C#

using System;
using System.Windows.Forms;

public class Test : Form, IMessageFilter
{
  public static void Main()
  {

    Test t = new Test();
    Application.AddMessageFilter(t);
    Application.Run(t);
    Application.RemoveMessageFilter(t);
  }

  public Test()
  {
    this.Text = "Test";
  }

  bool IMessageFilter.PreFilterMessage(ref Message m)
  {
    if (m.Msg == WM_NCLBUTTONDOWN)
      MessageBox.Show("You clicked the Window frame");

    return false;
  }

  private const int WM_NCLBUTTONDOWN = 0x00a1;
}

static void Foo(ref string str) {
    str = "Yo";
}
 
static void Main(string[] args)
{
    string s = null;
    Foo(ref s);
    Console.WriteLine(s); // Will print "Yo" and not null
}