泛型类型的异常

Question

public class clsA<T, T1>
    where T : class
    where T1 : class
{
    public T GetEntity()
    {
        return (T) new clsB<T1>();            
    }
}

I am getting error Cannot convert type clsB<T1> to T

Please help

Answer 1

That's because T could be anything -- string for example, and that would cause the cast to fail. If T cannot be a variable type, then do not include it in the generic argument list.

If you want to allow T to be clsB<T1> or any subtype, and you want to construct an instance of that class, then do this instead:

public class clsA<T, T1>
    where T : clsB<T1>, new()
    where T1 : class
{
    public T GetEntity()
    {
        return new T();
    }
}

Then, given the type:

public class clsC : clsB<string> { }

Invoking GetEntity() on an object of type clsA<clsC, string> will construct and return an instance of clsC .