Why Does This Maintainability Index Increase?

Question

I would be appreciative if someone could explain to me the difference between the following two pieces of code in terms of Visual Studio's Code Metrics rules. Why does the Maintainability Index increase slightly if I don't encapsulate everything within using ( )?

Sample 1 (MI score of 71)

public static String Sha1(String plainText)
{
    using (SHA1Managed sha1 = new SHA1Managed())
    {
        Byte[] text = Encoding.Unicode.GetBytes(plainText);
        Byte[] hashBytes = sha1.ComputeHash(text);
        return Convert.ToBase64String(hashBytes);    
    }
}

Sample 2 (MI score of 73)

public static String Sha1(String plainText)
{
    Byte[] text, hashBytes;
    using (SHA1Managed sha1 = new SHA1Managed())
    {
        text = Encoding.Unicode.GetBytes(plainText);
        hashBytes = sha1.ComputeHash(text);
    }
    return Convert.ToBase64String(hashBytes);   
}

I understand metrics are meaningless outside of a broader context and understanding, and programmers should exercise discretion. While I could boost the score up to 76 with return Convert.ToBase64String(sha1.ComputeHash(Encoding.Unicode.GetBytes(plainText))), I shouldn't. I would clearly be just playing with numbers and it isn't truly any more readable or maintainable at that point. I am curious though as to what the logic might be behind the increase in this case. It's obviously not line-count.

Answer 1

Having your variables all laid out at the top so you know what's in the function is more "maintainable", at least that's what whoever decides the rules for the code metrics thinks.

Whether that's actually true? Totally depends on the team working on the code. It seems you already know this by the tone of the question, but take almost all code metrics with a grain of salt, they're what someone thinks is best, that may not be true for teams outside of microsoft...do what's best for your team, not what some calculator tells you.

I wouldn't make changes that are detrimental to your and your team's coding performance (unless it's for actual performance or improved error handling, etc) that you think are less readable for getting a few points on the metrics board.

All that being said, if it gives you a very low maintainability, there probably is something worth looking at or breaking down into smaller chunks, as a very low score is probably not so acceptable, for pretty much any team.

Answer 2

This is an old question, but I just thought I'd add that the MI is partially based on Halstead volume, which is based on a count of 'operators' and 'operands'. If declaration of a variable by type is an 'operator', this would mean that Sample 2 has fewer operators, thus changing the score. In general, because the MI is a statistical measurement, it is of limited usefulness when dealing with small sample sizes (like a single short method.)