无法转换为通用类型C＃

Question

I have the following scenario

public class A
{
}

public class BA : A
{

}

//other subtypes of A are defined

public class AFactory
{
    public T Create<T>() where T : A
    {
        //work to calculate condition
        if (condition)
            return new BA();
        //return other subtype of A
    }
}

The following compilation error is thrown:

Error CS0029 Cannot implicitly convert type 'B' to 'T'

What's wrong?

Answer 1

Well the cast could easily fail. Suppose I have:

public class AB : A {}

B b = new B();
AB ab = b.Create<AB>();

That would end up trying to assign a B reference to a variable of type AB . Those are incompatible.

It sounds like you probably shouldn't make Create a generic method. Or maybe you should make A generic:

public abstract class A<T> where T : A
{
    public abstract T Create();
}

public class B : A<B>
{
    public override B Create()
    {
        return new B();
    }
}

That would work - but we don't know what you're trying to achieve, so it may not actually help you.

Alternatively you could keep your current design, but use:

public T Create<T>() where T :  A
{
    return (T) (object) new B();
}

That will then fail if you call Create with a type argument of anything other than object , A or B , which sounds a little odd to me...

Answer 2

T can be any class derived from A . There could also be a

class C : A { }

It matches the generic constraint, but you can't return an instance of B from Create<C>() method.

Answer 3

Right. For creating new B class you should use this code:

public class A
{
}

public class B : A
{
    public static T Create<T>() where T : A, new()
    {
        return new T();
    }
}

public class Example
{
    public void DoWork()
    {

        B b = B.Create<B>();
        A a = B.Create<A>();
    }
}