สอบถามเรื่องการ Search ใน Combobox ด้วยครับ พิมพ์ตัวอักษรอะไรก็ได้

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 f = new Form2();
            f.ShowDialog();
            textBox1.Text = (!string.IsNullOrEmpty(f.postID)) ? f.postID : textBox1.Text;
        }
    }

public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }
        public string postID = "";

        private void Form2_Load(object sender, EventArgs e)
        {
            // TODO: This line of code loads data into the 'dATA_ALSDataSet.DataTable1' table. You can move, or remove it, as needed.
            this.dataTable1TableAdapter.Fill(this.dATA_ALSDataSet.DataTable1);

        }

        private void dataTable1DataGridView_CellClick(object sender, DataGridViewCellEventArgs e)
        {
            postID = this.dataTable1DataGridView[1, e.RowIndex].Value.ToString();
            Close();
        }

        private void button1_Click(object sender, EventArgs e)
        {

            dataTable1BindingSource.Filter = "PostCode like '%" + textBox1.Text + "%' OR Post_Province like '%" + textBox1.Text + "%' OR Post_Amphur like '%" + textBox1.Text + "%' OR Post_PostOffice like '%" + textBox1.Text + "%'";
        }
    }

 private void dataTable1DataGridView_CellClick(object sender, DataGridViewCellEventArgs e)
        {
            postID ="ตู้ ปณ." +  this.dataTable1DataGridView[4, e.RowIndex].Value.ToString() + Environment.NewLine +
                    "อ." + this.dataTable1DataGridView[3, e.RowIndex].Value.ToString() + Environment.NewLine +
                    "จ." + this.dataTable1DataGridView[2, e.RowIndex].Value.ToString() + Environment.NewLine +
                    "รหัส " + this.dataTable1DataGridView[1, e.RowIndex].Value.ToString();

            Close();
        }