arglist.cs

using System;

class ArgListFun {
  static void Main() {
    var arg2 = 1;
    string arg3;
    var arg1 = M(__arglist(123, ref arg2, out arg3));

    Console.WriteLine(arg1); // 123
    Console.WriteLine(arg2); // 43
    Console.WriteLine(arg3); // "abc"
  }

  static int M(__arglist) {
    var iterator = new ArgIterator(__arglist);
    var arg1 = __refvalue(iterator.GetNextArg(), int);
    __refvalue(iterator.GetNextArg(), int) += 42;
    __refvalue(iterator.GetNextArg(), string) = "abc";

    return arg1;
  }
}

Is it C# 7 ?

__arglist, __refvalue,__makeref and __reftype keywords available in earlier versions