C# generic with constant

问题:

Is there something similar to this C++ template?

template <int A>
class B
{
    int f()
    {
        return A;
    }
}

I want to make every instance of B<1>, B<2>, etc (eg tuple) a different type.

回答1:

The short answer is no.

It doesn't fit the way C# generics, as apposed to C++ templates, work.

The .net generics are not a language feature, they are a runtime feature. The runtime knows how to instantiate generics from special generic bytecode which is rather restricted compared to what C++ templates can describe.

Compare this with C++ templates, which basically instantiate the whole AST of the class using substituted types. It'd be possible to add AST based instantiation to the runtime, but it'd certainly be much more complex than the current generics.

Without features like value-type arrays (which only exist in unsafe code), recursive template instantiation or template specialization using such parameters wouldn't be very useful either.

回答2:

A workaround to this limitation is to define a class which itself exposes the literal value you are interested in. For example:

public interface ILiteralInteger
{
   int Literal { get; }
}

public class LiteralInt10 : ILiteralInteger
{
   public int Literal { get { return 10; } }
}

public class MyTemplateClass< L > where L: ILiteralInteger, new( )
{
   private static ILiteralInteger MaxRows = new L( );

   public void SomeFunc( )
   {
      // use the literal value as required
      if( MaxRows.Literal ) ...
   }
}

Usage:

var myObject = new MyTemplateClass< LiteralInt10 >( );

回答3:

C# does not support non-type generic parameters like C++ does.

C# generics are far simpler and less capable than C++ templates. MSDN has a succinct list of Differences Between C++ Templates and C# Generics.

回答4:

C# generics are specialized at run-time, whereas C++ templates are processed at compile-time to make an entirely new type. Given this, the runtime simply doesn't have the features to process non-type parameters (it's not just a C# issue).