Solutionizing .NET

In this installment of my Hacking LINQ series we’ll take a look at providing an IEqualityComparer for use in a LINQ join clause.

The Problem

Many of the Standard Query Operators require comparing sequence elements and the default query providers are kind enough to give us overloads that accept a suitable comparer. Among these operators, Join and GroupJoin have perhaps the most useful query syntax:

 var res = from s in States
          join a in AreaCodes
            on s.Abbr equals a.StateAbbr
          select new { s.Name, a.AreaCode };

While a bit more verbose, I find the intent much easier to read then the method equivalent:

var res = States.Join(AreaCodes,
                      s => s.Abbr, a => a.StateAbbr,
                      (s, a) => new { s.Name, a.AreaCode });

Or maybe I’ve just spent too much time in SQL. Either way, I thought it would be useful to support joins by a comparer.

The Goal

We will use another extension method to specify how the join should be performed:

var res = from s in States
          join a in AreaCodes.WithComparer(StringComparer.OrdinalIgnoreCase)
            on s.Abbr equals a.StateAbbr
          select new { s.Name, a.AreaCode };

We can also support the same syntax for group joins:

var res = from s in States
          join a in AreaCodes.WithComparer(StringComparer.OrdinalIgnoreCase)
            on s.Abbr equals a.StateAbbr into j
          select new { s.Name, Count = j.Count() };

The Hack

As with most LINQ hacks, we’re going to use the result of WithComparer to call a specialized version of Join or GroupJoin, in this case by providing a replacement for the join’s inner sequence:

var res = States.Join(AreaCodes.WithComparer(StringComparer.OrdinalIgnoreCase),
                      s => s.Abbr, a => a.StateAbbr,
                      (s, a) => new { s.Name, a.AreaCode });

Eventually leading to this method call:

var res = States.Join(AreaCodes,
                      s => s.Abbr, a => a.StateAbbr,
                      (s, a) => new { s.Name, a.AreaCode },
                      StringComparer.OrdinalIgnoreCase);

Since we need both the inner collection we’re extending and the comparer, we can guess our extension method will be implemented something like this:

public static JoinComparerProvider<T, TKey> WithComparer<T, TKey>(
    this IEnumerable<T> inner, IEqualityComparer<TKey> comparer)
{
    return new JoinComparerProvider<T, TKey>(inner, comparer);
}

With a trivial provider implementation:

public sealed class JoinComparerProvider<T, TKey>
{
    internal JoinComparerProvider(IEnumerable<T> inner, IEqualityComparer<TKey> comparer)
    {
        Inner = inner;
        Comparer = comparer;
    }

    public IEqualityComparer<TKey> Comparer { get; private set; }
    public IEnumerable<T> Inner { get; private set; }
}

The final piece is our Join overload:

public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(
    this IEnumerable<TOuter> outer,
    JoinComparerProvider<TInner, TKey> inner,
    Func<TOuter, TKey> outerKeySelector,
    Func<TInner, TKey> innerKeySelector,
    Func<TOuter, TInner, TResult> resultSelector)
{
    return outer.Join(inner.Inner, outerKeySelector, innerKeySelector,
                      resultSelector, inner.Comparer);
}

Implementations of GroupJoin and their IQueryable counterparts are similarly trivial.