我有一个 ASP.NET MVC 应用程序，每次在 UI 中按下一个按钮时它都会调用 ASP.NET Web API REST 服务。
每次按下此按钮，DumpWarehouseDataIntoFile 方法都会被执行。
public class MyClass
{
private static HttpClient client = new HttpClient();
public async Task DumpWarehouseDataIntoFile(Warehouse myData, string path, string filename)
{
try
{
//Hosted web API REST Service base url
string Baseurl = "http://XXX.XXX.XX.X:YYYY/";
//using (var client = new HttpClient()) --> I have declared client as an static variable
//{
//Passing service base url
client.BaseAddress = new Uri(Baseurl);
client.DefaultRequestHeaders.Clear();
//Define request data format
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
// Serialize parameter to pass to the asp web api rest service
string jsonParam = Newtonsoft.JsonConvert.SerializeObject(myData);
//Sending request to find web api REST service resource using HttpClient
var httpContent = new StringContent(jsonParam, Encoding.UTF8, "application/json");
HttpResponseMessage Res = await client.PostAsync("api/Warehouse/DumpIntoFile", httpContent);
//Checking the response is successful or not which is sent using HttpClient
if (Res.IsSuccessStatusCode)
{
// Some other sftuff here
}
//}
}
catch (Exception ex)
{
// Do some stuff here
} // End Try
} // End DumpWarehouseDataIntoFile method
} // End class
仓库类对象:
public class Warehouse
{
public DataTable dt { get; set; }
public string Filepath { get; set; }
}
我在 this post 找到了那种模式:
using (var myClient = new HttpClient())
{
}
不推荐使用，因为它会导致套接字耗尽(System.Net.Sockets.SocketException)。建议使用 HttpClient 作为静态变量并重用它，因为它有助于减少套接字的浪费。所以我使用了静态变量。
这种方法(在我的场景中)的问题是它只在第一次按下按钮时起作用，下次按下按钮并执行 DumpWarehouseDataIntoFile 方法时，抛出以下异常:
An unhandled exception has occurred while executing the request.
System.InvalidOperationException: This instance has already started one or more requests. Properties can only be modified before sending the first request.
如错误所述，基地址等属性只能在发送第一个请求之前修改一次。
我用谷歌搜索并找到了一些建议的解决方案:
因此，单例模式似乎是一个不错的选择，正如提议的那样here .下面是 Alper 提出的单例:
using System;
using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;
//You need to install package Newtonsoft.Json > https://www.nuget.org/packages/Newtonsoft.Json/
using Newtonsoft.Json;
using Newtonsoft.Json.Serialization;
public class MyApiClient : IDisposable
{
private readonly TimeSpan _timeout;
private HttpClient _httpClient;
private HttpClientHandler _httpClientHandler;
private readonly string _baseUrl;
private const string ClientUserAgent = "my-api-client-v1";
private const string MediaTypeJson = "application/json";
public MyApiClient(string baseUrl, TimeSpan? timeout = null)
{
_baseUrl = NormalizeBaseUrl(baseUrl);
_timeout = timeout ?? TimeSpan.FromSeconds(90);
}
public async Task<string> PostAsync(string url, object input)
{
EnsureHttpClientCreated();
using (var requestContent = new StringContent(ConvertToJsonString(input), Encoding.UTF8, MediaTypeJson))
{
using (var response = await _httpClient.PostAsync(url, requestContent))
{
response.EnsureSuccessStatusCode();
return await response.Content.ReadAsStringAsync();
}
}
}
public async Task<TResult> PostAsync<TResult>(string url, object input) where TResult : class, new()
{
var strResponse = await PostAsync(url, input);
return JsonConvert.DeserializeObject<TResult>(strResponse, new JsonSerializerSettings
{
ContractResolver = new CamelCasePropertyNamesContractResolver()
});
}
public async Task<TResult> GetAsync<TResult>(string url) where TResult : class, new()
{
var strResponse = await GetAsync(url);
return JsonConvert.DeserializeObject<TResult>(strResponse, new JsonSerializerSettings
{
ContractResolver = new CamelCasePropertyNamesContractResolver()
});
}
public async Task<string> GetAsync(string url)
{
EnsureHttpClientCreated();
using (var response = await _httpClient.GetAsync(url))
{
response.EnsureSuccessStatusCode();
return await response.Content.ReadAsStringAsync();
}
}
public async Task<string> PutAsync(string url, object input)
{
return await PutAsync(url, new StringContent(JsonConvert.SerializeObject(input), Encoding.UTF8, MediaTypeJson));
}
public async Task<string> PutAsync(string url, HttpContent content)
{
EnsureHttpClientCreated();
using (var response = await _httpClient.PutAsync(url, content))
{
response.EnsureSuccessStatusCode();
return await response.Content.ReadAsStringAsync();
}
}
public async Task<string> DeleteAsync(string url)
{
EnsureHttpClientCreated();
using (var response = await _httpClient.DeleteAsync(url))
{
response.EnsureSuccessStatusCode();
return await response.Content.ReadAsStringAsync();
}
}
public void Dispose()
{
_httpClientHandler?.Dispose();
_httpClient?.Dispose();
}
private void CreateHttpClient()
{
_httpClientHandler = new HttpClientHandler
{
AutomaticDecompression = DecompressionMethods.Deflate | DecompressionMethods.GZip
};
_httpClient = new HttpClient(_httpClientHandler, false)
{
Timeout = _timeout
};
_httpClient.DefaultRequestHeaders.UserAgent.ParseAdd(ClientUserAgent);
if (!string.IsNullOrWhiteSpace(_baseUrl))
{
_httpClient.BaseAddress = new Uri(_baseUrl);
}
_httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue(MediaTypeJson));
}
private void EnsureHttpClientCreated()
{
if (_httpClient == null)
{
CreateHttpClient();
}
}
private static string ConvertToJsonString(object obj)
{
if (obj == null)
{
return string.Empty;
}
return JsonConvert.SerializeObject(obj, new JsonSerializerSettings
{
ContractResolver = new CamelCasePropertyNamesContractResolver()
});
}
private static string NormalizeBaseUrl(string url)
{
return url.EndsWith("/") ? url : url + "/";
}
}
using (var client = new MyApiClient("http://localhost:8080"))
{
var response = client.GetAsync("api/users/findByUsername?username=alper").Result;
var userResponse = client.GetAsync<MyUser>("api/users/findByUsername?username=alper").Result;
}
我在这里看到的问题是，如果您多次调用上面的代码(在我的例子中是每次按下 UI 上的按钮并调用 DumpWarehouseDataIntoFile 方法)，您将创建并实例化MyApiClient 每次都会创建一个新的 HttpClient 实例，我想重用 HttpClient，而不是创建它的许多实例。
按照提议创建一种工厂 here尼科。在他建议的代码下方:
public interface IHttpClientFactory
{
HttpClient CreateClient();
}
public class HttpClientFactory : IHttpClientFactory
{
static string baseAddress = "http://example.com";
public HttpClient CreateClient()
{
var client = new HttpClient();
SetupClientDefaults(client);
return client;
}
protected virtual void SetupClientDefaults(HttpClient client)
{
client.Timeout = TimeSpan.FromSeconds(30); //set your own timeout.
client.BaseAddress = new Uri(baseAddress);
}
}
public HomeController(IHttpClientFactory httpClientFactory)
{
_httpClientFactory = httpClientFactory;
}
readonly IHttpClientFactory _httpClientFactory;
public IActionResult Index()
{
var client = _httpClientFactory.CreateClient();
//....do your code
return View();
}
每次调用 CreateClient 时，您都会再次创建一个新的 HttpClient 实例。您不重用 HttpClient 对象。
按照说明使用 IHttpClientFactory 发出 HTTP 请求 here .
问题是它仅适用于 .NET Core，不适用于标准 ASP.NET Framework，尽管它似乎可以通过安装 this nuget package 获得。 .它似乎自动有效地管理 HttpClient 实例，我想将它应用到我的场景中。我想避免
重新发明轮子。
我从未使用过IHttpClientFactory，我也不知道如何使用它:配置一些功能，如基地址、设置请求 header 、创建HttpClient 实例以及然后在其上调用 PostAsync 作为参数传递 HttpContent。
我认为这是最好的方法，所以有人可以告诉我我需要做的必要步骤，以便在 DumpWarehouseDataIntoFile 方法中做同样的事情，但使用 IHttpClientFactory?我有点迷茫，我不知道如何应用 IHttpClientFactory 来执行与我在 DumpWarehouseDataIntoFile 方法中执行的操作相同的操作。
此处未提出的任何其他解决方案以及一些代码片段将不胜感激。
最佳答案
HttpClient在以下情况下，HttpClient 可以抛出 InvalidOperationException:
BaseAddress setter 时Timeout setter 时MaxResponseContentBufferSize setter 时为了避免这些，您可以在每个请求级别上设置前两个，例如:
CancellationTokenSource timeoutSource = new CancellationTokenSource(2000);
await httpClient.GetAsync("http://www.foo.bar", timeoutSource.Token);
HttpClientFactory您可以将 .NET Framework 中的 IHttpClientFactory 与 following trick 一起使用:
AddHttpClient registers DefaultHttpClientFactory 为 IHttpClientFactoryvar serviceProvider = new ServiceCollection().AddHttpClient().BuildServiceProvider();
container.RegisterInstance(serviceProvider.GetService<IHttpClientFactory>());
container.ContainerScope.RegisterForDisposal(serviceProvider);
此示例使用 SimpleInjector，但相同的概念可以应用于任何其他 DI 框架。
关于c# - HttpClient : This instance has already started one or more requests. 属性只能在发送第一个请求之前修改，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/65269262/