C#

int l;
if (somebool)
{
  l = 0;
}
// do more stuff, but not to somebool
if (somebool)
{
  COnsole.WriteLine(l);
}

using System;
using System.Drawing;
using System.Windows.Forms;

public class Test : Form, IMessageFilter
{
  static void Main(string[] args)
  {
    Test t = new Test();
    Application.AddMessageFilter(t);
    Application.Run(t);
    Application.RemoveMessageFilter(t);
  }

  public Test()
  {
    TextBox tb = new TextBox();
    tb.Location = new Point(8, 8);
    this.Controls.Add(tb);

    tb = new TextBox();
    tb.Location = new Point(8, 30);
    this.Controls.Add(tb);
  }

  private const int WM_KEYDOWN = 0x0100;
  private const int WM_LBUTTONDOWN = 0x0201;
  private const int WM_RBUTTONDOWN = 0x0204;
  private const int WM_XBUTTONDOWN = 0x020b;

  public bool PreFilterMessage(ref Message m)
  {
    if (m.Msg == WM_KEYDOWN ||
      m.Msg == WM_LBUTTONDOWN ||
      m.Msg == WM_RBUTTONDOWN ||
      m.Msg == WM_XBUTTONDOWN)
      return true;

    return false;
  }
}