Extending string with Extension Methods (C# 3.0)?

Question

I know I can extend the string class like so:

public static class EMClass
{
   public static int ToInt32Ext(this string s)
   {
      return Int32.Parse(s);
   }
   public static int ToInt32Static(string s)
   {
      return Int32.Parse(s);
   }
}

And use it like this:

string x = "12";
x.ToInt32Ext()

But how can I make it to where I can use it like this:

int x = string.ToInt32("15");

Remember that the first argument in the method signature, (this T argt), represents the type, and not the instance of the type! — Claus Jørgensen, Aug 18 '09 at 18:09
@Claus This is wrong - the first parameter contains the instance the extension method was invoked on (or null if the extension method was invoked on null). — Daniel Brückner, Aug 18 '09 at 18:13

score 12 · Accepted Answer · answered Aug 18 '09 at 18:06

12

I don't think you can do "static" extension methods.

answered Aug 18 '09 at 18:06

Andy Gaskell

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That's right. This feature has been suggested to Microsoft many times (Microsoft Connect) but won't be added in the near future. – Meta-Knight Aug 18 '09 at 18:18
I really, really want static extension methods. – Callum Rogers Aug 18 '09 at 19:02

score 3 · Answer 2 · answered Aug 18 '09 at 18:14

3

Dude, do you have any problems with Int32.Parse() ?

answered Aug 18 '09 at 18:14

Leahn Novash

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Agree- even if the code was actually possibly, this is an excellent example of a useless extension method. – RichardOD Aug 18 '09 at 18:47

score 3 · Answer 3 · edited May 23 '17 at 12:32

3

What I did in our project at work is created a class called Strings which is where I put all of our extension methods. That way you have something that visually looks similar to string.

You can call the extension method in your example like so too (not all people might like this, but I use this myself depending on the code I'm working on):

int x = "15".ToInt32();

edited May 23 '17 at 12:32

Community

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answered Aug 18 '09 at 18:16

Brian Surowiec

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score 1 · Answer 4 · answered Aug 18 '09 at 18:20

Extension methods only apply to instances of a class, not the class itself. You can't extend a class with a class method using an extension. Note that you should easily be able to do what you want with an extension method, just using.

var fifteen = "15".ToInt32Ext();

Brian Rudolph · Answer 5 · 2009-08-18T18:45:10.510

1

You can't really do this... the closest you are going to get is this:

public static class Ext
{
    public static class String
    {
        public static int ToInt32(string val)
        {
            return Int32.Parse(val);
        }
    }
}

//Then call:
Ext.String.ToInt32("32");

Which i'm surprised the compiler even allows you to name a class "String"

Just for entertainment, i thought of another fun way to do it, though i would never recommend this. Add:

using String = NameSpace1.Ext.String;

//Then you can access it as:
String.ToInt32("32"); //Yipes

edited Aug 18 '09 at 18:45

answered Aug 18 '09 at 18:30

Brian Rudolph

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Why a surprise? You can have a lot of class with the same name, just not in the same namespace ;) – Patrick Desjardins Aug 18 '09 at 18:32
You *almost* got my +1. I was with you until you've created another String, I find it very confusing. But your answer is correct: if he wants a static method, it's easiest to create a static class - he has to use a class anyway. – Kobi Aug 18 '09 at 18:56
Well, he wanted an answer of how he could do it. The simple answer is "no you can't do that", unfortunately thats not entirely true. You could do it, i would just not recommend it at all. Surely, future developers on the proejct would hate you for it. – Brian Rudolph Aug 18 '09 at 19:08

score 0 · Answer 6 · answered Aug 18 '09 at 21:29

Personally, I don't see a lot of use for an extension-method wrapper to a method call that's already a short one-liner. On the other hand, wrapping the Int32.TryParse pattern to return a nullable int can be pretty convenient:

    public static int? ToInt32(this string input)
    {
        if (String.IsNullOrEmpty(input))
            return null;

        int parsed;
        if (Int32.TryParse(input, out parsed))
            return parsed;

        return null;
    }

This allows you to parse a value that might not be present, or might not contain an integer, and provide a default value in a single line of code:

int id = Request.QueryString["id"].ToInt32() ?? 0;

Extending string with Extension Methods (C# 3.0)?

6 Answers6