private void Form1_Load(object sender, EventArgs e)
{
comboBox1.SelectedIndexChanged -= comboBox1_SelectedIndexChanged;
SqlCommand cmd = new SqlCommand("select * from Country", con);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds);
comboBox1.DataSource = ds.Tables[0];
comboBox1.SelectedIndex = -1;
comboBox1.DisplayMember = "County";
comboBox1.ValueMember = "Countryid";
comboBox1.SelectedIndexChanged += comboBox1_SelectedIndexChanged;
}