通用类型。 没有装箱转换或类型参数转换

Question

I have the following structure of my code :

public interface ICode { }

public interface IPreparedCode<TCode>
    where TCode : ICode { }

public interface ILayer<TPreparedCode>
    where TPreparedCode : IPreparedCode<ICode> { }

public interface IContext<TCode, TPreparedCode, TLayer>
    where TCode : ICode
    where TPreparedCode : IPreparedCode<TCode>
    where TLayer : ILayer<TPreparedCode> { }

and i have error "There is no boxing conversion or type parameter conversion from TPreparedCode to IPreparedCode<ICode>".

Answer 1

The generic parameter for ILayer is constrained to be IPreparedCode<ICode> , but the generic parameter you are giving it, is TPreparedCode , which is constrained to IPreparedCode<TCode> . Notice the difference? IPreparedCode<TCode> can't be converted to IPreparedCode<ICode> .

To fix this, you can add an extra generic parameter TCode to ILayer , and constrain TPreparedCode to IPreparedCode<TCode> instead.:

public interface ICode { }

public interface IPreparedCode<TCode>
    where TCode : ICode { }

public interface ILayer<TPreparedCode, TCode>
    where TCode : ICode
    where TPreparedCode : IPreparedCode<TCode> { }

public interface IContext<TCode, TPreparedCode, TLayer>
    where TCode : ICode
    where TPreparedCode : IPreparedCode<TCode>
    where TLayer : ILayer<TPreparedCode, TCode> { }