项目总结一：HttpClient DelegatingHandler管道扩展 生命周期问题

在项目中自定义了一个HttpClientLoggingHandler用来记录httpclient发送请求的输入输出日志。运行后调用了几次抛出异常

The 'InnerHandler' property must be null. 'DelegatingHandler' instances provided to 'HttpMessageHandlerBuilder' must not be reused or cached. 
Handler: 'EM.Passport.Badge.Service.Infrastructure.WebApi.HttpClientLoggingHandler'	

检查代码发现自己在注入HttpClientLoggingHandler时，生命周期选择的是AddSingleton。而异常错误很明显说明管道中的DelegatingHandler不能复用或缓存。所以将注入的生命周期修改为AddScoped。

问题解决！

附 HttpClientLoggingHandler 代码

    /// <summary>
    /// HttpClient 日志Handler
    /// </summary>
    public class HttpClientLoggingHandler : DelegatingHandler
    {
        private readonly ILogger<HttpClientLoggingHandler> _logger;

        public HttpClientLoggingHandler(ILogger<HttpClientLoggingHandler> logger)
        {
            _logger = logger;
        }
        protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {
            var response = await base.SendAsync(request, cancellationToken);
            //logging
            try
            {
                _logger.LogInformation($"requestUrl:{request.RequestUri}。request:{await request.Content.ReadAsStringAsync()}。response:{await response.Content.ReadAsStringAsync()}");
            }
            catch (Exception ex)
            {
                _logger.LogInformation($"requestUrl:{request.RequestUri}。request:{await request.Content.ReadAsStringAsync()}。logging error:{ex.Message}");
            }

            return response;
        }