Not able to retrieve the ID .. Help Required

i have a CustomerNameComboBox, on the selectedIndexChanged event, i want to show the record of the selected customer in the datagridview of the selected customer.. up til that, i have done.. but i am not able to get the ID of the Customer.. i want to use the ID of the Customer above.. can i please get to know how can i get the id of the customer? I also want to set the visibility of my CustomerNameComboBox to read only once the customer is selected. here is my code that i have been doing so far:

private void CustomerNameComboBox_SelectedIndexChanged(object sender, EventArgs e)
{
    if (CustomerNameComboBox.Items.Count > 0)
    {
        SqlConnection cn = new SqlConnection(@"Data Source=COMPAQ-PC-PC\SQLEXPRESS;Initial Catalog=Tyre Shop Management System;Integrated Security=True");
        SqlCommand cmd = new SqlCommand("Select * from CustomerInfo where ID = " + CustomerNameComboBox.SelectedValue + "", cn);
        MessageBox.Show(cmd.CommandText);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        da.Fill(dt);
        dgCustomerDetails.DataSource = dt;
        //if (CustomerNameComboBox.SelectedValue > 0)
        //{
        //    ReadOnlyAttribute ReadOnly = new ReadOnlyAttribute(true);
        //    CustomerNameComboBox.Visible = ReadOnly.IsReadOnly;
        //}
        //if (CustomerNameComboBox.ValueMember == CustomerNameComboBox.SelectedValue)
        //{
        //    ReadOnlyAttribute ReadOnly = new ReadOnlyAttr开发者_如何转开发ibute(true);
        //    CustomerNameComboBox.Visible = ReadOnly.IsReadOnly;
        //}   
    }
}


private void BindCombo(string query, ComboBox combo, string DisplayMember, string ValueMember)
{
    SqlConnection cn = new SqlConnection(@"Data Source=COMPAQ-PC-PC\SQLEXPRESS;Initial Catalog=Tyre Shop Management System;Integrated Security=True");
    SqlCommand cmd = new SqlCommand(query, cn);
    SqlDataAdapter da = new SqlDataAdapter(cmd);
    DataTable dt = new DataTable();
    da.Fill(dt);
    CustomerNameComboBox.DataSource = dt;
    CustomerNameComboBox.DisplayMember = DisplayMember;
    CustomerNameComboBox.ValueMember = ValueMember;
}

private void BindComboBoxAll()
{
    string CustomerQuery = "Select ID,FirstName from CustomerInfo";
    BindCombo(CustomerQuery, CustomerNameComboBox, "FirstName", "ID");
}

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you need to call the bind method after setting datasoure

 DataTable dt = new DataTable();
    da.Fill(dt);
    dgCustomerDetails.DataSource = dt;
dgCustomerDetails.DataBind();// Add this