Ok so this is one more,
I was working with SharePoint 2010 Modal dialogs for first time and was showing
my custom application page in modal dialog.
My application page was using custom master page and there were some buttons on
my custom application page which were causing post back and doing some changes
in user profile (basically accessing SharePoint site)
After everything was set up and I was ready to test.. and guess what .. After
clicking on those buttons I was getting error like .. Security Validation of
this page are invalid.. after some google I quickly got some results about this
error and some post were quite useful.
Firstly I got some posts which were describing about this error like, we should
add web.allowunsafeupdates = true; in the code but this was not the case in my
situation
After some more results I found that we should have to have <SharePoint:FormDigestControl>
included on our page layout / master page and error was solved
You need to register assembly like this
<%@ Register
Tagprefix="SharePoint"
Namespace="Microsoft.SharePoint.WebControls"
Assembly="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral,
PublicKeyToken=71e9bce111e9429c" %>
And then add control
There is already placeholder for this control in SharePoint master page by
default, but If you are using your own master page then you should make sure
that you add this control
<SharePoint:FormDigest ID="MyFormDigest" runat="server"/>
What this control is all about?
Well.. msdn says that,
The FormDigest control generates a security validation, or message digest, to
help prevent the type of attack whereby a user is tricked into posting data to
the server without knowing it. The security validation is specific to a user,
site, and time period and expires after a configurable amount of time. When the
user requests a page, the server returns the page with security validation
inserted. When the user then submits the form, the server verifies that the
security validation has not changed.
For more Information:
Here