The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.

If you are getting the following error while Attaching a EventHandler on a Sharepoint List, than follow these three steps to resolve that issue..

Error: The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.
Solution: Add the following Lines in you List Update code:
1. SPSecurity.RunWithElevatedPrivileges(delegate(){ - See Line 01
2. Add site.RootWeb.AllowUnsafeUpdates = true; , See Line 04
3. Add site.WebApplication.FormDigestSettings.Enabled = false;, See line 10
Sample Code

SPSecurity.RunWithElevatedPrivileges(delegate(){
using (SPSite site = new SPSite(URL))
{
site.RootWeb.AllowUnsafeUpdates = true;
using (SPWeb siteWeb = site.OpenWeb())
{
SPList list = siteWeb.Lists["TestList"];

SPEventReceiverDefinition defItemAdding = list.EventReceivers.Add();
site.WebApplication.FormDigestSettings.Enabled = false;
defItemAdding.Assembly = "WSPBuilderProject1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=e0a58138cf93116a";
defItemAdding.Class = "WSPBuilderProject1.TestListEH";
defItemAdding.Name = "AddingEventHandler";
defItemAdding.Type = SPEventReceiverType.ItemAdding;
defItemAdding.SequenceNumber = 1001;
//defItemAdding.Synchronization = SPEventReceiverSynchronization.Synchronous;

defItemAdding.Update();
site.WebApplication.FormDigestSettings.Enabled = true;

}
site.RootWeb.AllowUnsafeUpdates = false;
}
});

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