asp.net page jump implementation code

Request.UrlReferrer
Note: if the previous 1 page navigates to the current page using the document.location method, Request.UrlReferrer returns a null value
If there are two pages of A and B, directly request the A page in the browser, navigate to the B page in the Page_Load event in the A page,
Request.UrlReferrer returns empty. Because the page is not initialized in the Page_load event, the information of the current page cannot be recorded,
Navigate to the b page and you won't get the information from page 1

 
protected void Page_Load(object sender, EventArgs e) 
{ 
if (!IsPostBack) 
{ 
if (Request.UrlReferrer != null) 
{ 
ViewState["UrlReferrer"] = Request.UrlReferrer.ToString(); 
} 
if (Session["user"] != null) 
{ 
if (Request.UrlReferrer != null) 
{ 
Response.Redirect(Request.UrlReferrer.ToString()); 
} 
else 
{ 
Response.Redirect("/"); 
} 
} 
username.Value = Request.Form["Uname"]; 
pass.Attributes.Add("value", Request.Form["password"]); 
} 
} 
/// <summary> 
///  Individual member login  
/// </summary> 
protected void userLog() 
{ 
Lovetrip.BLL.Manage.Users bllu = new Lovetrip.BLL.Manage.Users(); 
Lovetrip.Model.Manage.Users modeu = bllu.Login(username.Value.Trim(), pass.Text.Trim()); 
if (modeu != null) 
{ 
Session["user"] = modeu; 
Session["userType"] = 1; 
Command.Public.MoveUserSession(1); 
bbsLogin(modeu.unick); 
if (Request.UrlReferrer != null) 
{ 
Response.Redirect(ViewState["UrlReferrer"].ToString()); 
} 
else 
{ 
Response.Redirect("/"); 
} 
} 
else 
{ 
Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert(' Incorrect account or password! ')", true); 
} 
}