There seems to be significant confusion in the C# community regarding spawning a FFmpeg process within a GUI app (e.g., developed with the Avalonia framework) and capturing its standard output. The following articles are top-ranked on Google but do not provide a straightforward solution:
- https://stackoverflow.com/questions/50680393/c-sharp-process-standardinput-and-standardoutput-with-ffmpeg-freezes
- https://stackoverflow.com/questions/46491619/using-ffmpeg-in-c-cant-get-the-ffmpeg-output
- https://www.codeproject.com/Questions/492381/StartInfo-RedirectStandardOutp
The trick is to read from stderr (!!!!) after setting the redirection flags and do this in a separate process to not block the GUI:
Process process = new Process();
process.StartInfo.FileName = "ffmpeg"; // or "ffmpeg.exe" if on MS Windows
process.StartInfo.Arguments = args; // ffmpeg args go here
process.StartInfo.UseShellExecute = false;
process.StartInfo.RedirectStandardInput = true; // so that we can terminate the process
process.StartInfo.RedirectStandardOutput = true;
process.StartInfo.RedirectStandardError = true;
process.Start();
Task.Run(() => {
// read all text at once and write it to a file
string output = process.StandardError.ReadToEnd();
File.WriteAllText("ffmpeg-output.txt", output);
});
If needed, it is simple to modify the above snippet so that the output is read in a line-by-line fashion.
If we would like to terminate the process, we can do this by writing q to its stdin.
The following line of code works:
process.StandardInput.WriteLine("q");