title: Server cannot modify cookies after HTTP headers have been sent, how Fix?
body: <p>i want to auto login my users at page_load of masterpage if UserName and Password exist in cookies!<br>
  so i wrote the code below :  </p>
  
  <pre><code>        protected void Page_Load(object sender, EventArgs e)
          {
              LoadDataFromCookieIfExistAndLogin();
          }
  
  private void LoadDataFromCookieIfExistAndLogin()
  {
      string Query = Request.Url.Query.ToString();
      string[] Ar_Query = new string[2];
      string[] splitter = { "%2f" };
      Ar_Query = Query.Split(splitter, System.StringSplitOptions.None);
      string[] Ar_new_Query = new string[2];
      int minLength = Math.Min(Ar_Query.Length, Ar_new_Query.Length);
      Array.Copy(Ar_Query, Ar_new_Query, minLength);
      if (string.IsNullOrEmpty(Ar_new_Query[1]))
      {
          Ar_new_Query[1] = string.Empty;
      }
  
      if ((Request.QueryString["ReturnURL"] != null) &amp;&amp; (Ar_new_Query[1].ToString().ToUpper() == "ADMIN"))
      {
          Session.Clear();
          FormsAuthentication.SignOut();
      }
      else if ((Request.QueryString["ReturnURL"] != null) &amp;&amp; (Ar_new_Query[1].ToString().ToUpper() == "ELMAH.AXD"))
      {
          Session.Clear();
          FormsAuthentication.SignOut();
      }
      else
      {
          HttpCookie Situation_Cookie = Request.Cookies["Situation"];
          if (Situation_Cookie != null)
          {
              if (Situation_Cookie["Login"] == "Yes")
              {
                  HttpCookie Data_Cookie = Request.Cookies["Data"];
                  if (Data_Cookie != null)
                  {
                      string UserName = Data_Cookie["UserName"].ToString();
                      string PassWord = ata_Cookie["PassWord"].ToString();
  
                      string HashedPass = FormsAuthentication.HashPasswordForStoringInConfigFile(PassWord, "MD5");
                      DataSet dsUsers = DataLayer.Users.SelectRowForLogin_FromCookie(UserName, HashedPass);
                      if (dsUsers.Tables["Users"].Rows.Count &gt; 0)
                      {
                          DataRow drUsers = dsUsers.Tables["Users"].Rows[0];
  
                          if (Session["User_ID"] == null)
                          {
                              Session["UserName"] = UserName;
                              Session["Password"] = PassWord;
                              Session["User_ID"] = drUsers["ID"].ToString();
                              Session["UserType_ID"] = drUsers["UserType_ID"].ToString();
                              DataLayer.OnlineUsers.UpdateRow_UserID_By_SessionID(
                                                                                   Session["User_ID"],
                                                                                   Session.SessionID);
                          }
                          if (!HttpContext.Current.User.Identity.IsAuthenticated)
                          {
                              FormsAuthentication.SetAuthCookie(drUsers["ID"].ToString(), true);
                          }
                      }
                  }
              }
          }
      }
  }
  </code></pre>
  
  <p>also for understanding my login codes i am using RoleProvider Like Below :  </p>
  
  <pre><code>  using System;
      using System.Collections.Generic;
      using System.Linq;
      using System.Web;
      using System.Web.Security;
      using System.Data;
  
      namespace NiceFileExplorer.Classes
      {
          public class NiceFileExplorerRoleProvider : RoleProvider
          {
              public override void AddUsersToRoles(string[] usernames, string[] roleNames)
              {
                  throw new NotImplementedException();
              }
  
              public override string ApplicationName
              {
                  get
                  {
                      throw new NotImplementedException();
                  }
                  set
                  {
                      throw new NotImplementedException();
                  }
              }
  
              public override void CreateRole(string roleName)
              {
                  throw new NotImplementedException();
              }
  
              public override bool DeleteRole(string roleName, bool throwOnPopulatedRole)
              {
                  throw new NotImplementedException();
              }
  
              public override string[] FindUsersInRole(string roleName, string usernameToMatch)
              {
                  throw new NotImplementedException();
              }
  
              public override string[] GetAllRoles()
              {
                  throw new NotImplementedException();
              }
  
              //public override string[] GetRolesForUser(string username)
              public override string[] GetRolesForUser(string User_ID)
              {
                  string[] UserTypes = new string[1];
                  DataSet dsUser = DataLayer.Users.SelectRowWithUserTypeInfo(int.Parse(User_ID));
                  if (dsUser.Tables["Users"].Rows.Count &gt; 0)
                  {
                      DataRow drUser = dsUser.Tables["Users"].Rows[0];
                      UserTypes[0] = drUser["Name"].ToString();
                  }
                  if (User_ID == "-255")
                  {
                      UserTypes[0] = "Administrators";
                  }
                  return UserTypes;
              }
  
              public override string[] GetUsersInRole(string roleName)
              {
                  throw new NotImplementedException();
              }
  
              public override bool IsUserInRole(string username, string roleName)
              {
                  throw new NotImplementedException();
              }
  
              public override void RemoveUsersFromRoles(string[] usernames, string[] roleNames)
              {
                  throw new NotImplementedException();
              }
  
              public override bool RoleExists(string roleName)
              {
                  throw new NotImplementedException();
              }
          }
  
  }
  </code></pre>
  
  <p>sometimes i have the error below :  </p>
  
  <p><strong>System.Web.HttpException: Server cannot modify cookies after HTTP headers have been sent.</strong>  </p>
  
  <pre><code>System.Web.HttpUnhandledException (0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown. ---&gt; System.Web.HttpException (0x80004005): Server cannot modify cookies after HTTP headers have been sent.
     at System.Web.HttpCookieCollection.Add(HttpCookie cookie)
     at System.Web.Security.FormsAuthentication.SetAuthCookie(String userName, Boolean createPersistentCookie, String strCookiePath)
     at NiceFileExplorer.en.Site1.Page_Load(Object sender, EventArgs e)
     at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)
     at System.Web.UI.Control.LoadRecursive()
     at System.Web.UI.Control.LoadRecursive()
     at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
     at System.Web.UI.Page.HandleError(Exception e)
     at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
     at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
     at System.Web.UI.Page.ProcessRequest()
     at System.Web.UI.Page.ProcessRequest(HttpContext context)
     at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
     at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&amp; completedSynchronously)
  </code></pre>
  
  <p>it seems the problem is for the line below :  </p>
  
  <blockquote>
    <p>FormsAuthentication.SetAuthCookie(drUsers["ID"].ToString(), true);  </p>
  </blockquote>
  
  <p>what this error mean and how can i prevent it?  </p>