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Based on mjolka's test it is about 2 times slower than the original.

After reading Dmitry's interresting and fast approach to the problem, I had the idea,why not skipping the Dictionary<T> and use a IList<T> instead.

Based on mjolka's test it is about 2 times slower than the original.

After reading Dmitry's interresting and fast approach to the problem, I had the idea,why not skipping the Dictionary<T> and use a IList<T> instead.

We will

• create a new List<T> by passing the source to the constructor
• iterate over other and remove each item of other from the List<T>

Each T we use, should implement the IEquatable<T> interface to assure the equality of two T.

public static bool SetEqual<T>(this IEnumerable<T> source, IEnumerable<T> other, IEqualityComparer<T> comparer = null)
{
 if (source == null)
 {
 return other == null;
 }
 if (other == null)
 {
 return false;
 }
 ICollection<T> sourceCollection = source as ICollection<T>;
 ICollection<T> otherCollection = other as ICollection<T>;
 if (sourceCollection != null && otherCollection != null)
 {
 if (sourceCollection.Count != otherCollection.Count)
 {
 return false;
 }
 }
 IList<T> sourceList = new List<T>(source);
 foreach (T item in other)
 {
 if (!sourceList.Remove(item)) { return false; }
 }
 return sourceList.Count == 0;
} 

Updated timing
1000 / 1000: 00:00:00.1250604 HashSet (Timed on my computer)
1000 / 1000: 00:00:00.2211285 Original (Timed on my computer)
1000 / 1000: 00:00:00.4917692 MyDictionary (Timed on my computer)
1000 / 1000: 00:00:00.1868693 Dmitry's Dictionary (Timed on my computer)
1000 / 1000: 00:00:00.0074877 IList (Timed on my computer)

This had been very slow, as for each call to Remove() the List has been iterated over. I initially did a timing which evaluated to beeing the fastest, but unfortunately I did my timing with an changed version of the test mjolka provided. It is the fastest, but only if both IEnumerable will contain absolutly unique items. Both for the single IEnumerable and each item to the other IEnumerable.

Thanks to @dmitry for pointing this out. If you want to see the implementation, check the edit history.

Update