Value of type T cannot be converted to

Question

This is likely a a novice question  but google surprisingly did not provide an answer   I have this rather artificial method  T HowToCast lt T gt  T t        if  typeof T     typeof string                 T newT1    some text           T newT2    string t             return t      Coming from a C   background I have expected this to work  However  it fails to compile with  Cannot implicitly convert type  T  to string  and  Cannot convert type  T  to string  for both of the above assignments   I am either doing something conceptually wrong or just have the wrong syntax  Please help me sort this one out   Thank you

User · Accepted Answer

Even though it s inside of an if block  the compiler doesn t know that T is string  Therefore  it doesn t let you cast   For the same reason that you cannot cast DateTime to string   You need to cast to object   which any T can cast to   and from there to string  since object can be cast to string   For example   T newT1    T  object  some text   string newT2    string  object t

User · Answer

If you re checking for explicit types  why are you declaring those variables as T s   T HowToCast lt T gt  T t        if  typeof T     typeof string                 var newT1    some text           var newT2   t     this builds but I m not sure what it does under the hood          var newT3   t ToString       for sure the string you want             return t

User · Answer

Both lines have the same problem  T newT1    some text   T newT2    string t    The compiler doesn t know that T is a string and so has no way of knowing how to assign that  But since you checked you can just force it with  T newT1    some text  as T  T newT2   t     you don t need to cast the t since it s already a string  also need to add the constraint  where T   class

User · Answer

You will also get this error if you have a generic declaration for both your class and your method   For example the code shown below gives this compile error     public class Foo  lt T gt         T var       public  lt T gt  void doSomething Class  lt T gt  cls  throws InstantiationException  IllegalAccessException           this var   cls newInstance               This code does compile  note T removed from method declaration    public class Foo  lt T gt         T var       public void doSomething Class  lt T gt  cls  throws InstantiationException  IllegalAccessException           this var   cls newInstance

User · Answer

Change this line   if  typeof T     typeof string     For this line   if  t GetType      typeof string

User · Answer

I know similar code that the OP posted in this question from generic parsers  From a performance perspective  you should use Unsafe As lt TFrom  TResult gt  ref TFrom source   which can be found in the System Runtime CompilerServices Unsafe NuGet package  It avoids boxing for value types in these scenarios  I also think that Unsafe As results in less machine code produced by the JIT than casting twice  using  TResult   object  actualString   but I haven t checked that out   public TResult ParseSomething lt TResult gt  ParseContext context        if  typeof TResult     typeof string                 var token   context ParseNextToken            string parsedString   token ParseToDotnetString            return Unsafe As lt string  TResult gt  ref parsedString             else if  typeof TResult     typeof int                 var token   context ParseNextToken            int parsedInt32   token ParseToDotnetInt32               This will not box which might be critical to performance         return Unsafe As lt int  TResult gt  ref parsedInt32                 other cases omitted for brevity s sake     Unsafe As will be replaced by the JIT with efficient machine code instructions  as you can see in the official CoreFX repo

[c#] Value of type 'T' cannot be converted to

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