Is it appropriate to use extension methods on types that you own?

Question

I have the following classes in my solution with the Request class being allowed to create Response instances:

public class Request
{
    public Guid Id {get;} 

    public int UserId {get;}

    public string SomeInfo {get;}

    public Request(Guid id, int userId, string someInfo)
    {
        Id = id;
        UserId = userId;
        SomeInfo = someInfo;
    }

    public Response GetResponse(string content)
    {
        return new Response(requestId, content);
    }
}

public class Response
{
    public Guid Id {get;}

    public Guid RequestId {get;}

    public string Content {get;}

    public Response(Guid requestId, string content)
    {
         Id = Guid.NewGuid();
         RequestId = requestId;
         Content = content;
    }
}

Let's say I remove the GetResponse method and move its code into an extension method, like this:

public static class RequestExtensions
{
     public static Response GetResponse(this Request request, string content)
     {
           return new Response(request.Id, content);
     }
}

Is it an appropriate use of the extension methods? I understand the need of those in the context where you do not have access to the type. But does that make sense to use an extension method on a type I do own?

Answer 1

In my mind (though this question is fairly opinion-based) doing this makes sense in one scenario:

• GetResponse needs to live in a different assembly than Response for some reason (and there are valid ones)

Otherwise you can just use a partial class if you want the code files separate for some reason.