.Net FtpWebRequest and 503 (bad sequence of commands) errors
Mar 26 2009 · .NET Platform
I’ve been working on some FTP code in C# with the .Net framework’s FTP classes and frustrated that I was occasionally getting 503 (bad sequence of commands) errors. Turns out these errors will pop up if the FtpWebRequest.KeepAlive property is set to true, which it is by default. Unfortunately, this doesn’t really address why this is occurring in the first place. I’m doing a lot of multithreaded stuff, so perhaps that’s the culprit (i.e. 2+ threads using the same connection, layering their calls on top of each other). However, I’ve found some forum posts (such as this one) indicating others are having the same issue doing seemingly serial, non-asynchronous stuff.