Instantly Error-Monitoring of Network Based on C# .NET and ICMP Zhang,Xun1 Jing,Xijun2 Abstract: According to the theory of Internet Control Message Protocol and using Visual C# .NET as the programming tool, we can send data packets to the monitored objects and listen to their responding time to judge the condition of intercommunicating lines, so we can find the errors of the lines instantly. Key words: ICMP, Visual C#.NET, multi-thread, error-monitoring