Dropdownlist in header template of gridview

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C#3.0

C#

ASP.NET

hi,

I have dropdownlist in my header template of gridview. Actually, its a kind of filter. Since it is a child control now, that's why wrote the following..

protected void gvDisplayCal_RowDataBound(object sender, GridViewRowEventArgs e)
        { 
           if (e.Row.RowType == DataControlRowType.Header)
             {
              impactFilter =(DropDownList)gvDisplayCal.FindControl("ddlImpact");
             }


        }

On selected Index Changed property of dropdownlist I wrote,

protected void ddlImpact_SelectedIndexChanged(object sender, EventArgs e)
        {

            DataTable dt = (DataTable)Cache["dt"];
            if (impactFilter.SelectedItem.Value.ToString() == "All")
            {
                dt.DefaultView.RowFilter = "COUNTRYCOL<>''";
                gvDisplayCal.DataSource = dt;
                gvDisplayCal.DataBind();
            }
            else
            {
                dt.DefaultView.RowFilter = "COL1='" + impactFilter.SelectedItem.Value.ToString() + "'";
                gvDisplayCal.DataSource = dt;
                gvDisplayCal.DataBind();
            }


        }

FYI, ddlImpact is the dropdownlist in my gridview and impactFilter is the another dropdownlist which catches ddlImpact.

My impactFilter appears to be empty/null.

How do I make it working?? Any suggestions please??