Extension methods + Console apps

Question

Ok, i'm well and truly stumped here.

I've written extension methods before, and never had any problems. However, i've never had to use them in Console Apps. The following code won't compile and I have no idea why! I created a simple console app to try it out and it just won't work:

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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = "hello";

            Console.WriteLine(s.TestMethod());

            Console.Read();
        }
    }

    public static class ExtensionTest
    {
        public static string TestMethod(this string input)
        {
            return input.ToUpper();
        }
    }
}

Can anyone see what's wrong here?

The first error i'm running into is "Type expected" on line 21, which is:

(this string input)

I know i could quite easily just change to:

Console.WriteLine(ExtensionTest.TestMethod(s));

But i'd really like to know why i can't write this extension method in a console app.

Thanks for any help!

Answer 1

Just created a new Console application using that exact text and it compiled and worked as expected. Sure it's targeting the right framework?

Answer 2

It seems, your project is set up to compile for v2.0 of .NET framework. Try switching to v3.5 in project properties and referencing System.Core assembly..

Answer 3

You sure you are using C#3.0? The code doesn't have a problem in it.

If you are targeting .NET 2.0 with C# 3.0 you can still ue extension methods by defining the following type in your project:

namespace System.Runtime.CompilerServices
{
     [AttributeUsage(AttributeTargets.Method)]
     public class ExtensionAttribute : Attribute
     {
         public ExtensionAttribute() { }
     }
}

Answer 4

Ensure your project targets .NET Framework v3.0 or higher and make sure System.Core is a referenced assembly in your project.