This code is written in ComboBox KeyUp event as we have tocheck after every new character is typed. 

private void comboBox1_KeyUp(objectsender, KeyEventArgs e)
        {
            intindex;
            stringactual;
            stringfound;
 
            // Donothing for certain keys, such as navigation keys.
            if((e.KeyCode == Keys.Back) ||
            (e.KeyCode == Keys.Left) ||
            (e.KeyCode == Keys.Right) ||
            (e.KeyCode == Keys.Up) ||
            (e.KeyCode == Keys.Down) ||
            (e.KeyCode == Keys.Delete) ||
            (e.KeyCode == Keys.PageUp) ||
            (e.KeyCode == Keys.PageDown) ||
            (e.KeyCode == Keys.Home) ||
            (e.KeyCode == Keys.End))
            {
                return;
            }
 
            // Storethe actual text that has been typed.
            actual = this.comboBox1.Text;
 
            // Findthe first match for the typed value.
            index = this.comboBox1.FindString(actual);
 
            // Getthe text of the first match.
            if(index > -1)
            {
                found = this.comboBox1.Items[index].ToString();
 
                //Select this item from the list.
                this.comboBox1.SelectedIndex= index;
 
                //Select the portion of the text that was automatically
                //added so that additional typing replaces it.
                this.comboBox1.SelectionStart= actual.Length;
                this.comboBox1.SelectionLength= found.Length;
            }
        }