ASP.NET Core 8+MediatR报错CS0535：PerformanceBehaviour未实现接口成员

这个错误是因为MediatR v10+版本调整了IPipelineBehavior<TRequest, TResponse>.Handle方法的参数顺序，你的代码里参数顺序和接口定义不匹配，导致编译器判定未实现接口成员。

问题根源 #

旧版MediatR的Handle方法参数顺序是：
Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)

而MediatR v10及以后的版本，接口定义的参数顺序改为：
Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken cancellationToken)

你的代码沿用了旧的参数顺序，和新版本接口不兼容。

修复步骤 #

调整Handle方法的参数顺序，把RequestHandlerDelegate<TResponse> next移到第二个位置，CancellationToken cancellationToken放在最后。

修正后的代码：

using MediatR;
using Microsoft.Extensions.Logging;
using System.Diagnostics;

namespace SpecMaker.Application.Common.Behaviours
{
    public class PerformanceBehaviour<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
        where TRequest : IRequest<TResponse>
    {
        private readonly Stopwatch _timer;
        private readonly ILogger<TRequest> _logger;

        public PerformanceBehaviour(ILogger<TRequest> logger)
        {
            _timer = new Stopwatch();
            _logger = logger;
        }

        // 调整参数顺序：next放在第二个位置，cancellationToken在最后
        public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken cancellationToken)
        {
            _timer.Start();

            var response = await next();

            _timer.Stop();

            var elapsedMilliseconds = _timer.ElapsedMilliseconds;

            if (elapsedMilliseconds > 600)
            {
                var requestName = typeof(TRequest).Name;
                var userName = string.Empty;

                _logger.LogWarning("SpecMaker Long Running Request: {Name} ({ElapsedMilliseconds} milliseconds) {@Request}",
                    requestName, elapsedMilliseconds, request);
            }
            return response;
        }
    }
}

内容的提问来源于stack exchange，提问作者Gbenga