Can I add extension methods to an existing static class?

Question

I'm a fan of extension methods in C#, but haven't had any success adding an extension method to a static class, such as Console.

For example, if I want to add an extension to Console, called 'WriteBlueLine', so that I can go:

Console.WriteBlueLine("This text is blue");

I tried this by adding a local, public static method, with Console as a 'this' parameter... but no dice!

public static class Helpers {
    public static void WriteBlueLine(this Console c, string text)
    {
        Console.ForegroundColor = ConsoleColor.Blue;
        Console.WriteLine(text);
        Console.ResetColor();
    }
}

This didn't add a 'WriteBlueLine' method to Console... am I doing it wrong? Or asking for the impossible?

Answer 1

You are indeed asking the impossible.

Answer 2

You can't add static methods to a type. You can only add (pseudo-)instance methods to an instance of a type.

The point of the this modifier is to tell the C# compiler to pass the instance on the left-side of the . as the first parameter of the static/extension method.

In the case of adding static methods to a type, there is no instance to pass for the first parameter.

Answer 3

You may be asking the impossible as it stands, however this is certainly something that would be as technically possible to do as instance-based extension methods if they provided a syntax for it. Sounds like the idea simply got missed by the C# architects. Maybe something similar to partial classes that could package extension methods for a class (without actually changing the class itself):

// NOTE: this is an imaginary (not valid) syntax!
public extension class Console
{
    public static void WriteBlueLine( string text)
    {
        // ...
    }
}

This might also have been a somewhat cleaner (others' opinions might well differ on this) way to package instance-based extension methods as well. There would be namespacing issues, but with some thought (which I clearly have not done much of on this) I think they could be worked out.

Unfortunately, you came up with the idea too late!

Answer 4

I tried to do this with System.Environment back when I was learning extension methods and was not successful. The reason is, as others mention, because extension methods require an instance of the class.

Answer 5

Nope. Extension method definitions require an instance of the type you're extending. Its unfortunate; I'm not sure why its required...

Answer 6

No. Extension methods require an instance of an object. You can however, write a static wrapper around the ConfigurationManager interface. If you implement the wrapper, you don't need an extension method since you can just add the method directly.

 public static class ConfigurationManagerWrapper
 {
      public static ConfigurationSection GetSection( string name )
      {
         return ConfigurationManager.GetSection( name );
      }

      .....

      public static ConfigurationSection GetWidgetSection()
      {
          return GetSection( "widgets" );
      }
 }

Answer 7

Yea I think MS made a mistake here.

Their decision does not make sense and forces programmers to write (as described above) a pointless wrapper class.

Here is a good example: Trying to extend static MS Unit testing class Assert: I want 1 more Assert method AreEqual(x1,x2).

The only way to do this is to point to different classes or write a wrapper around 100s of different Assert methods. Why!?

If the decision was been made to allow extensions of instances I see no logical reason to not allow static extensions. The arguments about sectioning libraries does not stand up once instances can be extended.

Answer 8

Maybe you could add a static class with your custom namespace and the same class name:

using CLRConsole = System.Console;

namespace ExtensionMethodsDemo
{
    public static class Console
    {
        public static void WriteLine(string value)
        {
            CLRConsole.WriteLine(value);
        }

        public static void WriteBlueLine(string value)
        {
            System.ConsoleColor currentColor = CLRConsole.ForegroundColor;

            CLRConsole.ForegroundColor = System.ConsoleColor.Blue;
            CLRConsole.WriteLine(value);

            CLRConsole.ForegroundColor = currentColor;
        }

        public static System.ConsoleKeyInfo ReadKey(bool intercept)
        {
            return CLRConsole.ReadKey(intercept);
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                Console.WriteBlueLine("This text is blue");   
            }
            catch (System.Exception ex)
            {
                Console.WriteLine(ex.Message);
                Console.WriteLine(ex.StackTrace);
            }

            Console.WriteLine("Press any key to continue...");
            Console.ReadKey(true);
        }
    }
}

Answer 9

You CAN do this if you are willing to "frig" it a little by making a variable of the static class and assigning it to null. However, this method would not be available to static calls on the class, so not sure how much use it would be:

Console myConsole = null;
myConsole.WriteBlueLine("my blue line");

public static class Helpers {
    public static void WriteBlueLine(this Console c, string text)
    {
        Console.ForegroundColor = ConsoleColor.Blue;
        Console.WriteLine(text);
        Console.ResetColor();
    }
}