LavaBlast.Linq.Extensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace System.Linq
{
	public class KeyEqualityComparer<T> : IEqualityComparer<T>
	{
		private readonly Func<T, T, bool> comparer;
		private readonly Func<T, object> keyExtractor;

		// Enable to only specify the key to compare with: y => y.CustomerID
		public KeyEqualityComparer(Func<T, object> keyExtractor) : this(keyExtractor, null) { }
		// Enable to specify how to tel if two objects are equal: (x, y) => y.CustomerID == x.CustomerID
		public KeyEqualityComparer(Func<T, T, bool> comparer) : this(null, comparer) { }

		public KeyEqualityComparer(Func<T, object> keyExtractor, Func<T, T, bool> comparer)
		{
			this.keyExtractor = keyExtractor;
			this.comparer = comparer;
		}

		public bool Equals(T x, T y)
		{
			if (comparer != null)
				return comparer(x, y);
			else
			{
				var valX = keyExtractor(x);
				if (valX is IEnumerable<object>) // The special case where we pass a list of keys
					return ((IEnumerable<object>)valX).SequenceEqual((IEnumerable<object>)keyExtractor(y));

				return valX.Equals(keyExtractor(y));
			}
		}

		public int GetHashCode(T obj)
		{
			if (keyExtractor == null)
				return obj.ToString().ToLower().GetHashCode();
			else
			{
				var val = keyExtractor(obj);
				if (val is IEnumerable<object>) // The special case where we pass a list of keys
					return (int)((IEnumerable<object>)val).Aggregate((x, y) => x.GetHashCode() ^ y.GetHashCode());

				return val.GetHashCode();
			}
		}
	}

	public class Comparer<T> : IComparer<T>
	{
		private readonly Func<T, T, int> _comparer;

		public Comparer(Func<T, T, int> comparer)
		{
			if (comparer == null)
				throw new ArgumentNullException("comparer");
			_comparer = comparer;
		}

		public int Compare(T x, T y)
		{
			return _comparer(x, y);
		}
	}

	public static class GeneralExtensions
	{
		public static IEnumerable<T> Distinct<T>(this IEnumerable<T> list, Func<T, object> keyExtractor)
		{
			return list.Distinct(new KeyEqualityComparer<T>(keyExtractor));
		}

		public static bool Contains<T>(this IEnumerable<T> list, T item, Func<T, object> keyExtractor)
		{
			return list.Contains(item, new KeyEqualityComparer<T>(keyExtractor));
		}

		public static IEnumerable<T> Except<T>(this IEnumerable<T> list, IEnumerable<T> except, Func<T, object> keyExtractor)
		{
			return list.Except(except, new KeyEqualityComparer<T>(keyExtractor));
		}

		public static IEnumerable<T> Intersect<T>(this IEnumerable<T> list, IEnumerable<T> toIntersect, Func<T, object> keyExtractor)
		{
			return list.Intersect(toIntersect, new KeyEqualityComparer<T>(keyExtractor));
		}

		public static IOrderedEnumerable<T> OrderBy<T, TKey>(this IEnumerable<T> list, Func<T, TKey> keySelector, Func<TKey, TKey, int> comparer)
		{
			return list.OrderBy(keySelector, new Comparer<TKey>(comparer));
		}

		public static IOrderedEnumerable<T> OrderByDescending<T, TKey>(this IEnumerable<T> list, Func<T, TKey> keySelector, Func<TKey, TKey, int> comparer)
		{
			return list.OrderByDescending(keySelector, new Comparer<TKey>(comparer));
		}

		public static bool SequenceEqual<T>(this IEnumerable<T> list, IEnumerable<T> sequenceToEqual, Func<T, object> keyExtractor)
		{
			return list.SequenceEqual(sequenceToEqual, new KeyEqualityComparer<T>(keyExtractor));
		}

		public static IEnumerable<TSource> Union<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second, Func<TSource, object> keyExtractor)
		{
			return first.Union(second, new KeyEqualityComparer<TSource>(keyExtractor));
		}

		public static IEnumerable<T> ToEnumerable<T>(this T o)
		{
			return new List<T>() { o };
		}

		public static bool IsNullOrEmpty(this string value)
		{
			return string.IsNullOrEmpty(value);
		}

		public static string FormatIt(this string val, params object[] args)
		{
			return string.Format(val, args);
		}
	}
}

Is this based on the answers to this StackOverflow question? http://stackoverflow.com/questions/98033/wrap-a-delegate-in-an-iequalitycomparer

Yeah, EtienneT blogged about it here: http://blog.lavablast.com/post/2010/05/05/Lambda-IEqualityComparer3cT3e.aspx

Thanks! I didn't see that post