Ok. I built a custom ffmpeg wrapper but it was just a dirty hack. it was not encapsulated clean.... it worked but it was just tooo ugly to develop some extension... wrapping looked something like this: THIS DOES NOT WORK it is refactored to death: class MencoderSharp { public static bool mencoder(System.Uri infovdrpath, string ratio, System.Uri destinationpath, BackgroundWorker worker, bool deleteConverted,string audioparameter, string videoparamter, bool testRun, DoWorkEventArgs e) { int returncode = 0; foreach (string sourcepath in Directory.GetFiles(infovdrpath.ToString().Substring(0, infovdrpath.ToString().Length - 9), "0*.vdr")) { Process p = new Process(); //p.StartInfo.WorkingDirectory = @"C:\whatever"; p.StartInfo.FileName = @"mencoder.exe"; // http://msdn.microsoft.com/de-de/library/system.diagnostics.processstar