Overload constructors with normal and generic parameters

Question

I have a Response class with two constructors. One receive a Result object and other a generic Result object.

How to declare the generic constructor of Response class to get the generic object?

In this sample code, the non-generic constructor is used.

var result = new Result<SomeModel>();
var response = new Response(result); // should use the generic constructor but uses the normal

public interface IModel
{
}

public class SomeModel : IModel
{
    public string Text { get; set; }
}

public class Result<T> : Result where T : class, IModel
{
    public T Model { get; set; }
}

public class Result
{
    public bool Success { get; set; }
}

public class Response
{
    public Response(Result<IModel> data)
    {
    }

    public Response(Result data)
    {
    }
}

Try this and look at the message why it won't compile: `new Response((Result)result)` -- this is a (co|contra)variance thing. `Result` isn't `Result`. — 15ee8f99-57ff-4f92-890c-b56153, Apr 13 '18 at 17:23

score 3 · Accepted Answer · answered Apr 13 '18 at 17:26

A Result<IModel> isn't the same as a Result<SomeModel>.

You have two options: first, you can make Response generic as Response<T> where T : IModel and then change the constructor to take a Result<T>.

Alternately, you can define an IResult<out T> interface (implemented by Result<T> with only a getter for the model property, and change the constructor of Response to take an IResult<IModel>; this is called "covariance".

Overload constructors with normal and generic parameters

1 Answers1