How to set SelectedValue of DropDownList in Edit form

See more:

C#

DropDownList

SelectedValue

Hello

I have a problem in Drop Downstairs control please help me

Here are tow Drop Down-list control which one of theme Bind to Province TB anther one is Bind to District TB,

Now in Edit page form I want show the Province Name and District Name of that ID in Drop Down-list which redirected from main page by Query string
the drop downlist display the First ID like 1

Here is my code

public void ReadSchool()
  {
      string strcmd = "select *from School where S_ID=@SIID";
      SqlCommand cmd = new SqlCommand(strcmd, con);

      cmd.Parameters.AddWithValue("@SIID", Request.QueryString["SID"].ToString());
      con.Open();
      SqlDataReader dr = cmd.ExecuteReader();
      while (dr.Read())
      {

          TextNameschool.Text = dr["SName"].ToString();
          TextCode.Text = dr["Code"].ToString();
          DropGrade.SelectedValue = dr["Grade"].ToString();
          Dropgender.SelectedValue = dr["Gender"].ToString();
          DropType.SelectedValue = dr["Type"].ToString();

          DropProvince.SelectedValue = dr["ProvinceID"].ToString();
          DropDistrict.SelectedValue = dr["DistrictID"].ToString();