C# Error CS1112 – Do not use ‘System.Runtime.CompilerServices.ExtensionAttribute’. Use the ‘this’ keyword instead.

C# Error

CS1112 – Do not use ‘System.Runtime.CompilerServices.ExtensionAttribute’. Use the ‘this’ keyword instead.

Reason for the Error & Solution

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

This error is generated when the 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 .

To correct this error

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

Example

The following example generates CS1112:

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