Essential .NET, Volume 1: The Common Language Runtime

Enforcement

As important as security policy is, it spends most of its life lying dormant until it is time for enforcement. The CLR itself sometimes implicitly enforces security policy; however, security policy is most often enforced explicitly by trusted libraries that wish to protect a secure resource. One enforces security policy by demanding that all callers have been granted a particular permission or set of permissions. To that end, both the IPermission interface and the PermissionSet class support a Demand method to allow explicit policy enforcement.

The Demand method triggers a stack walk in which the permissions of every method are inspected. The CLR calculates the permissions of each method by running the evidence from the method's assembly ...