data fetch second time on page_load giving error....

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C#

data fetch second time on page_load giving error....
means first time when i am fetching data from other page its properly fetched.
code is:-public partial class UpdatePage : System.Web.UI.Page
{

string sEcode = "";
string semployeename = "";
string sdepartment = "";
string slocation = "";
string sdeptcode = "";
protected void Page_Load(object sender, EventArgs e)
{

Library lib = new Library();
sEcode = Request.QueryString["cc"].ToString();
semployeename = Request.QueryString["cd"].ToString();
sdepartment = Request.QueryString["ce"].ToString();
slocation = Request.QueryString["cf"].ToString();


txtempcode.Text = sEcode;
txtEmployeename.Text = semployeename;
txtdepartment.Text = sdepartment;
txtlocation.Text = slocation;



}
but while second time fetching data from other page its giving error
like :-
public partial class UpdatePage : System.Web.UI.Page
{

string sEcode = "";
string semployeename = "";
string sdepartment = "";
string slocation = "";
string sdeptcode = "";
protected void Page_Load(object sender, EventArgs e)
{

Library lib = new Library();
sEcode = Request.QueryString["cc"].ToString();
semployeename = Request.QueryString["cd"].ToString();
sdepartment = Request.QueryString["ce"].ToString();
slocation = Request.QueryString["cf"].ToString();


txtempcode.Text = sEcode;
txtEmployeename.Text = semployeename;
txtdepartment.Text = sdepartment;
txtlocation.Text = slocation;

below two line fetching data second time.
Below two lines shall i can write in this block
or i have to some thing else.

error msg is :-Object reference not set to an instance of an object.
sdeptcode = Request.QueryString["co"].ToString();
txtDeptcode.Text = sdeptcode;
}


please someone help me how to write for fetching data while reload second time....