.NET Framework 4.7.2 WinForms项目HttpClientFactory无法正常使用求助

我查遍了能找到的HttpClientFactory资料，试了几天都没解决，来求助。我想在.NET 4.7.2 WinForms项目里给REST API客户端实现HttpClientFactory，尝试了类型化客户端和命名客户端两种方式，但每次实例化都会触发空引用错误，以下是我的操作步骤：

类型化客户端实现 #

我创建了如下类：

Imports System.Net.Http
Imports Microsoft.Extensions.Http
Imports Microsoft.Extensions.DependencyInjection

Public Class TypedCustomHTTPClient

    Public Property _Client() As HttpClient
    Public Sub New(ByVal httpClient As HttpClient)
        'httpClient.BaseAddress = New Uri("https://api.google.com/")
        httpClient.DefaultRequestHeaders.Add("Accept", "application/json")
        httpClient.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample")
        _Client = httpClient

    End Sub


End Class

注册客户端 #

在项目的Main子程序中，我注册了类型化客户端和命名客户端（不确定方式是否正确）：

Imports Microsoft.Extensions.DependencyInjection
Imports Microsoft.Extensions.Hosting
Imports Microsoft.Extensions.Http
Imports Polly


Module MainModule


    Public Sub Main()


        Dim seviceColl As New ServiceCollection

        '--------注册命名HttpClient
        seviceColl.AddHttpClient("s2sfHTTPClient", Sub(c)
                                                            'c.BaseAddress = New Uri("https://api.google.com/")
                                                            c.DefaultRequestHeaders.Add("Accept", "application/json")
                                                        End Sub).AddPolicyHandler(PolHolder.httpRetryWithReauthorizationPolicy())
        seviceColl.AddHttpClient("GitHub", Sub(httpClient)
                                                    httpClient.BaseAddress = New Uri("https://api.github.com/")

                                                    ' using Microsoft.Net.Http.Headers;
                                                    ' GitHub API需要两个请求头
                                                    ''httpClient.DefaultRequestHeaders.Add(HeaderNames.Accept, "application/vnd.github.v3+json")
                                                    ''httpClient.DefaultRequestHeaders.Add(HeaderNames.UserAgent, "HttpRequestsSample")
                                                End Sub)


       '注册类型化HttpClient

        seviceColl.AddHttpClient(Of TypedCustomHTTPClient)().SetHandlerLifetime(TimeSpan.FromMinutes(5)).AddPolicyHandler(PolHolder.httpRetryWithReauthorizationPolicy()) '设置生命周期为5分钟
       
        '构建服务提供器
        Dim serviceprovider = servicecoll.BuildServiceProvider
        '
        Application.Run(New Form1()) ''//这里用你的主窗体
    End Sub

End Module

使用时出错 #

当我尝试使用类型化客户端或者通过CreateClient方法使用命名客户端时，在CreateClient行出现空引用错误：

Private Property _httpClientFactory As IHttpClientFactory
Public Function TestQuery2(ByVal soqlQuery As String) As String
        Dim customclient = _httpClientFactory.CreateClient("s2sfHTTPClient")
        'Using customclient._Client '= New HttpClient()
        '
        Dim restRequest As String = InstanceUrl + API_ENDPOINT & "query/?q=" & soqlQuery
            Dim request = New HttpRequestMessage(HttpMethod.[Get], restRequest)
            request.Headers.Add("Authorization", "Bearer " & AuthToken)
            request.Headers.Accept.Add(New MediaTypeWithQualityHeaderValue("application/json"))
            request.Headers.Add("X-PrettyPrint", "1")
        Dim response = customclient.SendAsync(request).Result
        Return response.Content.ReadAsStringAsync().Result
        'End Using
    End Function

请问我哪里操作错了？

解决方案 #

核心问题 #

你定义的_httpClientFactory只是一个私有属性，没有从服务容器中获取实例，所以它的值是Nothing（VB.NET的空引用），调用CreateClient自然会报错。

具体修复步骤 #

1. 全局保存服务容器实例
   在Main模块中，构建完服务提供器后，把它保存到全局可访问的变量中：

   Module MainModule
       Public Shared ServiceProvider As IServiceProvider
   
       Public Sub Main()
           Dim serviceColl As New ServiceCollection
           '... 你的HttpClient注册代码 ...
           ServiceProvider = serviceColl.BuildServiceProvider
           Application.Run(New Form1())
       End Sub
   End Module
   

   注意：修正了你代码中seviceColl和servicecoll的拼写不一致问题，避免潜在报错

2. 在窗体中获取IHttpClientFactory实例
   在Form1的构造函数中，从全局服务容器获取IHttpClientFactory：

   Public Class Form1
       Private _httpClientFactory As IHttpClientFactory
   
       Public Sub New()
           InitializeComponent()
           '从全局服务容器获取实例
           _httpClientFactory = MainModule.ServiceProvider.GetService(Of IHttpClientFactory)()
       End Sub
   
       '你的TestQuery2方法放在这里
       Public Function TestQuery2(ByVal soqlQuery As String) As String
           '现在_httpClientFactory不为空
           Dim customclient = _httpClientFactory.CreateClient("s2sfHTTPClient")
           '... 剩余代码不变 ...
       End Function
   End Class
   

3. 更规范的依赖注入方式
   如果你想严格遵循依赖注入规范，可以让Form1通过构造函数接收IHttpClientFactory，然后从服务容器解析窗体：

   '修改Main模块代码
   Public Sub Main()
       Dim serviceColl As New ServiceCollection
       '... HttpClient注册代码 ...
       '注册Form1，声明它依赖IHttpClientFactory
       serviceColl.AddTransient(Of Form1)()
       
       ServiceProvider = serviceColl.BuildServiceProvider
       '从服务容器解析Form1，而非直接New
       Dim mainForm = ServiceProvider.GetService(Of Form1)()
       Application.Run(mainForm)
   End Module
   
   '修改Form1构造函数
   Public Class Form1
       Private _httpClientFactory As IHttpClientFactory
   
       '构造函数注入IHttpClientFactory
       Public Sub New(httpClientFactory As IHttpClientFactory)
           InitializeComponent()
           _httpClientFactory = httpClientFactory
       End Sub
   
       'TestQuery2方法不变
   End Class
   

额外优化建议 #

• 不要用.Result阻塞异步方法，WinForms中建议使用Async/Await避免UI线程卡顿：

  Public Async Function TestQuery2(ByVal soqlQuery As String) As Task(Of String)
       Dim customclient = _httpClientFactory.CreateClient("s2sfHTTPClient")
       Dim restRequest As String = InstanceUrl + API_ENDPOINT & "query/?q=" & soqlQuery
       Dim request = New HttpRequestMessage(HttpMethod.[Get], restRequest)
       request.Headers.Add("Authorization", "Bearer " & AuthToken)
       request.Headers.Accept.Add(New MediaTypeWithQualityHeaderValue("application/json"))
       request.Headers.Add("X-PrettyPrint", "1")
       Dim response = Await customclient.SendAsync(request)
       Return Await response.Content.ReadAsStringAsync()
   End Function
  

内容的提问来源于stack exchange，提问作者MartinT