public static void Main()
    {
        VersionAssembly240 obj1 = new VersionAssembly240();
        VersionAssembly250 obj2 = new VersionAssembly250();
        VersionAssembly260 obj3 = new VersionAssembly260();

        ChooseRightInstance(null, obj2, null, "v25");
    }
    public static void ChooseRightInstance(VersionAssembly240 obj1, VersionAssembly250 obj2, VersionAssembly260 obj3, string version)
    {
        IVersionAssembly obj;   //declare the object outside the if, as an instance of the interface

        if (version == "v24")
        {
            obj = obj1;
        }
        else if (version == "v25")
        {
            obj = obj2;
        }
        else
        {
            obj = obj3;
        }

        Console.WriteLine(obj.example());   //can call the example() method because the interface defines it
    }