如何使用C# 捕获进程输出

Intro

很多时候我们可能会需要执行一段命令获取一个输出，遇到的比较典型的就是之前我们需要用 FFMpeg 实现视频的编码压缩水印等一系列操作，当时使用的是 FFMpegCore 这个类库，这个类库的实现原理是启动另外一个进程，启动 ffmpeg 并传递相应的处理参数，并根据进程输出获取处理进度

为了方便使用，实现了两个帮助类来方便的获取进程的输出，分别是 ProcessExecutor 和 CommandRunner，前者更为灵活，可以通过事件添加自己的额外事件订阅处理，后者为简化版，主要是只获取输出的场景，两者的实现原理大体是一样的，启动一个 Process，并监听其输出事件获取输出

ProcessExecutor

使用示例，这个示例是获取保存 nuget 包的路径的一个示例：

using var executor = new ProcessExecutor("dotnet", "nuget locals global-packages -l");
var folder = string.Empty;
executor.OnOutputDataReceived += (sender, str) =>
{
  if(str is null)
    return;

  Console.WriteLine(str);

  if(str.StartsWith("global-packages:"))
  {
    folder = str.Substring("global-packages:".Length).Trim();
  }
};
executor.Execute();

Console.WriteLine(folder);

ProcessExecutor 实现代码如下：

public class ProcessExecutor : IDisposable
{
  public event EventHandler<int> OnExited;

  public event EventHandler<string> OnOutputDataReceived;

  public event EventHandler<string> OnErrorDataReceived;

  protected readonly Process _process;

  protected bool _started;

  public ProcessExecutor(string exePath) : this(new ProcessStartInfo(exePath))
  {
  }

  public ProcessExecutor(string exePath, string arguments) : this(new ProcessStartInfo(exePath, arguments))
  {
  }

  public ProcessExecutor(ProcessStartInfo startInfo)
  {
    _process = new Process()
    {
      StartInfo = startInfo,
      EnableRaisingEvents = true,
    };
    _process.StartInfo.UseShellExecute = false;
    _process.StartInfo.CreateNoWindow = true;
    _process.StartInfo.RedirectStandardOutput = true;
    _process.StartInfo.RedirectStandardInput = true;
    _process.StartInfo.RedirectStandardError = true;
  }

  protected virtual void InitializeEvents()
  {
    _process.OutputDataReceived += (sender, args) =>
    {
      if (args.Data != null)
      {
        OnOutputDataReceived?.Invoke(sender, args.Data);
      }
    };
    _process.ErrorDataReceived += (sender, args) =>
    {
      if (args.Data != null)
      {
        OnErrorDataReceived?.Invoke(sender, args.Data);
      }
    };
    _process.Exited += (sender, args) =>
    {
      if (sender is Process process)
      {
        OnExited?.Invoke(sender, process.ExitCode);
      }
      else
      {
        OnExited?.Invoke(sender, _process.ExitCode);
      }
    };
  }

  protected virtual void Start()
  {
    if (_started)
    {
      return;
    }
    _started = true;

    _process.Start();
    _process.BeginOutputReadLine();
    _process.BeginErrorReadLine();
    _process.WaitForExit();
  }

  public async virtual Task SendInput(string input)
  {
    try
    {
      await _process.StandardInput.WriteAsync(input!);
    }
    catch (Exception e)
    {
      OnErrorDataReceived?.Invoke(_process, e.ToString());
    }
  }

  public virtual int Execute()
  {
    InitializeEvents();
    Start();
    return _process.ExitCode;
  }

  public virtual async Task<int> ExecuteAsync()
  {
    InitializeEvents();
    return await Task.Run(() =>
    {
      Start();
      return _process.ExitCode;
    }).ConfigureAwait(false);
  }

  public virtual void Dispose()
  {
    _process.Dispose();
    OnExited = null;
    OnOutputDataReceived = null;
    OnErrorDataReceived = null;
  }
}

CommandExecutor

上面的这种方式比较灵活但有些繁琐，于是有了下面这个版本

使用示例：

[Fact]
public void HostNameTest()
{
  if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
  {
    return;
  }

  var result = CommandRunner.ExecuteAndCapture("hostname");

  var hostName = Dns.GetHostName();
  Assert.Equal(hostName, result.StandardOut.TrimEnd());
  Assert.Equal(0, result.ExitCode);
}

实现源码：

public static class CommandRunner
{
  public static int Execute(string commandPath, string arguments = null, string workingDirectory = null)
  {
    using var process = new Process()
    {
      StartInfo = new ProcessStartInfo(commandPath, arguments ?? string.Empty)
      {
        UseShellExecute = false,
        CreateNoWindow = true,

        WorkingDirectory = workingDirectory ?? Environment.CurrentDirectory
      }
    };

    process.Start();
    process.WaitForExit();
    return process.ExitCode;
  }

  public static CommandResult ExecuteAndCapture(string commandPath, string arguments = null, string workingDirectory = null)
  {
    using var process = new Process()
    {
      StartInfo = new ProcessStartInfo(commandPath, arguments ?? string.Empty)
      {
        UseShellExecute = false,
        CreateNoWindow = true,

        RedirectStandardOutput = true,
        RedirectStandardError = true,

        WorkingDirectory = workingDirectory ?? Environment.CurrentDirectory
      }
    };
    process.Start();
    var standardOut = process.StandardOutput.ReadToEnd();
    var standardError = process.StandardError.ReadToEnd();
    process.WaitForExit();
    return new CommandResult(process.ExitCode, standardOut, standardError);
  }
}

public sealed class CommandResult
{
  public CommandResult(int exitCode, string standardOut, string standardError)
  {
    ExitCode = exitCode;
    StandardOut = standardOut;
    StandardError = standardError;
  }

  public string StandardOut { get; }
  public string StandardError { get; }
  public int ExitCode { get; }
}

More

如果只要执行命令获取是否执行成功则使用 CommandRunner.Execute 即可，只获取输出和是否成功可以用 CommandRunner.ExecuteAndCapture 方法，如果想要进一步的添加事件订阅则使用 ProcessExecutor

Reference

https://github.com/rosenbjerg/FFMpegCore
https://github.com/WeihanLi/WeihanLi.Common/blob/dev/src/WeihanLi.Common/Helpers/ProcessExecutor.cs
https://github.com/WeihanLi/WeihanLi.Common/blob/dev/test/WeihanLi.Common.Test/HelpersTest/ProcessExecutorTest.cs
https://github.com/WeihanLi/WeihanLi.Common/blob/dev/test/WeihanLi.Common.Test/HelpersTest/CommandRunnerTest.cs

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