Prevents callers higher in the call stack from using the code that calls this method to access all resources except for the resource specified by the current instance.
The CodeAccessPermission.PermitOnly method should be used only to protect resources from accidental access by fully trusted code. It should not be used to protect resources from intentional misuse by untrusted code. For example, if method A issues a CodeAccessPermission.PermitOnly for a permission and then calls method B, method B can overtly override the CodeAccessPermission.PermitOnly by issuing an CodeAccessPermission.Assert. The called method is always higher in the stack. Therefore, if method B tries to access a protected resource, the security system begins checking for permissions with it because method B is the immediate caller, and then walks down the stack to confirm that there is no CodeAccessPermission.Deny or CodeAccessPermission.PermitOnly lower in the stack. Method B, which is trying to access the resource, can stop the stack walk immediately by using the CodeAccessPermission.Assert method. In that case, the CodeAccessPermission.PermitOnly placed on the stack by method A (the calling method) is never discovered.
CodeAccessPermission.PermitOnly is similar to CodeAccessPermission.Deny, in that both cause stack walks to fail when they would otherwise succeed. The difference is that CodeAccessPermission.Deny specifies permissions that will cause the stack walk to fail, but CodeAccessPermission.PermitOnly specifies the only permissions that do not cause the stack walk to fail.
Call this method to ensure that your code can be used to access only the specified resources. The call to CodeAccessPermission.PermitOnly is effective until the calling code returns to its caller. Only one CodeAccessPermission.PermitOnly can be active on a frame. An attempt to call CodeAccessPermission.PermitOnly when an active CodeAccessPermission.PermitOnly exists on the frame results in a System.Security.SecurityException. Call CodeAccessPermission.RevertPermitOnly or CodeAccessPermission.RevertAll to remove an active CodeAccessPermission.PermitOnly.
CodeAccessPermission.PermitOnly is ignored for a permission not granted because a demand for that permission will not succeed. However, if code lower on the call stack later calls CodeAccessPermission.Demand for that permission, a System.Security.SecurityException is thrown when the stack walk reaches the code that tried to call CodeAccessPermission.PermitOnly. This is because the code that called CodeAccessPermission.PermitOnly has not been granted the permission, even though it called CodeAccessPermission.PermitOnly for that permission. The call stack is typically represented as growing down, so that methods higher in the call stack call methods lower in the call stack.