WebPart is raising the following error "Invalid data has been used to update the list item.The field you are trying to update may be read only"

Question

I have created a farm solution and then i deploy it to SharePoint server, the code looks as follow, and i use it to update a page info values (as the current page values represents old info):-
[ToolboxItemAttribute(false)]
public partial class VisualWebPart1 : WebPart
// Uncomment the following SecurityPermission attribute only when doing Performance Profiling using
// the Instrumentation method, and then remove the SecurityPermission attribute when the code is ready
// for production. Because the SecurityPermission attribute bypasses the security check for callers of
// your constructor, it's not recommended for production purposes.
// [System.Security.Permissions.SecurityPermission(System.Security.Permissions.SecurityAction.Assert, UnmanagedCode = true)]
public VisualWebPart1()
protected override void OnInit(EventArgs e)
base.OnInit(e);
InitializeControl();
using (SPSite site = new SPSite(SPContext.Current.Site.Url))
using (SPWeb web = site.OpenWeb())
SPList list = web.Lists["Pages"];
web.AllowUnsafeUpdates = true;
foreach (SPListItem items in list.Items)
items["Author"] = "SharePoint";
items["Created"] = "01/08/2014 01:44 PM";
items.Update();
list.Update();
web.AllowUnsafeUpdates = false;
protected void Page_Load(object sender, EventArgs e)
but when i try adding this web part to a page i got the following error:-
Invalid data has been used to update the list item.The field you are trying to update may be read only
so can anyone advice?

Answer 1

i only changed lines bitween 
web.AllowUnsafeUpdates = true;
and
web.AllowUnsafeUpdates = false;
and other parts of code remains without change
so it will updates all pages in current web
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