C＃-创建一个其构造函数采用通用参数的类，导致转换异常

Question

I have IInfo and its generic version:

public interface IInfo
{
    IInput Input { get; }
}

public interface IInfo<T> : IInfo where T : IInput
{
    new T Input { get; }
}

Class implementation:

public class Info : IInfo<IInput>
{
    public IInput Input { get; }    

    public Info (IInput input) {}
}

Factory to create IOutput from IInput:

public class GenericFactory<TInput, TOutput> where TInput : IInput where TOutput : IOutput
{
    public IOutput Create(IInfo info)
    {
        ConstructorInfo cInfo = typeof(TOutput).GetConstructor(new[] { typeof(IInfo<TInput>) });
        object output = cInfo.Invoke(new object[] {cInfo});
    }
}

To test the above code:

public class TestInput : IInput 
{       
}

public abstract class AbstractOutput<TInput> : IOutput where TInput : IInput
{       
}

public class TestOutput: AbstractOutput<TestInput>
{
    public TestOutput(IInfo<TestInput> info)
    {

    }       
}

public void Test()
{
    IInput input = new TestInput();
    IInfo info = new Info(input);

    var factory = new GenericFactory<TestInput, TestOutput>();
    IOutput output = factory.Create(info);
}

I get the following error:

Object of type 'Info' cannot be converted to type'Info<TestInput>'.

Side note: I'm open to any suggestions to simplify/re-write the code in a different way.

Answer 1

public TestOutput(IInfo<TestInput> info)
{

}

Is expecting an IInfo<TestInput> explicitly . However, you're trying to call it with IInfo<IInput> (which is what Info is designed to be).

To make it clear, you could also write:

IInput input = new OtherInput();
IInfo info = new Info(input);

var factory = new GenericFactory<TestInput, TestOutput>();
IOutput output = factory.Create(info);

And now you've provided IInfo<OtherInput> to something expecting IInfo<TestInput>

You would need to make IInfo<T> contravariant to allow it to be cast, for example:

public interface IInfo<in T> : IInfo 
    where T : IInput
{
    //new T Input { get; }
}

But note that it's illegal to return T when with a contravariant interface. The alternative is to make Info generic, and change Create to accept IInfo<TInput> . That latter gives you the benefit of a compile-time error when trying to pass IInfo<OtherInput> to Create() , rather than a run-time error