MediatR泛型Pipeline Behavior注册无效，如何正确注册？

• 问题根源：你定义的LoggingPipeLineBehavior泛型约束过于具体，仅匹配IRequest<Result<TResult>>类型的请求，而MediatR在解析开放泛型Behavior时，无法正确将这种约束与具体请求（如CreateArtistCommand）关联，导致Behavior未被实例化执行。

• 修复方案：

  1. 调整Behavior的泛型定义，使其适配所有请求类型，内部针对Result类型的返回做日志处理：
     首先确保你的Result<T>有一个非泛型基类（用于统一判断错误状态），示例基类：

     public abstract class ResultBase
     {
         public bool IsError { get; protected set; }
         public Error FirstError { get; protected set; }
     }
     
     public class Result<T> : ResultBase 
     {
         // 你的Result<T>原有实现
         public T Value { get; protected set; }
     }
     

     然后修改Behavior：

     public class LoggingPipeLineBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
         where TRequest : IRequest<TResponse>
     {
         private readonly ILogger<LoggingPipeLineBehavior<TRequest, TResponse>> _logger;
     
         public LoggingPipeLineBehavior(ILogger<LoggingPipeLineBehavior<TRequest, TResponse>> logger)
         {
             _logger = logger;
         }
     
         public async Task<TResponse> Handle(
             TRequest request,
             RequestHandlerDelegate<TResponse> next,
             CancellationToken cancellationToken)
         {
             _logger.LogInformation("Starting request {@RequestName}, {@DateTimeUtc}",
                 typeof(TRequest).Name,
                 DateTime.UtcNow);
             
             var result = await next();
     
             if (result is ResultBase resultBase && resultBase.IsError)
             {
                 _logger.LogError("Request failure {@RequestName}, {@Error} {@DateTimeUtc}",
                     typeof(TRequest).Name,
                     resultBase.FirstError,
                     DateTime.UtcNow);
             }
             
             _logger.LogInformation("Completed request {@RequestName}, {@DateTimeUtc}",
                 typeof(TRequest).Name,
                 DateTime.UtcNow);
             
             return result;
         }
     }
     

  2. 保持原注册方式不变即可：

     var assembly = typeof(DependencyInjection).Assembly;
     
     services.AddMediatR(configuration =>
     {
         configuration.RegisterServicesFromAssembly(assembly);
         configuration.AddOpenBehavior(typeof(LoggingPipeLineBehavior<,>));
     });
     

• 替代方案（无需非泛型Result基类）：
  若不想修改Result类型结构，可通过反射判断返回类型是否为Result<T>并提取错误信息：

  public async Task<TResponse> Handle(
      TRequest request,
      RequestHandlerDelegate<TResponse> next,
      CancellationToken cancellationToken)
  {
      _logger.LogInformation("Starting request {@RequestName}, {@DateTimeUtc}",
          typeof(TRequest).Name,
          DateTime.UtcNow);
      
      var result = await next();
  
      var resultType = result?.GetType();
      if (resultType != null && resultType.IsGenericType && resultType.GetGenericTypeDefinition() == typeof(Result<>))
      {
          var isErrorProp = resultType.GetProperty(nameof(Result<object>.IsError));
          var firstErrorProp = resultType.GetProperty(nameof(Result<object>.FirstError));
          
          if (isErrorProp != null && (bool)isErrorProp.GetValue(result) == true && firstErrorProp != null)
          {
              var error = firstErrorProp.GetValue(result);
              _logger.LogError("Request failure {@RequestName}, {@Error} {@DateTimeUtc}",
                  typeof(TRequest).Name,
                  error,
                  DateTime.UtcNow);
          }
      }
      
      _logger.LogInformation("Completed request {@RequestName}, {@DateTimeUtc}",
          typeof(TRequest).Name,
          DateTime.UtcNow);
      
      return result;
  }
  

内容的提问来源于stack exchange，提问作者qimbe