populate one dropdownlist based on the selected of another dropdownlist

hi
i am using c# asp.net
i have one datagrid in my form and i bound two dropdown list using coding. i loaded my database values in my first dropdown list by using this code:

private void Page_Load(object sender, System.EventArgs e)
{// Put user code to initialize the page here
if(!Page.IsPostBack)
{ BindData();
PopulateList();
}
}
public void BindData()
{
SqlDataAdapter ad = new SqlDataAdapter("SELECT Course_NAME FROM JTSISControlManager_Course",conn);
DataSet ds = new DataSet();
ad.Fill(ds,"JTSISControlManager_Course");
DataGrid2.DataSource = ds;
DataGrid2.DataBind();

}
public DataSet PopulateList()
{

SqlDataAdapter ad = new
SqlDataAdapter("SELECT Course_Name FROM JTSISControlManager_Course", conn);

DataSet ds = new DataSet();
ad.Fill(ds,"JTSISControlManager_Course");
return ds;
}

here is the html coding:

<form id="Form1" method="post" runat="server">
<asp:datagrid id="DataGrid2" style="Z-INDEX: 101; LEFT: 328px; POSITION: absolute; TOP: 64px"
runat="server" Width="408px" AutoGenerateColumns="False" DataKeyField="Course_NAME">
<Columns>
<asp:BoundColumn DataField="Course_NAME" HeaderText="Course_NAME" />
<asp:TemplateColumn>
<HeaderTemplate>
<aspropDownList ID="Dropdownlist1" Runat="server" DataTextField="Course_NAME" OnSelectedIndexChanged ="DropDown_SelectedIndexChanged" DataSource =" <%#PopulateList()%> " AutoPostBack="True" />
<aspropDownList ID="Dropdownlist3" Runat="server" DataTextField="Dept_NAME" AutoPostBack="True" />
</HeaderTemplate>
</asp:TemplateColumn>
</Columns>
</asp:datagrid></form>

then using the following code i retreived my first dropdownlist value

protected void DropDown_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList list = (DropDownList)sender;
li=list.SelectedValue.Trim();
Response.Write(li);

}

my need is i want to use the retreived value of the first dropdownlist in query and fill the values in my second dropdownlist of the datagrid.
tell me how to use that retreived value of first dropdown list in next qurey and how to fill the second dropdownlist .so please help me to do this. give example coding to do my requirement
Edit/Delete MessageHi karthikeyan,

In my application I've Make and Model dropdowns. Model dropdown gets populated as soon as we select a make in a dropdown, it gets all the models for that make;

Here is my code;

Populating Model Dropdown
private void ddlMake_SelectedIndexChanged(object sender, System.EventArgs e)
{
if (ddlMake.SelectedValue != "")
{
PopulateFieldsInDropDowns("SELECT cat.DisplayText Model, to_char(cat.CATEGORYID) ModelID FROM TDCATEGORIES cat WHERE cat.CATEGORYLEVEL=4 and parentid= " + ddlMake.SelectedValue + " ORDER BY cat.DisplayText", ddlModel);
}
else
ddlModel.Items.Clear();
}

Populate Method
private void PopulateFieldsInDropDowns(string sqlQuery, DropDownList drpList)
{
try {
conn.Open();
OleDbCommand cmCommand = new OleDbCommand(sqlQuery, conn);
OleDbDataReader dreader = cmCommand.ExecuteReader();

drpList.Items.Clear();
drpList.Items.Add(new ListItem("", ""));

while (dreader.Read())
{
drpList.Items.Add(new ListItem(dreader.GetString(0), dreader.GetString(1)));
}

dreader.Close();

if (dreader != null)
dreader = null;

if (cmCommand != null)
cmCommand.Dispose();
}
catch
{
drpList.Items.Clear();
lblMessage.ForeColor = System.Drawing.Color.Red;
lblMessage.Text = "Cannot Get Data: Cannot Get Required Data.";
}
conn.Close();
}

It should rock.

thanks,
hi ahmarshi,

i tried which u given, but i got error:
System.NullReferenceException: Object reference not set to an instance of an object.

my first dropdown name is DropDownList1 and second name is ddlmodel
i modified my coding like this:
protected void DropDown_SelectedIndexChanged(object sender, System.EventArgs e)
{
DropDownList list = (DropDownList)sender;
li=list.SelectedValue.Trim();
Response.Write(li);

if (li != "")
{
PopulateFieldsInDropDowns("SELECT Dept_NAME from JTSISControlManager_Department where Course_ID in (select Course_ID from JTSISControlManager_Course where Course_NAME= '" + li + "')", ddlmodel);
}
else
ddlmodel.Items.Clear();
}
private void PopulateFieldsInDropDowns(string sqlQuery, DropDownList drpList)
{
try
{
conn.Open();
SqlCommand cmd = new SqlCommand(sqlQuery, conn);
SqlDataReader dreader =cmd.ExecuteReader();


drpList.Items.Clear();
drpList.Items.Add(new ListItem("", ""));

while (dreader.Read())
{
drpList.Items.Add(new ListItem(dreader.GetString(0), dreader.GetString(1)));
}

dreader.Close();

if (dreader != null)
dreader = null;

if (cmd!= null)
cmd.Dispose();
}
catch
{
drpList.Items.Clear();
//lblMessage.ForeColor = System.Drawing.Color.Red;
Response.Write ( "Cannot Get Data: Cannot Get Required Data.");
}
conn.Close();
}

i kept break point and run this but while comming to
this line
private void PopulateFieldsInDropDowns(string sqlQuery, System.Web.UI.WebControls.DropDownList drpList)
in the drplist comes <undefined value>.
then after comming to this line
drpList.Items.Add(new ListItem("", ""));
it automatically goes to catch.please tell me and give sugestion to rectify this prooblem.