Constructor fun


2 min read

At work, I have a small gripe about a Response class. It's minor, but it keeps bugging me.

The object is very simple, it returns either a generic value or an error message and a Boolean to determine which was returned. It's a very simple object. My gripe is that there is no constructor to distinguish between a value or error returned. Every time I need to instantiate a Response object, I need to set the IsError property.

So I added two constructors, one for a value and one for an error.

public class Response<T>
  public Response(T value)
    Value = value;
    IsError = false;
  public Response(string error)
    Error = error;
    IsError = true;
  public T Value { get; }
  public string Error { get; }
  public bool IsError { get; }

Only when I started using this, I noticed a problem. What if the value I want to return is a string. With other objects, there is no conflict, but with a string value, the constructor thinks it is an error that is passed along.

Fortunately, there is a very simple solution. When I initialise the Response, I can select the constructor to use by using the parameter name.

var text = "no error";
var errorResponse = new ResponseMessage(text); // IsError = true
var valueResponse = new ResponseMessage(value: text); // IsError = false

In the actual code, I left in an empty constructor, so older code compiles.

The lesson I learned here is to think how I will use the classes I create and guide the instantiation process. It will help me down the line. Besides, fixing small gripes like this are quick wins that make me feel better about the code I produce.

All code can be found in this gist.

UPDATE: A friend, Wesley Cabus, took this idea a bit further and thought about how to do this a little cleaner. The problem he sees, is that it becomes difficult in a string case to distinguish between a value (new Response(value: "value")) and an error (new Response("error")) and I fully agree. So if you need a solution to this problem and are not tied to an existing solution, go check out Wesleys blogpost.