C#

using System;
using System.Windows.Forms;

namespace YourNameSpace
{
	public class ACComboBox : ComboBox
	{		
		protected string oldText = "";
		protected override void OnKeyPress(KeyPressEventArgs e)
		{			
			if(!Char.IsControl(e.KeyChar))
			{
				int possibleMatch=0;
				string textBefore = Text;

				possibleMatch = FindString(Text);					
				if(possibleMatch >= 0)
				{						
					oldText = Text;
					SelectedIndex = possibleMatch;						
					Select(textBefore.Length,Text.Length-textBefore.Length);
				}
			}
			base.OnKeyPress(e);			
		}

		protected override void OnTextChanged(EventArgs e)
		{
			oldText = Text;
			base.OnTextChanged(e);
		}
	}
}

private void comboBox_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
// Auto Complete routine for combobox
int where = comboBox.FindString(comboBox.Text);
int start = comboBox.Text.Length;
if ((where >= 0) && (e.KeyChar >= 32))
{
comboBox.Text = ((string) comboBox.Items[where]);
comboBox.Select(start, comboBox.Text.Length);
}
}

This is within pre tags