Here is an Example, Hope it is exactly what you need...
DropDownList1 Has Collection:
Uttar Pradesh
Delhi
Rajasthan
Now User will select any one of above 3 Items
if (DropDownList1.SelectedItem.Text == "Uttar Pradesh")
{
if (DropDownList2.Items.Count > 0)
{
DropDownList2.Items.Clear();
}
DropDownList2.Items.Add("Kanpur");
DropDownList2.Items.Add("Allahabad");
DropDownList2.Items.Add("Banaras");
}
else if (DropDownList1.SelectedItem.Text == "Delhi")
{
if (DropDownList2.Items.Count > 0)
{
DropDownList2.Items.Clear();
}
DropDownList2.Items.Add("New Delhi");
DropDownList2.Items.Add("Old Delhi");
}
else if (DropDownList1.SelectedItem.Text == "Rajasthan")
{
if (DropDownList2.Items.Count > 0)
{
DropDownList2.Items.Clear();
}
DropDownList2.Items.Add("Jaipur");
DropDownList2.Items.Add("Ajmer");
}