找不到适合类型“RestDataService”的构造函数

Question

运行 .Net Core 2.0 API 端点时出现以下错误。

找不到适合类型“RestDataService”的构造函数。确保类型是具体的，并且为公共构造函数的所有参数注册了服务。

 public partial class RestDataService : IRestDataService
    {
        private static HttpClient _client;
        private static AppConfiguration _configuration;
        private const short MaxRetryAttempts = 3;
        private const short TimeSpanToWait = 2;

        public RestDataService(AppConfiguration configuration)
        {
            _client = configuration.HttpClient;
            _configuration = configuration;
        }
........

我的创业班是这样的：

  // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {

            var config = new AppConfiguration
            {
                Environment = Configuration["environment"],
            };

            services.AddMvc().AddJsonOptions(o => o.SerializerSettings.NullValueHandling = NullValueHandling.Include);
            services.AddMemoryCache();
            services.AddCors();
            services.AddAutoMapper(AppDomain.CurrentDomain.GetAssemblies());
                services.AddSingleton(Configuration);
                services.AddSingleton(config);
            services.AddLogging();
            services.AddTransient<IRestDataService, RestDataService>();

services.AddHttpClient<IRestDataService, RestDataService>()
                .AddPolicyHandler(request => request.Method == HttpMethod.Get ? retryPolicy : noOp);

任何建议，摆脱这个？构造函数已经是公共的并且所有参数都在启动文件中注册

Answer 1

我收到错误消息“找不到适合类型 '<type>' 的构造函数。” 在意外生成受保护的构造函数而不是 公共构造函数之后。将其标记回公共修复它。

Answer 2

对于AddHttpClient，您需要为 提供HttpClient参数RestDataService。而且，您需要注册AppConfiguration.

1. RestDataService

   public class RestDataService: IRestDataService
   {
       private static HttpClient _client;
       private static AppConfiguration _configuration;
       private const short MaxRetryAttempts = 3;
       private const short TimeSpanToWait = 2;
   
       public RestDataService(AppConfiguration configuration
           , HttpClient client)
       {
           _client = configuration.HttpClient;
           _configuration = configuration;
       }
   }
   

2. Startup.cs

   var config = new AppConfiguration
   {
       Environment = Configuration["environment"],
   };
   
   services.AddSingleton(typeof(AppConfiguration), config);
   services.AddHttpClient<IRestDataService, RestDataService>();
   

Answer 3

我刚刚遇到这个问题，因为我不小心注册HttpClient到了interface.

public void ConfigureServices(IServiceCollection services) {
  // !! Wrong, this is done automatically by AddHttpClient<IMyService, MyService>()
  services.AddTransient<IMyService, MyService>();

  // !! There is no suitable constructor for IMyService, since it is an interface.
  services.AddHttpClient<IMyService>();

  // Instead, let AddHttpClient manage your service's lifetime, 
  // and tell it the implementation.
  // It is registered with a Transient lifetime as described below.
  services.AddHttpClient<IMyService, MyService>();
}

重要上下文，来源： https: //www.stevejgordon.co.uk/ihttpclientfactory-patterns-using-typed-clients-from-singleton-services

在ConfigureServices 方法中定义类型化客户端时，类型化服务将在瞬态范围内注册。这意味着每次需要一个实例时，DI 容器都会创建一个新实例。发生这种情况的原因是 HttpClient 实例被注入到类型化客户端实例中。该 HttpClient 实例的寿命是短暂的，以便 HttpClientFactory 可以确保底层处理程序（和连接）被释放和回收。

Answer 4

您必须定义要用于 IRestDataService 的具体接口类。所以，这样定义。

services.AddTransient<IRestDataService, RestDataService>();

删除 AppConfiguration 之前的 static 关键字。

private readonly AppConfiguration _configuration;