expect call (socket.receive) and assign new buffer

May i know how to assign new value to the buffer receive. eg,

int Read = mSocket.Receive(mBuffer, 0, 4, SocketFlags.None)

mockSocket.ExpectAndReturn("Receive", 4).Args(Check.IsTypeof(typeof(System.Array)), 0, 4, SocketFlags.None);

For your information, the socket will store the value received in mBuffer. May i know how to assign new value to the mBuffer. According to the guidelines, Assign class only can used in ref or out parameter. Therefore, may i know which method can be used in this case? thanks