如何捕获自定义启动的WebAPI Core服务的Kestrel标准输出？

问题 #

我搭建了一个WebAPI Core服务，启动时标准输出会打印以下内容：

info: Microsoft.Hosting.Lifetime[14]
      Now listening on: http://localhost:5000
info: Microsoft.Hosting.Lifetime[14]
      Now listening on: https://localhost:5001
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.

我希望将这些输出（以及其他输出）捕获为字符串。我采用非标准方式启动服务，自定义了HostTestService类：

public static class HostTestService
{
    private static WebApplication? currentApp;

    public static async Task<List<string>?> StartService()
    {
        if (currentApp != null) { await StopService(); }

        var builder = WebApplication.CreateBuilder();
        var app = builder.Build();
        app.Start();
        
        var server = app.Services.GetService<IServer>();
        var addressFeature = server?.Features.Get<IServerAddressesFeature>();
        var addresses = addressFeature?.Addresses.ToList();
        
        // Set the Current app so that calls to shut it down will now succeed.
        currentApp = app;

        return addresses;
    }

    public static async Task StopService()
    {
        if (currentApp != null)
        {
            await currentApp.StopAsync(TimeSpan.FromSeconds(10));
            await currentApp.DisposeAsync();
            currentApp = null;
        }
    }
}

我在NUnit测试中调用HostTestService.StartService()启动服务，但需要捕获该服务的输出，且不希望重定向所有标准输出（避免影响其他依赖标准输出的功能）。

解决方案 #

方法1：添加自定义日志提供程序（推荐） #

ASP.NET Core的日志系统支持自定义提供程序，我们可以创建一个专门收集日志的提供程序，直接捕获服务的所有日志输出，完全不影响全局标准输出。

1. 实现自定义日志提供程序和日志类：

public class CapturedLog
{
    public LogLevel Level { get; set; }
    public string Message { get; set; } = string.Empty;
}

public class CustomLoggerProvider : ILoggerProvider
{
    public List<CapturedLog> CapturedLogs { get; } = new();

    public ILogger CreateLogger(string categoryName)
    {
        return new CustomLogger(this, categoryName);
    }

    public void Dispose() { }

    private class CustomLogger : ILogger
    {
        private readonly CustomLoggerProvider _provider;
        private readonly string _categoryName;

        public CustomLogger(CustomLoggerProvider provider, string categoryName)
        {
            _provider = provider;
            _categoryName = categoryName;
        }

        public IDisposable? BeginScope<TState>(TState state) where TState : notnull => null;

        public bool IsEnabled(LogLevel logLevel) => true;

        public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception? exception, Func<TState, Exception?, string> formatter)
        {
            var message = formatter(state, exception);
            _provider.CapturedLogs.Add(new CapturedLog
            {
                Level = logLevel,
                Message = $"[{_categoryName}] {message}"
            });
        }
    }
}

2. 修改HostTestService，添加日志提供程序并返回捕获的日志：

public static class HostTestService
{
    private static WebApplication? currentApp;
    private static CustomLoggerProvider? _loggerProvider;

    public static async Task<(List<string>? Addresses, List<CapturedLog> CapturedLogs)> StartService()
    {
        if (currentApp != null) { await StopService(); }

        _loggerProvider = new CustomLoggerProvider();
        
        var builder = WebApplication.CreateBuilder();
        // 添加自定义日志提供程序
        builder.Logging.ClearProviders(); // 可选：移除默认控制台提供，避免重复输出
        builder.Logging.AddProvider(_loggerProvider);
        
        var app = builder.Build();
        app.Start();
        
        var server = app.Services.GetService<IServer>();
        var addressFeature = server?.Features.Get<IServerAddressesFeature>();
        var addresses = addressFeature?.Addresses.ToList();
        
        currentApp = app;

        return (addresses, _loggerProvider.CapturedLogs);
    }

    public static async Task StopService()
    {
        if (currentApp != null)
        {
            await currentApp.StopAsync(TimeSpan.FromSeconds(10));
            await currentApp.DisposeAsync();
            currentApp = null;
            _loggerProvider = null;
        }
    }
}

3. 在NUnit测试中获取日志：

[Test]
public async Task TestServiceOutput()
{
    var result = await HostTestService.StartService();
    var capturedLogs = result.CapturedLogs;
    // 遍历capturedLogs即可获取所有输出内容
    foreach (var log in capturedLogs)
    {
        Console.WriteLine($"Captured: {log.Message}"); // 这里用全局输出是测试用，实际可以直接处理字符串
    }
    
    await HostTestService.StopService();
}

方法2：临时重定向控制台输出流 #

如果需要保留默认日志格式但仅捕获该服务的输出，可以临时替换Console.Out为内存流，启动服务后再恢复原流。这种方式要注意线程安全，避免影响其他输出。

修改HostTestService如下：

public static class HostTestService
{
    private static WebApplication? currentApp;
    private static StringWriter? _outputWriter;
    private static TextWriter? _originalOut;

    public static async Task<(List<string>? Addresses, string CapturedOutput)> StartService()
    {
        if (currentApp != null) { await StopService(); }

        // 保存原始输出流，创建内存流捕获输出
        _originalOut = Console.Out;
        _outputWriter = new StringWriter();
        Console.SetOut(_outputWriter);
        
        var builder = WebApplication.CreateBuilder();
        var app = builder.Build();
        app.Start();
        
        // 恢复原始输出流，避免影响后续操作
        Console.SetOut(_originalOut);
        
        var server = app.Services.GetService<IServer>();
        var addressFeature = server?.Features.Get<IServerAddressesFeature>();
        var addresses = addressFeature?.Addresses.ToList();
        
        currentApp = app;
        var capturedOutput = _outputWriter.ToString();

        return (addresses, capturedOutput);
    }

    public static async Task StopService()
    {
        if (currentApp != null)
        {
            await currentApp.StopAsync(TimeSpan.FromSeconds(10));
            await currentApp.DisposeAsync();
            currentApp = null;
            _outputWriter?.Dispose();
            _outputWriter = null;
            _originalOut = null;
        }
    }
}

说明 #

• 方法1是ASP.NET Core推荐的方式，更灵活，能精准控制捕获的日志级别、分类，完全隔离全局输出。
• 方法2适合需要保留原始控制台输出格式的场景，但要注意启动服务过程中如果有其他线程写入控制台，可能会被意外捕获，所以建议在测试中确保只有当前服务启动时使用该方式。

内容的提问来源于stack exchange，提问作者Vaccano