Can I add extension methods to an existing static class?

No. Extension methods require an instance variable (value) for 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" );
      }
 }

Can you add static extensions to classes in C#? No but you can do this:

public static class Extensions
{
    public static T Create<T>(this T @this)
        where T : class, new()
    {
        return Utility<T>.Create();
    }
}

public static class Utility<T>
    where T : class, new()
{
    static Utility()
    {
        Create = Expression.Lambda<Func<T>>(Expression.New(typeof(T).GetConstructor(Type.EmptyTypes))).Compile();
    }
    public static Func<T> Create { get; private set; }
}

Here's how it works. While you can't technically write static extension methods, instead this code exploits a loophole in extension methods. That loophole being that you can call extension methods on null objects without getting the null exception (unless you access anything via @this).

So here's how you would use this:

    var ds1 = (null as DataSet).Create(); // as oppose to DataSet.Create()
    // or
    DataSet ds2 = null;
    ds2 = ds2.Create();

    // using some of the techniques above you could have this:
    (null as Console).WriteBlueLine(...); // as oppose to Console.WriteBlueLine(...)

Now WHY did I pick calling the default constructor as an example, and AND why don't I just return new T() in the first code snippet without doing all of that Expression garbage? Well todays your lucky day because you get a 2fer. As any advanced .NET developer knows, new T() is slow because it generates a call to System.Activator which uses reflection to get the default constructor before calling it. Damn you Microsoft! However my code calls the default constructor of the object directly.

Static extensions would be better than this but desperate times call for desperate measures.

It's not possible.

And yes, 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 being 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.

I stumbled upon this thread while trying to find an answer to the same question the OP had. I didn't find the answer I wanted, but I ended up doing this.

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

And I use it like this:

ConsoleColor.Cyan.WriteLine("voilà");

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);
        }
    }
}

As of C#7 this isn't supported. There are however discussions about integrating something like that in C#8 and proposals worth supporting.

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

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.

As for extension methods, extension methods themselves are static; but they are invoked as if they are instance methods. Since a static class is not instantiable, you would never have an instance of the class to invoke an extension method from. For this reason the compiler does not allow extension methods to be defined for static classes.

Mr. Obnoxious wrote: "As any advanced .NET developer knows, new T() is slow because it generates a call to System.Activator which uses reflection to get the default constructor before calling it".

New() is compiled to the IL "newobj" instruction if the type is known at compile time. Newobj takes a constructor for direct invocation. Calls to System.Activator.CreateInstance() compile to the IL "call" instruction to invoke System.Activator.CreateInstance(). New() when used against generic types will result in a call to System.Activator.CreateInstance(). The post by Mr. Obnoxious was unclear on this point... and well, obnoxious.

This code:

System.Collections.ArrayList _al = new System.Collections.ArrayList();
System.Collections.ArrayList _al2 = (System.Collections.ArrayList)System.Activator.CreateInstance(typeof(System.Collections.ArrayList));

produces this IL:

  .locals init ([0] class [mscorlib]System.Collections.ArrayList _al,
           [1] class [mscorlib]System.Collections.ArrayList _al2)
  IL_0001:  newobj     instance void [mscorlib]System.Collections.ArrayList::.ctor()
  IL_0006:  stloc.0
  IL_0007:  ldtoken    [mscorlib]System.Collections.ArrayList
  IL_000c:  call       class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle)
  IL_0011:  call       object [mscorlib]System.Activator::CreateInstance(class [mscorlib]System.Type)
  IL_0016:  castclass  [mscorlib]System.Collections.ArrayList
  IL_001b:  stloc.1

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.

It is not possible to write an extension method, however it is possible to mimic the behaviour you are asking for.

using FooConsole = System.Console;

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

This will allow you to call Console.WriteBlueLine(fooText) in other classes. If the other classes want access to the other static functions of Console, they will have to be explicitly referenced through their namespace.

You can always add all of the methods in to the replacement class if you want to have all of them in one place.

So you would have something like

using FooConsole = System.Console;

public static class Console
{
    public static void WriteBlueLine(string text)
    {
        FooConsole.ForegroundColor = ConsoleColor.Blue;
        FooConsole.WriteLine(text);
        FooConsole.ResetColor();
    }
    public static void WriteLine(string text)
    {
        FooConsole.WriteLine(text);
    }
...etc.
}

This would provide the kind of behaviour you are looking for.

*Note Console will have to be added through the namespace that you put it in.

yes, in a limited sense.

public class DataSet : System.Data.DataSet
{
    public static void SpecialMethod() { }
}

This works but Console doesn't because it's static.

public static class Console
{       
    public static void WriteLine(String x)
    { System.Console.WriteLine(x); }

    public static void WriteBlueLine(String x)
    {
        System.Console.ForegroundColor = ConsoleColor.Blue;
        System.Console.Write(.x);           
    }
}

This works because as long as it's not on the same namespace. The problem is that you have to write a proxy static method for every method that System.Console have. It's not necessarily a bad thing as you can add something like this:

    public static void WriteLine(String x)
    { System.Console.WriteLine(x.Replace("Fck","****")); }

or

 public static void WriteLine(String x)
    {
        System.Console.ForegroundColor = ConsoleColor.Blue;
        System.Console.WriteLine(x); 
    }

The way it works is that you hook something into the standard WriteLine. It could be a line count or bad word filter or whatever. Whenever you just specify Console in your namespace say WebProject1 and import the namespace System, WebProject1.Console will be chosen over System.Console as default for those classes in namespace WebProject1. So this code will turn all the Console.WriteLine calls into blue insofar as you never specified System.Console.WriteLine.

The following was rejected as an edit to tvanfosson's answer. I was asked to contribute it as my own answer. I used his suggestion and finished the implementation of a ConfigurationManager wrapper. In principle I simply filled out the ... in tvanfosson's answer.

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 NameValueCollection AppSettings
    {
        get { return ConfigurationManager.AppSettings; }
    }

    public static ConnectionStringSettingsCollection ConnectionStrings
    {
        get { return ConfigurationManager.ConnectionStrings; }
    }

    public static object GetSection(string sectionName)
    {
        return ConfigurationManager.GetSection(sectionName);
    }

    public static Configuration OpenExeConfiguration(string exePath)
    {
        return ConfigurationManager.OpenExeConfiguration(exePath);
    }

    public static Configuration OpenMachineConfiguration()
    {
        return ConfigurationManager.OpenMachineConfiguration();
    }

    public static Configuration OpenMappedExeConfiguration(ExeConfigurationFileMap fileMap, ConfigurationUserLevel userLevel)
    {
        return ConfigurationManager.OpenMappedExeConfiguration(fileMap, userLevel);
    }

    public static Configuration OpenMappedMachineConfiguration(ConfigurationFileMap fileMap)
    {
        return ConfigurationManager.OpenMappedMachineConfiguration(fileMap);
    }

    public static void RefreshSection(string sectionName)
    {
        ConfigurationManager.RefreshSection(sectionName);
    }
}

You can use a cast on null to make it work.

public static class YoutTypeExtensionExample
{
    public static void Example()
    {
        ((YourType)null).ExtensionMethod();
    }
}

The extension:

public static class YourTypeExtension
{
    public static void ExtensionMethod(this YourType x) { }
}

YourType:

public class YourType { }

unfotunately NO, you CANNOT extend static classes

https://onecompiler.com/csharp/3xvbe7axg

using System;

namespace HelloWorld
{
  public static class console_extensions {
    public static void EXTENSION(this object item) {
      System.Console.WriteLine("HELLO THERE!");
    }
  }
  
    public class Program
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Hello, World!");
            Console.EXTENSION();
            ((Console)null).EXTENSION();
            Console l = new Console();
            l.EXTENSION();
        }
    }
}

output

Compilation failed: 4 error(s), 0 warnings

HelloWorld.cs(16,12): error CS0117: `System.Console' does not contain a definition for `EXTENSION'
/usr/lib/mono/4.5/mscorlib.dll (Location of the symbol related to previous error)
HelloWorld.cs(17,5): error CS0716: Cannot convert to static type `System.Console'
HelloWorld.cs(18,4): error CS0723: `l': cannot declare variables of static types
/usr/lib/mono/4.5/mscorlib.dll (Location of the symbol related to previous error)
HelloWorld.cs(18,16): error CS0712: Cannot create an instance of the static class `System.Console'
/usr/lib/mono/4.5/mscorlib.dll (Location of the symbol related to previous error)

however you CAN pass null to the extension method

using System;

namespace HelloWorld
{
  public static class static_extensions {
      public static void print(this object item, int data = 0) {
      Console.WriteLine("EXT: I AM A STATIC EXTENSION!");
      Console.WriteLine("EXT: MY ITEM IS: " + item);
      Console.WriteLine("EXT: MY DATA IS: " + data);
      string i;
      if (item == null) {
        i = "null";
      } else {
        i = item.GetType().Name;
      }
      Console.WriteLine("EXT: MY TYPE IS: " + i + "\n");
    }
  }

    public class Program
    {
    
        public static void Main(string[] args)
        {
          // an extension method can be
          //   called directly
          //  (null is an instance)
          static_extensions.print(null);

          // an extension method can also be
          //   called directly with arguments
          //  (null is an instance)
          static_extensions.print(null, 1);
          
          // an extension method can also be
          //   called as part of an instance
          int x = 0; // initialize int
          x.print();
          
          // an extension method can also be
          //   called as part of an instance
          //   and with data
          int x2 = 0; // initialize int
          x2.print(2);
          
          // an extension method can also be
          //   called directly from null
          //   since `null` is an instance
          ((string)null).print();
          
          // an extension method can also be
          //   called directly from null
          //   and with data
          //   since `null` is an instance
          ((string)null).print(4);
        }
    }
}

live example: https://onecompiler.com/csharp/3xvbc8s6w

output:

EXT: I AM A STATIC EXTENSION!
EXT: MY ITEM IS: 
EXT: MY DATA IS: 0
EXT: MY TYPE IS: null

EXT: I AM A STATIC EXTENSION!
EXT: MY ITEM IS: 
EXT: MY DATA IS: 1
EXT: MY TYPE IS: null

EXT: I AM A STATIC EXTENSION!
EXT: MY ITEM IS: 0
EXT: MY DATA IS: 0
EXT: MY TYPE IS: Int32

EXT: I AM A STATIC EXTENSION!
EXT: MY ITEM IS: 0
EXT: MY DATA IS: 2
EXT: MY TYPE IS: Int32

EXT: I AM A STATIC EXTENSION!
EXT: MY ITEM IS: 
EXT: MY DATA IS: 0
EXT: MY TYPE IS: null

EXT: I AM A STATIC EXTENSION!
EXT: MY ITEM IS: 
EXT: MY DATA IS: 4
EXT: MY TYPE IS: null

Use this

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

  .....

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

Although the methods of Console are static, its static methods Write() and WriteLine() merely redirect the call to Console.Out.Write() and Console.Out.WriteLine() respectively. Out is an instance whose type derives from the abstract class TextWriter. This makes it possible to define extension methods for TextWriter:

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

    public static void WriteUppercase(this TextWriter writer, string text)
    {
        writer.Write(text.ToUpper());
    }
}

The method can then be invoked like this:

Console.Out.WriteBlueLine();

And the best part is that the type of the standard error stream instance Console.Error also derives from TextWriter which makes the same extension method also usable for Console.Error:

Console.Error.WriteBlueLine();

This can be quite useful if you have defined an extension method like WriteTable()(for writing a table out to the console) because you can also use it for the error stream or any other object of TextWriter.

Newer versions of C# allow this to be even shorter with a using static statement for Console to get red of the Console. prefix:

using static System.Console;

Out.WriteBlueLine("A blue line");
Error.WriteBlueLine("A blue line");

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();
    }
}