C#

private const int WM_KEYDOWN = 0x0100;
public bool PreFilterMessage(ref Message m)
{
    if(m.Msg == WM_KEYDOWN)
    {
        char c = (char)m.WParam;
        if(c == 'A')
            MessageBox.Show("A was hit");
    }
    return false;
}

using System;
using System.Windows.Forms;

public class Test : Form, IMessageFilter
{
  static void Main()
  {
    Test t = new Test();
    Application.AddMessageFilter(t);
    Application.AddMessageFilter(new MessageFilter());
    Application.Run(t);
  }

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

  public bool PreFilterMessage(ref Message m)
  {
    Print(this, m);
    return false;
  }

  internal static void Print(object obj, Message m)
  {
    Console.WriteLine("[{0}] 0x{1:x2} -> 0x{2:x2}",
      obj.GetType().Name, m.Msg, m.HWnd);
  }
}

public class MessageFilter : IMessageFilter
{
  public bool PreFilterMessage(ref Message m)
  {
    Test.Print(this, m);
    return false;
  }
}

[Test] Message.Msg -> Message.HWnd
[MessageFilter] Message.Msg -> Message.HWnd
[Test] Message.Msg -> Message.HWnd

<?xml version="1.0" standalone="yes"?>
<pairs>
  <pair>
    <x>3</x>
    <y>10</y>
  </pair>
  <pair>
    <x>4</x>
    <y>17</y>
  </pair>
</pairs>

private string GetInput()
{
	XmlDocument doc = new XmlDocument();
	StringBuilder sb = new StringBuilder();
	XmlNode buffer = null;
	doc.Load("../../data.xml");
	XmlNodeList nodes = doc.SelectNodes("//pairs/pair");
	if(nodes != null)
	{
		foreach(XmlNode node in nodes)
		{
			buffer = node.SelectSingleNode("x");
			if(buffer != null)
				sb.Append(buffer.InnerXml + " ");
			buffer = node.SelectSingleNode("y");
			if(buffer != null)
				sb.Append(buffer.InnerXml + " ");
		}
	}
	return(sb.ToString());
}