search in drop down list

Assume your ComboBox in DropDownList mode is 'comboBox1; your TextBox for trying to match an Item in the ComboBox is 'textBox1; your button for triggering the attempt to match and select is 'button1:

Define a Click EventHandler for 'button1:

private void button1_Click(object sender, EventArgs e)
{
    string theString = textBox1.Text;

    if (theString == string.Empty) return;

    object strAsObject = theString as object;

    if (comboBox1.Items.Contains(strAsObject))
    {
        comboBox1.SelectedItem = strAsObject;
    }
}

I am curious why you wish to provide a second means of selecting Items in the ComboBox.