.NET Fiddle

CSharp · Console

/Program.cs

using System;
using SomeOtherNamespace;
using name1;
using name1.name2;

// see: https://stackoverflow.com/a/66888084/1016343
namespace mainClass
{
    public static class mainClass
    {
        public static void Main()
        {
            var classFoo = new ClsFoo();
            var count = classFoo.Bar1() + classFoo.Bar2();
            Console.WriteLine($"count = {count}"); // output is 110
        } // main               
    } // class
} // namespace

namespace SomeOtherNamespace
{
    public class ClsFoo
    {
    } // class
} // namespace

namespace name1
{
    public static class FooExt
    {
        public static int Bar1(this ClsFoo foo)
        {
            return 10;
        } // method
    } // class
} // namespace

namespace name1.name2
{
    public static class FooExt
    {
        public static int Bar2(this ClsFoo foo)
        {
            return 100;
        } // method
    } // class
} // namespace