How Do I Slove Invalid Cast Exception

void addControl()
{
   foreach(GridViewRow row in GridView1.Rows)

   {

       int key=(int)GridView1.DataKeys[row.RowIndex].Value;
       if(row.RowType==DataControlRowType.DataRow)
       {
          DropDownList1=new DropDownList();
            DropDownList1.DataSource=list;
            DropDownList1.DataBind();
           string str=ht[key].ToString();
            DropDownList1.SelectedValue=str;
            row.Cells[2].Controls.Add(DropDownList1);
        }
}

       }