using System;

public class Program

{

    public static void Main()

    {

        TestSimpleChange("Simple Change");

        TestRefChange("Value Ref Change");

        TestClassChange("Class Change");

    }

    private static void TestSimpleChange(string message)

    {

        var i = 42;

        var j = i.Change(24);

        Console.WriteLine($"{message}: i = {i}; j = {j}");

    }

    private static void TestRefChange(string message)

    {

        var i = 42;

        var j = i.ChangeRef(24);

        Console.WriteLine($"{message}: i = {i}; j = {j}");

    }

    private static void TestClassChange(string message)

    {

        var i = new DemoClass(42);

        var j = i.ChangeClass(new DemoClass(24));

        Console.WriteLine($"{message}: i = {i}; j = {j}");

    }

    class DemoClass

    {

        private int _value;

        public DemoClass(int _value)

        {

            this._value = _value;

        }

        public override string ToString() => _value.ToString("D");

    }       

}

​

internal static class Extensions

{

    public static T Change<T>(this T obj, T newValue) where T : struct

    {

        obj = newValue;

        return obj;

    }

    public static T ChangeRef<T>(this ref T obj, T newValue) where T : struct

    {

        obj = newValue;

        return obj;

    }

    public static T ChangeClass<T>(this T obj, T newValue) where T : class

    {

        obj = newValue;

        return obj;

    }

    /* Compile-time error ref only for struct

    public static T ChangeRefClass<T>(this ref T obj, T newValue) where T : class

    {

        obj = newValue;

        return obj;

    }

    */

}