Multiple SDN applications can make several harmful interferences unintentionally, although each individual application may be properly developed. This paper proposes a Multiple SDN Applications Interference Detector (MSAID). To bridge the gap between the source code of applications and the actual interferences, we leverage symbolic execution and constraint solving to obtain how the event handler handles the input messages. We then analyze the complex interaction of multiple applications and present novel methods to identify the interferences. Finally, we evaluate its correctness and prove its usefulness with a series of SDN applications.