Operator Overloading with C# Extension Methods

This is not currently possible, because extension methods must be in static classes, and static classes can't have operator overloads. But the feature is being discussed for some future release of C#. Mads talked a bit more about implementing it in this video from 2017.

On why it isn't currently implemented, 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!

Further below 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.

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.

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.