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The method Remove from the interface ICollection<KeyValuePair<TKey, TValue>> is implemented differently in Mono than on the .NET Framework. Namely, the .NET method will also compare the values and remove only if the value is equal (.NET reference source available at: http://referencesource.microsoft.com/#mscorlib/system/Collections/Concurrent/ConcurrentDictionary.cs,1304 ). The Mono implementation just checks the key.
Although this safe remove trick seems a little hacky (it is not documented in MSDN library documentation, there is only a 3 yr old blog post on MSDN describing this, and the method is actually privately implemented so you really have to know this is there to be able to use it...), it is incredibly useful since otherwise removal from the dictionary, concurrent with adding to and updating it, is unsafe and requires using a lock in a lot of cases.
Also, this issue is very similar to the already resolved issue #16990, related to method Contains from the same interface. These two should probably behave similarly.
master mono uses reference source concurrent collections implementation