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I'm investigating an exception that a colleague just got while running an application through Visual Studio 2010: System.NullReferenceException was unhandled by user code Message=Object reference not set to an instance of an object. Source=mscorlib StackTrace: at System.Collections.Generic.GenericEqualityComparer`1.Equals(T x, T y) at System.Collections.Concurrent.ConcurrentDictionary`2.TryGetValue(TKey key, TValue& value) at xxxxxxx.xxxxxxx.xxxxxxx.RepositoryBase`2.GetFromCache(TIdentity id) Using .NET Reflector , I have looked at the code for GenericEqualityComparer<T>.Equals(T x, T y) , and I can't see any possible cause for a NullReferenceException . //GenericEqualityComparer<T>.Equals(T x, T y) from mscorlib 4.0.30319.269 public override bool Equals(T x, T y) { if (x != null) { return ((y != null) && x.Equals(y)); } if (y != null) { return false; } return true; } The type of T, TKey and TIdentity are all the same type in this stack trace . The type is a custom type called Identity that implements IEquatable<Identity> . It is immutable and cannot be constructed with null values for the fields that it uses in its implementation of Equals(Identity other) . It also overrides Equals(object obj) like this: public override bool Equals(object obj) { if ((object)this == obj) { return true; } return Equals(obj as Identity); } public bool Equals(Identity other) { if ((object)this == (object)other) { return true; } if ((object)other == null

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