ASP.NET Core 3.1中间件ILogger注入失效问题求助

我最近在学习ASP.NET Core 3.1的基础内容，尝试构建一个记录每个请求信息的中间件时，遇到了非常奇怪的日志行为，折腾了好一会儿才找到原因，分享给大家：

遇到的异常日志行为 #

• 情况1：指定名称创建Logger正常工作
  通过ILoggerFactory创建指定名称的Logger时，日志可以正常输出：

  public ContextRequestLoggingMiddleware( RequestDelegate next, ILoggerFactory loggerFactory ) 
  { 
      this._next = next; 
      _logger = loggerFactory.CreateLogger( "ContextRequestLoggingMiddleware" ); 
  }
  

  对应的InvokeAsync方法里执行_logger.LogInformation( "test from middleware" )能正常记录日志。

• 情况2：泛型Logger/注入ILogger无输出，注入ILogger报错
  当使用泛型方式创建Logger（_logger = loggerFactory.CreateLogger<ContextRequestLoggingMiddleware>();）或者直接注入ILogger<ContextRequestLoggingMiddleware>时，代码执行到日志语句却没有任何日志输出，也没有报错；如果尝试注入普通的ILogger，会直接抛出运行时错误：

  Unable to resolve service for type 'Microsoft.Extensions.Logging.ILogger' while attempting to activate 'Microsoft.AspNetCore.Builder.ContextRequestLoggingMiddleware'
  

• 情况3：调试无明显差异，控制器中ILogger正常
  调试时查看WatchList，没发现Logger对象有什么差异，而且控制器里的ILogger<T>是可以正常使用的。

我的项目相关代码 #

CreateHostBuilder代码 #

public static IHostBuilder CreateHostBuilder( string[] args ) => Host.CreateDefaultBuilder( args )
 .ConfigureLogging( ( hostContext, loggingBuilder ) => 
 { 
     loggingBuilder.ClearProviders(); 
     loggingBuilder.AddConfiguration( hostContext.Configuration.GetSection( "Logging" ) ); 
     loggingBuilder.AddCustomFileLogger(); 
 })
 .ConfigureWebHostDefaults( webBuilder => { webBuilder.UseStartup<Startup>(); } );

Startup类相关代码 #

public void ConfigureServices( IServiceCollection services ) 
{ 
    services.AddControllers(); 
    services.AddDbContext<NPAContext>( options => options.UseSqlServer( Configuration.GetConnectionString( cnnDB ) ) ); 
    services.AddAuthentication( options => 
    { 
        options.DefaultAuthenticateScheme = ApiKeyAuthenticationOptions.DefaultScheme; 
        options.DefaultChallengeScheme = ApiKeyAuthenticationOptions.DefaultScheme; 
    })
    .AddApiKeySupport( options => { } ); 
    services.AddSingleton<List<JobEventDTO>>(); 
}

public void Configure( IApplicationBuilder app, IWebHostEnvironment env, ILoggerFactory loggerFactory ) 
{ 
    if ( env.IsDevelopment() ) { app.UseDeveloperExceptionPage(); } 
    app.UseHttpsRedirection(); 
    app.UseRouting(); 
    app.UseContextRequestLoggingMiddleware(); 
    app.UseAuthentication(); 
    app.UseAuthorization(); 
    app.UseEndpoints( endpoints => { endpoints.MapControllers(); } ); 
}

问题解决方法 #

最终发现问题出在中间件的命名空间上！我之前把中间件放在了Microsoft.AspNetCore.Builder命名空间下，把它改成自定义的命名空间（比如NPARetrieval.Middleware）之后，泛型CreateLogger<T>和注入ILogger<T>的方式都正常工作了。修改后的代码如下：

namespace NPARetrieval.Middleware 
{ 
    public class ContextRequestLoggingMiddleware 
    { 
        private readonly RequestDelegate _next; 
        private readonly ILogger _logger; 
        public ContextRequestLoggingMiddleware( RequestDelegate next, ILogger<ContextRequestLoggingMiddleware> logger ) 
        { 
            this._next = next; 
            _logger = logger; 
        } 
        public async Task InvokeAsync (HttpContext context ) 
        { 
            _logger.LogInformation( "test from middleware" ); 
            await _next( context ); 
        } 
    } 
}

内容的提问来源于stack exchange，提问作者Matus