Returning Null Generic Type

Question

I have a class:

public interface IA
{
    TValue? GetValue<TValue>();
}

public class A : IA
{
   public TValue? GetValue<TValue>()
   {
      ...

      if (value is not null)        
          return (TValue)value;

      return default(TValue);
   }
}

This is alright, but when I try to implement the interface explicitly:

public interface IA
{
    TValue? GetValue<TValue>();
}

public class A : IA
{
   TValue? IA.GetValue<TValue>()
   {
      ...

      if (value is not null)        
          return (TValue)value;

      return default(TValue);
   }
}

The compiler throws an error:

Cannot implicitly convert type TValue to TValue?

Is there a difference when implementing an interface explicitly? This seems to only be the case when dealing with a nullable generic return type.

Is this a bug or am I missing something?

Answer 1

You need to use default constraint:

TValue? IA.GetValue<TValue>() where TValue : default

To allow annotations for type parameters that are not constrained to reference types or value types