Why is this an issue?

Transparency attributes, SecurityCriticalAttribute and SecuritySafeCriticalAttribute are used to identify code that performs security-critical operations. The second one indicates that it is safe to call this code from transparent, while the first one does not. Since the transparency attributes of code elements with larger scope take precedence over transparency attributes of code elements that are contained in the first element a class, for instance, with a SecurityCriticalAttribute can not contain a method with a SecuritySafeCriticalAttribute.

This rule raises an issue when a member is marked with a System.Security security attribute that has a different transparency than the security attribute of a container of the member.

Noncompliant code example

using System;
using System.Security;

namespace MyLibrary
{

    [SecurityCritical]
    public class Foo
    {
        [SecuritySafeCritical] // Noncompliant
        public void Bar()
        {
        }
    }
}

Compliant solution

using System;
using System.Security;

namespace MyLibrary
{

    [SecurityCritical]
    public class Foo
    {
        public void Bar()
        {
        }
    }
}

Resources