Question

After upgrading my web app from .Net 3.5 to .Net4.0, I get a security transparency rules failed.

I’ve never even heard of this or incorporated it into my project.
Does anyone have any idea what this may be?

Answer 1

From Microsoft’s knowledge-base KB2635463:

Cause
This problem occurs because when the System.Web.UI.ScriptManager class
is first instantiated it builds a cache of method information for
classes in assemblies already loaded in the application. When it tries
to examine the constructor of the InstrumentedAttribute the exception
is thrown and not handled.

Resolution
To workaround this issue reliably, instantiate an instance of
ScriptManager at a point during application start up prior to the
first request actually being executed but after the assembly that has
the InstrumentedAttribute has been loaded.

For example, in global.asax add some new code to Application_Start:

private void Application_Start(HttpApplication application) 
{ 
    if (applicationStartupComplete) return; 
    try 
    { 
        object osm = new System.Web.UI.ScriptManager(); 
    } 
    catch(Exception)
    {}
}

---

Additionally, a post at the asp.net forums reveals this answer from a Microsoft employee:

We have seen a combination of issues that might cause this:

1) The
webserver has the trust level set less than “Full” at the machine
web.config (ref)

2) The webserver does not have the AJAX files (System.Web.Extensions)
installed into the GAC.

Solution 1: Set trust level to “Full” in the machine level web.config

Solution 2: Install AJAX files into the GAC

If refreshing resolves it – it might just be a timing issue loading
the extensions. Probably if you install into the GAC it will resolve
it.