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. 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.