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I am using C# 4.0 and Code Contracts and I have my own custom GameRoomCollection : IEnumerable<GameRoom> . I want to ensure, that no instances of GameRoomCollection will ever contain a null value element. I don't seem to be able to this, though. Instead of making a general rule, I have tried to do a plain and simple example. The AllGameRooms is an instance of GameRoomCollection . private void SetupListeners(GameRoom newGameRoom) { Contract.Requires(newGameRoom != null); //... } private void SetupListeners(Model model) { Contract.Requires(model != null); Contract.Requires(model.AllGameRooms != null); Contract.Assume(Contract.ForAll(model.AllGameRooms, g => g != null)); foreach (GameRoom gameRoom in model.AllGameRooms) SetupListeners(gameRoom);//<= Warning: Code Contracts: Requires unproven: newGameRoom != null } Can anyone see, why I haven't proven, that gameRoom is not null ? EDIT: Adding a reference for the object before iterating does not work either: IEnumerable<IGameRoom> gameRooms = model.AllGameRooms; Contract.Assume(Contract.ForAll(gameRooms, g => g != null)); foreach (IGameRoom gameRoom in gameRooms) SetupListeners(gameRoom);//<= Warning: Code Contracts: Requires unproven: newGameRoom != null EDIT2: However: If I convert the game room collection type to an array, it works fine: IGameRoom[] gameRoomArray = model.AllGameRooms.ToArray(); Contract.Assume(Contract.ForAll(gameRoomArray, g => g != null)); foreach (IGameRoom gameRoom in gameRoomArray) SetupListeners(gameRoom);//<= NO WARNING Is this caused by the fact, that you cannot define a rule for methods of the IEnumerable<T> interface? EDIT3: Ca

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