Elegant comparisons with the null coalescing operator

A while ago I commented on how I’d like a return? statement, which
only returned if the return value was non-null. The purpose of this was to remove
the irritation of implementing Equals and IComparable.CompareTo
on classes with several properties. For an example of the kind of thing I mean,
consider an Address class with properties Country,
State, City, ZipCode and HouseNumber.
(Apologies to readers who aren’t American – while I feel a traitor to my country for
using state instead of county and zip code instead of post code, I’m guessing there are
more readers from the US than from elsewhere.)

This Address class needs (for whatever reason) to be comparable to itself,
comparing the properties in the order described above, in normal string comparison order.
Let’s see how annoying that is without doing anything clever. (I haven’t included
any property implementations or constructors, but I’m sure you can all guess what they’d
look like. Similarly, I haven’t overridden object.Equals or object.Hashcode,
but the implementations are trivial.)

using System;

public sealed class Address : IComparable<Address>
{
    string country;
    string state;
    string city;
    string zipCode;
    int houseNumber;
    
    public int CompareTo(Address other)
    {
        if (other==null)
        {
            return 1;
        }
        int ret = country.CompareTo(other.country);
        if (ret != 0)
        {
            return ret;
        }
        ret = state.CompareTo(other.state);
        if (ret != 0)
        {
            return ret;
        }
        ret = city.CompareTo(other.city);
        if (ret != 0)
        {
            return ret;
        }
        ret = zipCode.CompareTo(other.zipCode);
        if (ret != 0)
        {
            return ret;
        }
        return houseNumber.CompareTo(other.houseNumber);
    }
}

That’s ignoring the possibility of any of the properties being null. If
we want to include that possibility, it’s worth having a static helper method which
copes with nulls, along the lines of object.Equals(object, object).

Now, if we don’t care about doing more comparisons than we really want to and
potentially creating an array each time, it wouldn’t be hard to implement a series
of overloaded methods along the lines of:

public static int ReturnFirstNonZeroElement(int first,
                                            int second,
                                            int third)
{
    return first != 0 ? first :
           second != 0 ? second :
           third;
}

(The array part would be when you implement ReturnFirstNonZeroElement(params int[] elements)
after you’d got enough overloads to get bored.)

That still ends up being a lot of code though, and it’s doing unnecessary comparisons.
I’m not keen on micro-optimisation, of course, but it’s the inelegance of it that
bothers me. It feels like there must be a way of doing it nicely. With
C# 2.0 and the null coalescing operator, we do. (At this point I’m reminded that
the irritation actually came when writing Java, which of course doesn’t have anything
similar. Grr.) For those who are unaware of the null coalescing operator (and it’s one of the
least well publicised new features in C# 2.0) see
my brief coverage of it.
Now consider the following helper method:

public static int? CompareFirstPass<T>(IComparable<T> first, T second) 
    where T : IComparable<T>
{
    if (first==null)
    {
        return -1;
    }
    // Assume CompareTo deals with second being null correctly
    int comparison = first.CompareTo(second);
    return comparison==0 ? (int?)null : comparison;
}

In short, this returns the result of the comparison if it’s non-zero, or null otherwise.
Now, with the null coalescing operator, this allows the Address class implementation
of CompareTo to be rewritten as:

public int CompareTo(Address other)
{        
    return other==null ? 1 :
           Helper.CompareFirstPass(country, other.country) ??
           Helper.CompareFirstPass(state, other.state) ??
           Helper.CompareFirstPass(city, other.city) ??
           Helper.CompareFirstPass(zipCode, other.zipCode) ??
           houseNumber.CompareTo(other.houseNumber);
}

It’s short, simple and efficient. Now, doesn’t that make you feel better? :)

3 thoughts on “Elegant comparisons with the null coalescing operator”

  1. Very nice! But I can already hear the angry howling of VB types who don’t even want to use the conditional operator…

    Like

  2. Imports System

    Module Module1

    Public Function IsDifferent(Of T As IComparable)(ByVal first As T, ByVal second As T, ByRef how As Integer) As Boolean
    If (first Is Nothing) Then
    how = -1
    Return False
    End If
    how = first.CompareTo(second)
    If how 0 Then
    Return True
    End If
    End Function

    Public NotInheritable Class Address
    Implements IComparable(Of Address)

    Private country As String
    Private state As String
    Private city As String
    Private zipCode As String
    Private houseNumber As Integer

    Public Function CompareTo(ByVal other As Address) As Integer Implements IComparable(Of Address).CompareTo
    If other Is Nothing Then
    Return 1
    End If
    Dim how As Integer = 0
    If IsDifferent(country, other.country, how) Then
    ElseIf IsDifferent(country, other.country, how) Then
    ElseIf IsDifferent(state, other.state, how) Then
    ElseIf IsDifferent(city, other.city, how) Then
    ElseIf IsDifferent(zipCode, other.zipCode, how) Then
    ElseIf IsDifferent(houseNumber, other.houseNumber, how) Then
    End If
    Return how
    End Function
    End Class

    End Module

    Like

  3. Yes, that’s a reasonably neat solution. I prefer the C# 2.0 version, but I’ll remember the solution using ByRef (which could be “out” in C#) for when I’m working in C# 1.1…

    Jon

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s