollection matches   regex Matches textValue     I m not assuming there ll be a single match as someone could paste text into the textbox  in which case catch textchanged  textBox1 TextChanged    new TextChangedEventHandler textBox1 TextChanged   private void textBox1 TextChanged object sender  EventArgs e        Regex regex   new Regex     0-9    -                     MatchCollection matches   regex Matches textBox1 Text       if  matches Count  gt  0             tell the user           and to validate single key presses  textBox1 KeyPress    new KeyPressEventHandler textBox1 KeyPress   private void textBox1 KeyPress object sender  System Windows Forms KeyPressEventArgs e           Check for a naughty character in the KeyDown event      if  System Text RegularExpressions Regex IsMatch e KeyChar ToString        0-9    -                                  Stop the character from being entered into the control since it is illegal          e Handled   true

User · Answer

You need to subscribe to the KeyDown event on the text box  Then something like this   private void textBox1 KeyDown object sender  System Windows Forms KeyEventArgs e        if   char IsControl e KeyChar           amp  amp   char IsDigit e KeyChar           amp  amp  e KeyChar         amp  amp  e KeyChar         amp  amp  e KeyChar     -          amp  amp  e KeyChar         amp  amp  e KeyChar         amp  amp  e KeyChar                 amp  amp  e KeyChar                       e Handled   true          return            e Handled false      return      The important thing to know is that if you changed the Handled property to true  it will not process the keystroke  Setting it to false will

User · Answer

You can probably use the KeyDown event  KeyPress event or KeyUp event  I would first try the KeyDown event I think   You can set the Handled property of the event args to stop handling the event

User · Answer

For your validation event IMO the easiest method would be to use a character array to validate textbox characters against  True - iterating and validating isn t particularly efficient  but it is straightforward   Alternately  use a regular expression of your whitelist characters against the input string  Your events are availalbe at MSDN here  http   msdn microsoft com en-us library system windows forms control lostfocus aspx

User · Answer

Intercept the KeyPressed event is in my opinion a good solid solution  Pay attention to trigger code characters  e KeyChar lower then 32  if you use a RegExp   But in this way is still possible to inject characters out of range whenever the user paste text from the clipboard  Unfortunately I did not found correct clipboard events to fix this   So a waterproof solution is to intercept TextBox TextChanged  Here is sometimes the original out of range character visible  for a short time  I recommend to implement both   using System Text RegularExpressions   private void Form1 Shown object sender  EventArgs e        filterTextBoxContent textBox1       string pattern       0-9    -             private void textBox1 KeyPress object sender  KeyPressEventArgs e        if e KeyChar  gt   32  amp  amp  Regex Match e KeyChar ToString    pattern  Success    e Handled   true       private void textBox1 TextChanged object sender  EventArgs e        filterTextBoxContent textBox1      private bool filterTextBoxContent TextBox textBox        string text   textBox Text       MatchCollection matches   Regex Matches text  pattern       bool matched   false       int selectionStart   textBox SelectionStart      int selectionLength   textBox SelectionLength       int leftShift   0      foreach  Match match in matches                if  match Success  amp  amp  match Captures Count  gt  0                        matched   true              Capture capture   match Captures 0                int captureLength   capture Length              int captureStart   capture Index - leftShift              int captureEnd   captureStart   captureLength               int selectionEnd   selectionStart   selectionLength               text   text Substring 0  captureStart    text Substring captureEnd  text Length - captureEnd                textBox Text   text               int boundSelectionStart   selectionStart  lt  captureStart   -1    selectionStart  lt  captureEnd   0   1               int boundSelectionEnd   selectionEnd  lt  captureStart   -1    selectionEnd  lt  captureEnd   0   1                if  boundSelectionStart    -1                                if  boundSelectionEnd    0                                        selectionLength -  selectionEnd - captureStart                                    else if  boundSelectionEnd    1                                        selectionLength -  captureLength                                              else if  boundSelectionStart    0                                if  boundSelectionEnd    0                                        selectionStart   captureStart                      selectionLength   0                                    else if  boundSelectionEnd    1                                        selectionStart   captureStart                      selectionLength -  captureEnd - selectionStart                                              else if  boundSelectionStart    1                                selectionStart -  captureLength                             leftShift                         textBox SelectionStart   selectionStart      textBox SelectionLength   selectionLength       return matched

User · Answer

private void txtuser KeyPress object sender  KeyPressEventArgs e                if   char IsLetter e KeyChar   amp  amp   char IsWhiteSpace e KeyChar   amp  amp   char IsControl e KeyChar                         e Handled   true

[c#] Only allow specific characters in textbox

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