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Compiler Error CS1112

Do not use 'System.Runtime.CompilerServices.ExtensionAttribute'. Use the 'this' keyword instead.

This error is generated when the ExtensionAttribute is used on a non-static class that contains extension methods. If this attribute is used on a static class, another error, such as CS0708: "Cannot declare instance members in a static class," might occur.

In C#, extension methods must be defined in a static class and the first parameter of the method is modified with the this keyword. Do not use the attribute at all in the source code. For more information, see Extension Methods (C# Programming Guide).

To correct this error

  • Remove the attribute and apply the this modifier to the first parameter of the method.

The following example generates CS1112:

// cs1112.cs
[System.Runtime.CompilerServices.ExtensionAttribute] // CS1112
public class Extensions
    public bool A(bool b) { return b; }

class A { }