comboBox1.Items.Add("a");
            comboBox1.Items.Add("ab");
            comboBox1.Items.Add("abc");
            comboBox1.Items.Add("abcd");
            comboBox1.Items.Add("abcde");
            comboBox1.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
            comboBox1.AutoCompleteSource = AutoCompleteSource.ListItems;