Extension method is not working,If method in nonstatic class?

Question

Just now i read about Extension methods.I created static method inside static class its working fine.

    static class ExtensionMethods
    {
        public static string splitFirstName(this string strName)
        {
            return strName.Split(" ".ToCharArray())[0];
        }
     }

But If i create static method inside Nonstatic class its not working.

    class NonStaticCls
    {
      public static string splitFirstName(this string strName)
      {
        return strName.Split(" ".ToCharArray())[0];
      }
    }

Please tell why its not working in nonstatic class.

score 6 · Accepted Answer · answered Aug 16 '13 at 06:28

Please tell why its not working in nonstatic class.

Because that's just how extension methods are specified. They must be declared in a non-nested, non-generic class.

From section 10.6.9 of the C# 5 specification:

When the first parameter of a method includes the this modifier, that method is said to be an extension method. Extension methods can only be declared in non-generic, non-nested static classes. The first parameter of an extension method can have no modifiers other than this, and the parameter type cannot be a pointer type.

Why do you want to declare it in a non-static class? What are you trying to achieve which can't be achieved equally well using a static class? (I can just about imagine some possibilities, but they're not things I've ever wanted to do in extension methods myself...)

`static class ExtensionMethods` isn't it declared in static class? — Guru Stron, Aug 16 '13 at 06:30
@GuruStron: Yes, read the question carefully - that version works, but the second code snippet doesn't. — Jon Skeet, Aug 16 '13 at 06:31

score 3 · Answer 2 · answered Aug 16 '13 at 06:28

It's a requirement that the class be static. This is reasonable. If it wasn't a static class, you'd be able (potentially) to create instances of that class. But then it has extension methods in it, possibly for other, completely unrelated classes. That could be very confusing.

score 2 · Answer 3 · answered Aug 16 '13 at 06:28

2

That's by design behavior. Extension method can only be declared in static class.

How to: Implement and Call a Custom Extension Method (C# Programming Guide)

To define and call the extension method

Define a static class to contain the extension method.

answered Aug 16 '13 at 06:28

MarcinJuraszek

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`static class ExtensionMethods` as I see method is defined in static class – Guru Stron Aug 16 '13 at 06:29
@GuruStron - re-read the question. The first code sample works (because it *is* `static`), the OP is asking why the *second* code sample doesn't work. – Damien_The_Unbeliever Aug 16 '13 at 06:31

score 2 · Answer 4 · edited May 23 '17 at 12:28

2

Looks like it's by design.

From MSDN;

The following list provides the main features of a static class:

Contains only static members.

Cannot be instantiated.

Is sealed.

Cannot contain Instance Constructors.

Check out this answer from Eric Lippert

Why are extension methods only allowed in non-nested, non-generic static class?

Also from AskJonSkeet

It's dictated in the language specification, section 10.6.9 of the C# 4 spec:

When the first parameter of a method includes the this modifier, that method is said to be an extension method. Extension methods can only be declared in non-generic, non-nested static classes. The first parameter of an extension method can have no modifiers other than this, and the parameter type cannot be a pointer type.

edited May 23 '17 at 12:28

Community

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answered Aug 16 '13 at 06:29

Soner Gönül

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You've listed features of a static class - not restrictions on extension methods. – Jon Skeet Aug 16 '13 at 06:36
@JonSkeet Thanks Jon, updated. Is [AskJonSkeet.com](http://askjonskeet.com/) still active by the way? – Soner Gönül Aug 16 '13 at 06:42
Yes, as far as I'm aware. (I don't run it.) – Jon Skeet Aug 16 '13 at 07:26

Ravi Gadag · Answer 5 · 2013-08-16T06:39:17.223

According to C# spec. you can read more here Extension Methods

Extension methods can only be declared in non-generic, non-nested static classes.

AS other pointed you, you need to include the static.

basically you will create a bunch of extension method as Utility Project or Helper Class

Creating static class you need not to instantiated, and invoke the method directly. for Eg. Math.Pow()
as per your question, Extension method work only when it's in Static class. and use of proper namespace.

Extension method is not working,If method in nonstatic class?

5 Answers5