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 · Answer

I know similar code that the OP posted in this question from generic parsers. From a performance perspective, you should use Unsafe.As<TFrom, TResult>(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<TResult>(ParseContext context)
{
    if (typeof(TResult) == typeof(string))
    {
        var token = context.ParseNextToken();
        string parsedString = token.ParseToDotnetString();
        return Unsafe.As<string, TResult>(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<int, TResult>(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:

User · 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

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

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

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

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

Examples related to c#

Examples related to .net

Examples related to generics

Examples related to casting