Question

In a .NET program that’s written to follow declarative style, what are some legitimate uses for ReferenceEquals()?

Answer 1

Not sure what you mean by “written to follow declarative style”, but ReferenceEquals is usually used when overriding the == operator. From http://msdn.microsoft.com/en-us/library/ms173147.aspx:

public static bool operator ==(ThreeDPoint a, ThreeDPoint b)
{
    // If both are null, or both are same instance, return true.
    if (System.Object.ReferenceEquals(a, b))
    {
        return true;
    }

    // If one is null, but not both, return false.
    if (((object)a == null) || ((object)b == null))
    {
        return false;
    }

    // Return true if the fields match:
    return a.x == b.x && a.y == b.y && a.z == b.z;
}

It is important to see the Note below for the justification:

Note: A common error in overloads of operator == is to use (a == b), (a == null), or (b == null) to check for reference equality. This
instead creates a call to the overloaded operator ==, causing an
infinite loop. Use ReferenceEquals or cast the type to Object, to
avoid the loop.