Compiler Error CS1109

Visual Studio 2008

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

Extension methods cannot be defined in nested classes.

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
        {
        }
    }
}

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