Immutable readonly reference types & FXCop Violation: Do not declare read only mutable reference types

Question

I have been trying to wrap my head around this FXCop violation "DoNotDeclareReadOnlyMutableReferenceTypes"

MSDN: http://msdn.microsoft.com/en-us/library/ms182302%28VS.80%29.aspx

Code from MSDN which would cause this violation:

namespace SecurityLibrary
{
    public class MutableReferenceTypes
    {
        static protected readonly StringBuilder SomeStringBuilder;

        static MutableReferenceTypes()
        {
            SomeStringBuilder = new StringBuilder();
        }
    }
}

From Jon's answer here and here , I understand that the field holding the reference to the object (in this case SomeStringBuilder) is readonly and not the object itself (which is created by new StringBuilder() )

So taking this example, how would I change the object itself, once the field has a reference to it ? I like Eric Lippert's example of how the readonly array can be changed, and would like to see something similar for any other mutable reference type

Answer 1

readonly means you can't change the reference post-construction.

The official FXCop stance is that it recommends that only types that can't be modified should be declared readonly. Therefore something like a string is okay because the value of the object can't be changed. However the value of StringBuilder can changed but making it readonly only prevents you from assigning the field to a different StringBuilder instance or null after the constructor runs.

I disagree with FXCop on this rule. As long as one understands that this is simply an enforcement that the reference may not change post-construction then there is no confusion.

Note that value-types are made immutable by the readonly keyword but reference types are not.

namespace SecurityLibrary
{
    public class MutableReferenceTypes
    {
        static protected readonly StringBuilder SomeStringBuilder;

        static MutableReferenceTypes()
        {
            // allowed
            SomeStringBuilder = new StringBuilder();
        }

        void Foo()
        {
            // not allowed
            SomeStringBuilder = new StringBuilder();
        }

        void Bar()
        {
            // allowed but FXCop doesn't like this
            SomeStringBuilder.AppendLine("Bar");
        }
    }
}