Switch case in C - a constant value is expected

Question

My code is as follows   public static void Output lt T gt  IEnumerable lt T gt  dataSource  where T   class          dataSourceName    typeof T  Name       switch  dataSourceName                case  string typeof CustomerDetails  Name ToString                  var t   123              break          default              Console WriteLine  Test              But this is not working  The case statement is giving me an error saying that a constant variable is expected  Please help guys thank you

User · Answer

Now you can use nameof   public static void Output lt T gt  IEnumerable lt T gt  dataSource  where T   class       string dataSourceName   typeof T  Name      switch  dataSourceName                case nameof CustomerDetails               var t   123              break          default              Console WriteLine  Test              nameof CustomerDetails  is basically identical to the string literal  CustomerDetails   but with a compile-time check that it refers to some symbol  to prevent a typo    nameof appeared in C  6 0  so after this question was asked

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switch is very picky in the sense that the values in the switch must be a compile time constant  and also the value that s being compared must be a primitive  or string now   For this you should use an if statement    The reason may go back to the way that C handles them in that it creates a jump table  because the values are compile time constants  and it tries to copy the same semantics by not allowing evaluated values in your cases

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There is this trick which was shared with me  don t ask for details - won t be able to provide them  but it works for me   switch  variable 1        case var value when value    variable 2     that s the trick         DoSomething            break      default          DoSomethingElse            break

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You can t use a switch statement for this as the case values cannot be evaluated expressions  For this you have to use an an if else      public static void Output lt T gt  IEnumerable lt T gt  dataSource  where T   class          dataSourceName    typeof T  Name       if string Compare dataSourceName  typeof CustomerDetails  Name ToString    true   0                var t   123            else if    case 2 conditional                    blah           else                 default case         Console WriteLine  Test              I also took the liberty of tidying up your conditional statement  There is no need to cast to string after calling ToString    This will always return a string anyway  When comparing strings for equality  bare in mind that using the    operator will result in a case sensitive comparison  Better to use string compare   0 with the last argument to set case sensitive on off

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See  C  switch statement limitations - why   Basically Switches cannot have evaluated statements in the case statement  They must be statically evaluated

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This seems to work for me at least when i tried on visual studio 2017   public static class Words        public const string temp    What        public const string temp2    the     var i    the    switch  i      case Words temp      break    case Words temp2      break

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You can only match to constants in switch statements     Example   switch  variable1        case 1     A hard-coded value            Code         break      default             Code         break      Successful      switch  variable1        case variable2             Code         break      default             Code         break      CS0150 A constant value is expected

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Johnnie  Please go through msdn guide on switch  Also  the C  language specification clearly defines the compile time error case          If the type of the switch expression is sbyte  byte  short  ushort    int  uint  long  ulong  bool  char  string  or an enum-type  or if it   is the nullable type corresponding to one of these types  then that is   the governing type of the switch statement            Otherwise  exactly one user-defined implicit conversion    6 4  must   exist from the type of the switch expression to one of the following   possible governing types  sbyte  byte  short  ushort  int  uint  long    ulong  char  string  or   a nullable type corresponding to one of   those types            Otherwise  if no such implicit conversion exists  or if more than   one such implicit conversion exists  a compile-time error occurs    Hope this helps

[c#] Switch case in C# - a constant value is expected

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