无法将类型隐式转换为类型参数 - C# generics

Question

I have declared a method with input and output type parameters. I have put some constrainsts on these types. See code below.

class Input { public int I { get; set; } }
    class InputType1 : Input { }
    class InputType2 : Input { }

    class Output { public int O { get; set; } }
    class OutputType1 : Output { }

    static TOutput Method<TInput, TOutput>(TInput input) 
        where TInput : Input 
        where TOutput : Output
    {
        var output = new Output() { O = input.I * 2 };
        return output; //  compile error: connot do conversion implicitly
    }

Why does the conversion can be done implicitly on the variable 'input' and not on the variable 'output'?

Answer 1

The compiler cannot infer out the concrete type of Output . The only thing the compiler knows the return type is some type that inherits from Output .

To create an instance of TOuptut change your code to

static TOutput Method<TInput, TOutput>(TInput input) 
        where TInput : Input 
        where TOutput : Output, new()
    {
        var output = new TOutput() { O = input.I * 2 };
        return output;
    }

Note the new() constraint requires that a parameterless constructor must exist on TOutput .