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I've got a generic type: class DictionaryComparer<TKey, TValue> : IEqualityComparer<IDictionary<TKey, TValue>> And a factory method that will (should) create an instance of this class for a given dictionary type. private static IEqualityComparer<T> CreateDictionaryComparer<T>() { Type def = typeof(DictionaryComparer<,>); Debug.Assert(typeof(T).IsGenericType); Debug.Assert(typeof(T).GetGenericArguments().Length == 2); Type t = def.MakeGenericType(typeof(T).GetGenericArguments()); return (IEqualityComparer<T>)Activator.CreateInstance(t); } Stripping away all of the extraneous stuff - even this code throws the same exception. private static object CreateDictionaryComparer() { Type def = typeof(DictionaryComparer<,>); Type t = def.MakeGenericType(new Type[] { typeof(String), typeof(object) }); return Activator.CreateInstance(t); } The Asserts pass so I know that T is generic and has two generic arguments. The line with MakeGenericType however excepts with: The number of generic arguments provided doesn't equal the arity of the generic type definition. Parameter name: instantiation I've done this sort of thing in the past and for the life of me can't figure out why this isn't working in this case. (plus I had to Google arity ).

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