C# Error CS1109 – Extension methods must be defined in a top level static class; {0} is a nested class

C# Error

CS1109 – Extension methods must be defined in a top level static class; {0} is a nested class

Reason for the Error & Solution

Extension Methods must be defined on top level static classes, ‘name’ is a nested class.

Extension methods cannot be defined in nested classes.

Example

The following example generates CS1109 because the class Extension is nested inside the class Out:

// cs1109.cs  
public class Test  
{  
}  
static class Out  
{  
    static class Extension  
    {  
        static void ExtMethod(this Test c) // CS1109  
        {  
        }  
    }  
}