Is it possible define an extension operator method?

Question

is it possible to define an extension method that at the same time is an operator? I want for a fixed class add the possibility to use a known operator that actually can't be applied. For this particular case i want to do this:

   somestring++;  //i really know that this string contains a numeric value

And i don't want to spread types conversions for all the code. I know that i could create wrapper class over an string and define that operator but i want to know if this kind of thing is possible to avoid search-and-replace every string declaration with MySpecialString.

Edited: as most have say string is sealed, so derivation isn't possible, so i modify "derived" to "wrapper", my mistake.

Solution 1

No, it is not possible to do from outside of the class. ++ operator should be defined inside class which is being incremented. You can either create your own class which will be convertible from string and will have ++ overload or you can forget about this idea and use regular methods.

Solution 2

That is not possible in C#, but why not a standard extension method?

 public static class StringExtensions {
     public static string Increment(this string s) {
          ....
     }
 }

I think somestring.Increment() is even more readable, as you're not confusing people who really dont expect to see ++ applied to a string.

Solution 3

A clear example of where this would be useful is to be able to extend the TimeSpan class to include * and / operators.

This is what would ideally work...

public static class TimeSpanHelper
{
    public static TimeSpan operator *(TimeSpan span, double factor)
    {
        return TimeSpan.FromMilliseconds(span.TotalMilliseconds * factor);
    }

    public static TimeSpan operator *(double factor, TimeSpan span)  // * is commutative
    {
        return TimeSpan.FromMilliseconds(span.TotalMilliseconds * factor);
    }

    public static TimeSpan operator /(TimeSpan span, double sections)
    {
        return TimeSpan.FromMilliseconds(span.TotalMilliseconds / factor);
    }

    public static double operator /(TimeSpan span, TimeSpan period)
    {
        return span.TotalMilliseconds / period.TotalMilliseconds);
    }

}

Solution 4

No, you can't have an extension method which is also an operator. Extension methods can only be declared in static classes, which can't have instances and according to the C# spec,

User-defined operator declarations always require at least one of the parameters to be of the class or struct type that contains the operator declaration. [7.3.2]

Therefore, it is impossible for an extension method to also be an overloaded operator.

Additionally, you can't override System.String since it is a sealed class.

Solution 5

The string class is sealed in C#, so creating a string-derived class actually isn't possible.

That being said, an extension method will of course work just fine (as will a standard static method in a helper class) but it won't be an operator, just ordinarily-named method.

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