Object Reference not set to an Instance of an Object in asp.net

In the below code, You got that error because your Session["User_ID "] was null and when you try to return a string representation of that you got the object reference  not set to an instance of an object error. There is difference between .Tostring() and casting Convert.ToString().

if (!Page.IsPostBack)

{          

  ViewState["purid"] = Request.QueryString["User_ID"].ToString();

}

Solution for Object Reference not set to an Instance of an Object in ASP.NET

ToString throw null reference error but Convert.ToString() does not throw null reference error.

if (!Page.IsPostBack)

{

  ViewState["purid"] = Convert.ToString(Request.QueryString["order_no_id"]);

}