C＃中的泛型编译错误（但在VB.NET中有效）

Question

I'm having an odd problem with generics. I receive the following compile errors:

The best overloaded method match has some invalid arguments

Argument '1': cannot convert from 'EntityBase' to 'T'

The error is in EntityWrapper.DoSomethingElse , see below:

public abstract class EntityBase
{
    public static bool DoSomething<T>(T entity, string someArg) where T : EntityBase
    {
        // implementation doesn't matter
        return true;
    }
}

public class EntityWrapper<T> where T : EntityBase
{
    private EntityBase _entity;

    public void DoSomethingElse()
    {

        EntityBase.DoSomething<T>(_entity, "some arg"); // <--- error here ---
    }

}

I have a VB.NET version of this code that compiles and executes just fine, so I would expect it to work in C#.

What am I missing here?

Finally, though it shouldn't matter, but this is VS2008, .NET 3.5.

Answer 1

Just omit the <T> . Type inference will figure it out:

EntityBase.DoSomething(_entity, "some arg"); 

Answer 2

Let me show you why your code is invalid: Imagine I create an EntityWrapper<MyEntity> , where MyEntity derives from BaseEntity :

var myWrapper = new EntityWrapper<MyEntity>();

What happens inside EntityWrapper? This:

EntityBase.DoSomething<T>(_entity, "some arg");

becomes

EntityBase.DoSomething<MyEntity>(_entity, "some arg");

which is invalid: DoSomething expects a MyEntity as its first argument, but you pass a BaseEntity . This is what the error Argument '1': cannot convert from 'EntityBase' to 'T' means.

---

How to fix this? In EntityWrapper, declare _entity as follows:

private T _entity;

This allows you to keep _entity statically typed to the concrete subtype of BaseEntity .

Answer 3

Maybe you could call the method as follows:

EntityBase.DoSomething<EntityBase>(_entity, "some arg");

The compiler fails to cast _entity (which is EntityBase ) to T (which is specified in the method call).