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    <p>It turns out Microsoft already has this covered in its testing framework: <a href="http://msdn.microsoft.com/en-us/library/ms243779.aspx" rel="noreferrer">CollectionAssert.AreEquivalent</a> </p> <blockquote> <p>Remarks</p> <p>Two collections are equivalent if they have the same elements in the same quantity, but in any order. Elements are equal if their values are equal, not if they refer to the same object.</p> </blockquote> <p>Using reflector, I modified the code behind AreEquivalent() to create a corresponding equality comparer. It is more complete than existing answers, since it takes nulls into account, implements IEqualityComparer and has some efficiency and edge case checks. plus, it's <em>Microsoft</em> :)</p> <pre><code>public class MultiSetComparer&lt;T&gt; : IEqualityComparer&lt;IEnumerable&lt;T&gt;&gt; { private readonly IEqualityComparer&lt;T&gt; m_comparer; public MultiSetComparer(IEqualityComparer&lt;T&gt; comparer = null) { m_comparer = comparer ?? EqualityComparer&lt;T&gt;.Default; } public bool Equals(IEnumerable&lt;T&gt; first, IEnumerable&lt;T&gt; second) { if (first == null) return second == null; if (second == null) return false; if (ReferenceEquals(first, second)) return true; if (first is ICollection&lt;T&gt; firstCollection &amp;&amp; second is ICollection&lt;T&gt; secondCollection) { if (firstCollection.Count != secondCollection.Count) return false; if (firstCollection.Count == 0) return true; } return !HaveMismatchedElement(first, second); } private bool HaveMismatchedElement(IEnumerable&lt;T&gt; first, IEnumerable&lt;T&gt; second) { int firstNullCount; int secondNullCount; var firstElementCounts = GetElementCounts(first, out firstNullCount); var secondElementCounts = GetElementCounts(second, out secondNullCount); if (firstNullCount != secondNullCount || firstElementCounts.Count != secondElementCounts.Count) return true; foreach (var kvp in firstElementCounts) { var firstElementCount = kvp.Value; int secondElementCount; secondElementCounts.TryGetValue(kvp.Key, out secondElementCount); if (firstElementCount != secondElementCount) return true; } return false; } private Dictionary&lt;T, int&gt; GetElementCounts(IEnumerable&lt;T&gt; enumerable, out int nullCount) { var dictionary = new Dictionary&lt;T, int&gt;(m_comparer); nullCount = 0; foreach (T element in enumerable) { if (element == null) { nullCount++; } else { int num; dictionary.TryGetValue(element, out num); num++; dictionary[element] = num; } } return dictionary; } public int GetHashCode(IEnumerable&lt;T&gt; enumerable) { if (enumerable == null) throw new ArgumentNullException(nameof(enumerable)); int hash = 17; foreach (T val in enumerable.OrderBy(x =&gt; x)) hash = hash * 23 + (val?.GetHashCode() ?? 42); return hash; } } </code></pre> <p>Sample usage:</p> <pre><code>var set = new HashSet&lt;IEnumerable&lt;int&gt;&gt;(new[] {new[]{1,2,3}}, new MultiSetComparer&lt;int&gt;()); Console.WriteLine(set.Contains(new [] {3,2,1})); //true Console.WriteLine(set.Contains(new [] {1, 2, 3, 3})); //false </code></pre> <p>Or if you just want to compare two collections directly:</p> <pre><code>var comp = new MultiSetComparer&lt;string&gt;(); Console.WriteLine(comp.Equals(new[] {"a","b","c"}, new[] {"a","c","b"})); //true Console.WriteLine(comp.Equals(new[] {"a","b","c"}, new[] {"a","b"})); //false </code></pre> <p>Finally, you can use your an equality comparer of your choice:</p> <pre><code>var strcomp = new MultiSetComparer&lt;string&gt;(StringComparer.OrdinalIgnoreCase); Console.WriteLine(strcomp.Equals(new[] {"a", "b"}, new []{"B", "A"})); //true </code></pre>
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    1. POComparing two collections for equality irrespective of the order of items in them
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    1. USOhad Schneider
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    1. POComparing two collections for equality irrespective of the order of items in them
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    1. COThanks for the answer, I didn't know Microsoft had it covered. For the matter, I actually use a variant of the answer on top which allows me to define how elements are compared for equality and checks for nulls.
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    2. CONo problem. It's also easy to add an IEqualityComparer<T> parameter to the above implementation in order to support a custom equality definition for T - In GetElementCounts(), simply use the Dictionary ctor that accepts IEqualityComparer<T>.
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    3. CONUnit also has a CollectionAssert.AreEquivalent() method. I'm curious which one came first, MS's or NUnit's.
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      1. USCristi Diaconescu
 

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