如何在没有死锁的情况下正确读取c＃进程标准错误流？

Question

It really feels like I'm following best practices according to MSDN . But I'm most likely missing something because my code hangs after continuing from this line: string errorOutput = cmd.StandardError.ReadToEnd(); . What am I doing wrong?

        var batchfile = File.OpenWrite("run.bat");
        StreamWriter writer = new StreamWriter(batchfile);
        writer.Write("dotnet run" + '\n');
        //writer.Write("set /p temp=\"Hit enter to continue\"" + '\n');
        writer.Close();

        var cmd = new Process();
        cmd.StartInfo.FileName = batchfile.Name;
        cmd.StartInfo.UseShellExecute = false;
        cmd.StartInfo.RedirectStandardError = true;

        cmd.Start();
        string errorOutput = cmd.StandardError.ReadToEnd();
        cmd.WaitForExit();

        var outputfile = File.OpenWrite("run_errorOut.txt");
        StreamWriter outputWriter = new StreamWriter(outputfile);
        outputWriter.Write(errorOutput);
        outputWriter.Close();

In case it helps: At the moment this code is running inside an xunit test inside a dotnetcore2.2 app (its target framework is 'netcoreapp2.2').

Answer 1

So I had a very similar problem not that long ago, doing almost exactly the same and what I found was that:

• Trying to read StandardError/Output without there being anything to read blocks until there is something to read (or deadlock if nothing is ever written - eg process has exited)
• cmd.HasExited is false until the StandardError/Output has been read so that cannot be used to detect when finished (and then read StandardError/Output)

The most reliable way I have found to do this is to use the provided events on Process :

var outData = new StringBuilder();
var errData = new StringBuilder();    

cmd.OutputDataReceived += (sender, args) => outData.Append(args.Data ?? string.Empty);
cmd.ErrorDataReceived += (sender, args) => errData.Append(args.Data ?? string.Empty);

And then after cmd.Start() == true :

cmd.BeginErrorReadLine();
cmd.BeginOutputReadLine();

To start the events firing.

After the process has exited you can call .ToString() on the StringBuilders to get the data:

Console.WriteLine(outData.ToString());
Console.WriteLine(errData.ToString());

(Note that .ToString() can be implicit when using Console.WriteLine )