How to access a security critical field from an anonymous delegate or lambda?

Question

Scenario

Let's say we've the next code:

[SecuritySafeCritical]
public void SomeMethod()
{
    SomeCriticalClass critical = new SomeCriticalClass();

    Action someDelegate = () => 
    {
         critical.Do();
    }

    someDelegate();
}

The SomeMethod signature has [SecuritySafeCritical] attribute.
SomeCriticalClass is some class that has the [SecurityCritical] attribute either in the class or method Do method-level.
We create an anonymous delegate auto-inferred to Action.

Problem

Calling critical.Do() causes a ~~MethodAccessException~~ FieldAccessException because a security transparent method (the anonymous method) is trying to access a security critical field (the critical SomeCriticalClass local variable).

Question

How you overcome this?

The easy way would be implementing an actual method marked with [SecuritySafeCritical] instead of using an anonymous delegate. But this moves us to pre-anonymous delegates and lambas era. I don't want this.

Other easy way would be just don't using security transparency. This isn't a solution.

Almost any available libraries both from Microsoft and open source community aren't designed with security transparency in mind. That is, any own custom code must interoperate with third-party libraries through [SecuritySafeCritical] or [SecurityCritical] methods/properties/delegates.

Actually I believe that security transparency is a good tool because it forces better and secure software designs, critical actions are very localized and the rest of the code works with minimal permissions.

In the specific case you've given, it's not clear that you need a lambda at all. You could just use Action someDelegate = critical.Do; couldn't you? — Jon Skeet
look at MSDN on this issue.. google the ERROR just as I did to get you to a good starting point on how to resolve this issue. msdn.microsoft.com/en-us/library/bb264475.aspx or msdn.microsoft.com/en-us/library/dd233102.aspx — MethodMan
@JonSkeet I'm my real-world scenario isn't that easy. Well, I can do it, but this wouldn't be in a single place, but in a lot of places. This is very bad because using security transparency prevents me from using modern and compact anonymous lambda delegates which saves a lot of code lines and makes the code smaller and yet readable. — Matías Fidemraizer
@MatíasFidemraizer: As you say at the bottom of your question - "critical actions are very localized" - if you don't have many places to do this, you can keep most of your code "modern", and just have more verbose code in a few places — Jon Skeet
@DJKRAZE Thanks for your contribution, but I really know how security transparency works. I'm asking this question because this is a real limitation of using it. — Matías Fidemraizer

Matías Fidemraizer Matías Fidemraizer · Accepted Answer · 2012-12-19T17:20:20

Sorry but I couldn't wait for other answerers... I got the solution!

By experimentation I could determine that marking with [SecuritySafeCritical] the class having the method that creates the anonymous method in its body, does the trick!!

In other words, or talking about the code:

[SecurityCritical]
public class SomeCriticalClass
{
      [SecurityCritical]
      public void Do()
      {
      }
}

[SecuritySafeCritical]
public sealed class SomeClass
{
    [SecuritySafeCritical]
    public void SomeMethod()
    {
          SomeCriticalClass critical = new SomeCriticalClass()

          // No more FieldAccessException!
          Action action = () => critical.Do();         
    }
}

I want to make some clarifications:

Marking the class SomeClass with [SecuritySafeCritical] doesn't mean that all declared methods will be [SecuritySafeCritical] by default. It means that the class can be used by partially-trusted callers. You still need to mark with [SecuritySafeCritical] attribute those methods, properties or fields that can be accessed by partially-trusted callers.
It seems that [SecuritySafeCritical] at class-level makes local variables and anonymous methods (maybe anonymous objects too!) security safe critical.

Yes! I hope both my question and my own answer will be useful for everyone, because I believe that the situation described in my question can happen often!

How to access a security critical field from an anonymous delegate or lambda?

Scenario

Problem

Question

1 Answers