Operator Overloading with C# Extension Methods

Question

I'm attempting to use extension methods to add an operater overload to the C# StringBuilder class. Specifically, given StringBuilder sb, I'd like sb += "text" to become equivalent to sb.Append("text").

Here's the syntax for creating an extension method for StringBuilder:

public static class sbExtensions
{
    public static StringBuilder blah(this StringBuilder sb)
    {
        return sb;
    }
} 

It successfully adds the blah extension method to the StringBuilder.

Unfortunately, operator overloading does not seem to work:

public static class sbExtensions
{
    public static StringBuilder operator +(this StringBuilder sb, string s)
    {
        return sb.Append(s);
    }
} 

Among other issues, the keyword this is not allowed in this context.

Are adding operator overloads via extension methods possible? If so, what's the proper way to go about it?

Answer 1

This is not currently possible, because extension methods must be in static classes, and static classes can't have operator overloads.

Mads Torgersen, C# Language PM says:

...for the Orcas release we decided to take the cautious approach and add only regular extension methods, as opposed to extention properties, events, operators, static methods, etc etc. Regular extension methods were what we needed for LINQ, and they had a syntactically minimal design that could not be easily mimicked for some of the other member kinds.

We are becoming increasingly aware that other kinds of extension members could be useful, and so we will return to this issue after Orcas. No guarantees, though!

Edit:

I just noticed, Mads wrote more in the same article:

I am sorry to report that we will not be doing this in the next release. We did take extension members very seriously in our plans, and spent a lot of effort trying to get them right, but in the end we couldn't get it smooth enough, and decided to give way to other interesting features.

This is still on our radar for future releases. What will help is if we get a good amount of compelling scenarios that can help drive the right design.

---

This feature is currently on the table (potentially) for C# 8.0. Mads talks a bit more about implementing it here.

Answer 2

If you control the places where you want to use this "extension operator" (which you normally do with extension methods anyway), you can do something like this:

class Program {

  static void Main(string[] args) {
    StringBuilder sb = new StringBuilder();
    ReceiveImportantMessage(sb);
    Console.WriteLine(sb.ToString());
  }

  // the important thing is to use StringBuilderWrapper!
  private static void ReceiveImportantMessage(StringBuilderWrapper sb) {
    sb += "Hello World!";
  }

}

public class StringBuilderWrapper {

  public StringBuilderWrapper(StringBuilder sb) { StringBuilder = sb; }
  public StringBuilder StringBuilder { get; private set; }

  public static implicit operator StringBuilderWrapper(StringBuilder sb) {
    return new StringBuilderWrapper(sb);
  }

  public static StringBuilderWrapper operator +(StringBuilderWrapper sbw, string s) { 
      sbw.StringBuilder.Append(s);
      return sbw;
  }

} 

The StringBuilderWrapper class declares an implicit conversion operator from a StringBuilder and declares the desired + operator. This way, a StringBuilder can be passed to ReceiveImportantMessage, which will be silently converted to a StringBuilderWrapper, where the + operator can be used.

To make this fact more transparent to callers, you can declare ReceiveImportantMessage as taking a StringBuilder and just use code like this:

  private static void ReceiveImportantMessage(StringBuilder sb) {
    StringBuilderWrapper sbw = sb;
    sbw += "Hello World!";
  }

Or, to use it inline where you're already using a StringBuilder, you can simply do this:

 StringBuilder sb = new StringBuilder();
 StringBuilderWrapper sbw = sb;
 sbw += "Hello World!";
 Console.WriteLine(sb.ToString());

I created a post about using a similar approach to make IComparable more understandable.

Answer 3

It appears this isn't currently possible - there's an open feedback issue requesting this very feature on Microsoft Connect:

http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=168224

suggesting it might appear in a future release but isn't implemented for the current version.

Answer 4

Though it's not possible to do the operators, you could always just create Add (or Concat), Subtract, and Compare methods....

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;    

namespace Whatever.Test
{
    public static class Extensions
    {
        public static int Compare(this MyObject t1, MyObject t2)
        {
            if(t1.SomeValueField < t2.SomeValueField )
                return -1;
            else if (t1.SomeValueField > t2.SomeValueField )
            {
                return 1;
            }
            else
            {
                return 0;
            }
        }

        public static MyObject Add(this MyObject t1, MyObject t2)
        {
            var newObject = new MyObject();
            //do something  
            return newObject;

        }

        public static MyObject Subtract(this MyObject t1, MyObject t2)
        {
            var newObject= new MyObject();
            //do something
            return newObject;    
        }
    }


}

Answer 5

Hah! I was looking up "extension operator overloading" with exactly the same desire, for sb += (thing).

After reading the answers here (and seeing that the answer is "no"), for my particular needs, I went with an extension method which combines sb.AppendLine and sb.AppendFormat, and looks tidier than either.

public static class SomeExtensions
{
    public static void Line(this StringBuilder sb, string format, params object[] args)
    {
        string s = String.Format(format + "\n", args);
        sb.Append(s);
    }

}

And so,

sb.Line("the first thing is {0}",first);
sb.Line("the second thing is {0}", second);

Not a general answer, but may be of interest to future seekers looking at this kind of thing.

Answer 6

Its possible to rigg it with a wrapper and extensions but impossible to do it properly. You end with garbage which totally defeats the purpose. I have a post up somewhere on here that does it, but its worthless.

Btw All numeric conversions create garbage in string builder that needs to be fixed. I had to write a wrapper for that which does work and i use it. That is worth perusing.