Convert string to nullable type int double etc

Question

I am attempting to do some data conversion  Unfortunately  much of the data is in strings  where it should be int s or double  etc      So what I ve got is something like   double  amount   Convert ToDouble strAmount     The problem with this approach is if strAmount is empty  if it s empty I want it to amount to be null  so when I add it into the database the column will be null  So I ended up writing this   double  amount   null  if strAmount Trim   Length gt 0        amount   Convert ToDouble strAmount       Now this works fine  but I now have five lines of code instead of one  This makes things a little more difficult to read  especially when I have a large amount of columns to convert   I thought I d use an extension to the string class and generic s to pass in the type  this is because it could be a double  or an int  or a long  So I tried this   public static class GenericExtension       public static Nullable lt T gt  ConvertToNullable lt T gt  this string s  T type  where T  struct               if  s Trim   Length  gt  0                        return  Nullable lt T gt  s                    return null            But I get the error  Cannot convert type  string  to  T    Is there a way around this  I am not very familiar with creating methods using generics

User · Answer

Let s add one more similar solution to the stack  This one also parses enums  and it looks nice  Very safe         lt summary gt           lt para gt More convenient than using T TryParse string  out T            Works with primitive types  structs  and enums          Tries to parse the string to an instance of the type specified          If the input cannot be parsed  null will be returned           lt  para gt           lt para gt          If the value of the caller is null  null will be returned          So if you have  string s   null   and then you try  s ToNullable              null will be returned  No null exception will be thrown            lt  para gt           lt author gt Contributed by Taylor Love  Pangamma  lt  author gt           lt  summary gt           lt typeparam name  T  gt  lt  typeparam gt           lt param name  p self  gt  lt  param gt           lt returns gt  lt  returns gt      public static T  ToNullable lt T gt  this string p self  where T   struct               if   string IsNullOrEmpty p self                         var converter   System ComponentModel TypeDescriptor GetConverter typeof T                if  converter IsValid p self   return  T converter ConvertFromString p self               if  typeof T  IsEnum    T t  if  Enum TryParse lt T gt  p self  out t   return t                      return null          https   github com Pangamma PangammaUtilities-CSharp blob master PangammaUtilities Extensions ToNullableStringExtension cs

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Another thing to keep in mind is that the string itself might be null   public static Nullable lt T gt  ToNullable lt T gt  this string s  where T  struct       Nullable lt T gt  result   new Nullable lt T gt         try               if   string IsNullOrEmpty s   amp  amp  s Trim   Length  gt  0                        TypeConverter conv   TypeDescriptor GetConverter typeof T                result    T conv ConvertFrom s                       catch          return result

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I like Joel s answer  but I ve modified it slightly as I m not a fan of eating exceptions            lt summary gt          Converts a string to the specified nullable type           lt  summary gt           lt typeparam name  T  gt The type to convert to lt  typeparam gt           lt param name  s  gt The string to convert lt  param gt           lt returns gt The nullable output lt  returns gt      public static T  ToNullable lt T gt  this string s  where T   struct               if  string IsNullOrWhiteSpace s               return null           TypeConverter conv   TypeDescriptor GetConverter typeof  T            return  T  conv ConvertFrom s                   lt summary gt          Attempts to convert a string to the specified nullable primative           lt  summary gt           lt typeparam name  T  gt The primitive type to convert to lt  typeparam gt           lt param name  data  gt The string to convert lt  param gt           lt param name  output  gt The nullable output lt  param gt           lt returns gt          True if conversion is successfull  false otherwise   Null and whitespace will         be converted to null and return true           lt  returns gt      public static bool TryParseNullable lt T gt  this string data  out T  output  where T   struct               try                       output   data ToNullable lt T gt                 return true                    catch                       output   null              return false

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There is a generic solution  for any type   Usability is good  but implementation should be improved  http   cleansharp de wordpress 2011 05 generischer-typeconverter   This allows you to write very clean code like this   string value   null  int  x   value ConvertOrDefault lt int  gt       and also   object obj   1     string value   null  int x   5  if  value TryConvert out x       Console WriteLine  TryConvert example      x     bool boolean    false  ConvertOrDefault lt bool gt     bool  nullableBoolean      ConvertOrDefault lt bool  gt     int integer   obj ConvertOrDefault lt int gt     int negativeInteger    -12123  ConvertOrDefault lt int gt     int  nullableInteger   value ConvertOrDefault lt int  gt     MyEnum enumValue    SecondValue  ConvertOrDefault lt MyEnum gt      MyObjectBase myObject   new MyObjectClassA    MyObjectClassA myObjectClassA   myObject ConvertOrDefault lt MyObjectClassA gt

User · Answer

public static class GenericExtension       public static T  ConvertToNullable lt T gt  this String s  where T   struct                try                       return  T  TypeDescriptor GetConverter typeof T   ConvertFrom s                     catch  Exception                        return null

User · Answer

Give this a shot     public delegate bool TryParseDelegate lt T gt  string data  out T output    public static T  ToNullablePrimitive lt T gt  this string data       TryParseDelegate lt T gt  func  where T struct       string IsNullOrEmpty data  return null       T output       if  func data  out output                 return  T  output             return null      Then call it like this     void doStuff         string foo    1 0        double  myDouble   foo ToNullablePrimitive lt double gt  double TryParse        foo    1        int  myInt   foo ToNullablePrimitive lt int gt  int TryParse        foo    haha        int  myInt2   foo ToNullablePrimitive lt int gt  int TryParse

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I wrote this generic type converter   It works with Nullable and standard values  converting between all convertible types - not just string   It handles all sorts of scenarios that you d expect  default values  null values  other values  etc      I ve been using this for about a year in dozens of production programs  so it should be pretty solid       public static T To lt T gt  this IConvertible obj                Type t   typeof T            if  t IsGenericType              amp  amp   t GetGenericTypeDefinition      typeof Nullable lt  gt                           if  obj    null                                return  T  object null                            else                               return  T Convert ChangeType obj  Nullable GetUnderlyingType t                                    else                       return  T Convert ChangeType obj  t                        public static T ToOrDefault lt T gt                    this IConvertible obj                try                       return To lt T gt  obj                     catch                       return default T                        public static bool ToOrDefault lt T gt                           this IConvertible obj                           out T newObj                try                       newObj   To lt T gt  obj               return true                    catch                       newObj   default T               return false                       public static T ToOrOther lt T gt                              this IConvertible obj                             T other                try                       return To lt T gt  obj                     catch                       return other                       public static bool ToOrOther lt T gt                                this IConvertible obj                               out T newObj                               T other                try                       newObj   To lt T gt  obj               return true                    catch                       newObj   other              return false                       public static T ToOrNull lt T gt                             this IConvertible obj                            where T   class               try                       return To lt T gt  obj                     catch                       return null                       public static bool ToOrNull lt T gt                         this IConvertible obj                        out T newObj                        where T   class               try                       newObj   To lt T gt  obj               return true                    catch                       newObj   null              return false

User · Answer

The generic answer provided by  Joel Coehoorn  is good    But  this is another way  without using those GetConverter    or try catch blocks     i m not sure but this may have better performance in some cases    public static class StrToNumberExtensions       public static short ToShort this string s  short defaultValue   0    gt  short TryParse s  out var v    v   defaultValue      public static int ToInt this string s  int defaultValue   0    gt  int TryParse s  out var v    v   defaultValue      public static long ToLong this string s  long defaultValue   0    gt  long TryParse s  out var v    v   defaultValue      public static decimal ToDecimal this string s  decimal defaultValue   0    gt  decimal TryParse s  out var v    v   defaultValue      public static float ToFloat this string s  float defaultValue   0    gt  float TryParse s  out var v    v   defaultValue      public static double ToDouble this string s  double defaultValue   0    gt  double TryParse s  out var v    v   defaultValue       public static short  ToshortNullable this string s  short  defaultValue   null    gt  short TryParse s  out var v    v   defaultValue      public static int  ToIntNullable this string s  int  defaultValue   null    gt  int TryParse s  out var v    v   defaultValue      public static long  ToLongNullable this string s  long  defaultValue   null    gt  long TryParse s  out var v    v   defaultValue      public static decimal  ToDecimalNullable this string s  decimal  defaultValue   null    gt  decimal TryParse s  out var v    v   defaultValue      public static float  ToFloatNullable this string s  float  defaultValue   null    gt  float TryParse s  out var v    v   defaultValue      public static double  ToDoubleNullable this string s  double  defaultValue   null    gt  double TryParse s  out var v    v   defaultValue      Usage is as following   var x1    123  ToInt      123 var x2    abc  ToInt      0 var x3    abc  ToIntNullable        int  null  int x4     string null  ToInt -1      -1 int x5    abc  ToInt -1      -1  var y    19 50  ToDecimal      19 50  var z1    invalid number string  ToDoubleNullable        double  null var z2    invalid number string  ToDoubleNullable 0       double  0

User · Answer

Another variation  This one     Does not swallow exceptions Throws a NotSupportedException if the type can not be converted from string  For instance  a custom struct without a type converter  Otherwise returns a  T  null if the string fails to parse  No need to check for null or whitespace      using System ComponentModel   public static Nullable lt T gt  ToNullable lt T gt  this string s  where T   struct       var ret   new Nullable lt T gt         var conv   TypeDescriptor GetConverter typeof T         if   conv CanConvertFrom typeof string                  throw new NotSupportedException               if  conv IsValid s                 ret    T conv ConvertFrom s              return ret

User · Answer

Here s something based on accepted answer  I removed the try catch to make sure all the exceptions are not swallowed and not dealt with  Also made sure that the return variable  in accepted answer  is never initialized twice for nothing   public static Nullable lt T gt  ToNullable lt T gt  this string s  where T  struct       if   string IsNullOrWhiteSpace s                 TypeConverter conv   TypeDescriptor GetConverter typeof T             return  T conv ConvertFrom s              return default Nullable lt T gt

User · Answer

There is no way around this   Nullable  as well as your method  is constrained to using only value types as it s argument   String is a reference type and hence is incompatible with this declaration

User · Answer

You might want to try   TypeConverter conv   TypeDescriptor GetConverter typeof int    conv ConvertFrom mystring     do your own null check and return int  if necessary  You ll also want to wrap that in a try

User · Answer

What about this   double  amount   string IsNullOrEmpty strAmount     double  null   Convert ToDouble strAmount     Of course  this doesn t take into account the convert failing

User · Answer

My example for anonimous types   private object ConvertNullable object value  Type nullableType        Type resultType   typeof Nullable lt  gt   MakeGenericType nullableType GetGenericArguments         return Activator CreateInstance resultType  Convert ChangeType value  nullableType GetGenericArguments   0             Type anonimousType   typeof Nullable lt int gt    object nullableInt1   ConvertNullable  5   anonimousType      or evident Type Nullable lt int gt  nullableInt2    Nullable lt int gt  ConvertNullable  5   typeof Nullable lt int gt

User · Answer

You can use the following with objects  unfortunately this does not work with strings though   double  amount    double  someObject    I use it for wrapping a session variable in a property  on a base page    so my actual usage is  in my base page    public int  OrganisationID       get   return  int  Session Constants Session Key OrganisationID         set   Session Constants Session Key OrganisationID    value        I m able to check for null in page logic   if  base OrganisationID    null         do stuff

User · Answer

You could try using the below extension method   public static T  GetValueOrNull lt T gt  this string valueAsString      where T   struct        if  string IsNullOrEmpty valueAsString           return null      return  T  Convert ChangeType valueAsString  typeof T        This way you can do this   double  amount   strAmount GetValueOrNull lt double gt     int  amount   strAmount GetValueOrNull lt int gt     decimal  amount   strAmount GetValueOrNull lt decimal gt

[c#] Convert string to nullable type (int, double, etc...)

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