使用 OData 连接服务在 Blazor 客户端应用程序中使用 OData

1. 创建了 netstandard2.1 blazor Web 程序集项目。
2. 将 Odata Connected Service (V.0.10.0) 添加到同一项目。
3. 生成 OData 代理类。
4. 从 Razor 页面的 Task OnInitializedAsync() 方法调用 odata 服务

从 Razor 页面调用 OData 服务：

protected override async Task OnInitializedAsync()
{
    await base.OnInitializedAsync();
    var context = new Container(new Uri("http://localhost/odata"));
    var result = await context.Students.ExecuteAsync();
}

浏览器异常：

blazor.webassembly.js:1 crit: Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
      Unhandled exception rendering component: Operation is not supported on this platform.
System.PlatformNotSupportedException: Operation is not supported on this platform.
  at System.Net.WebProxy.CreateDefaultProxy () <0x35560a0 + 0x00008> in <filename unknown>:0 
  at System.Net.Configuration.DefaultProxySectionInternal.GetSystemWebProxy () <0x3555f80 + 0x00000> in <filename unknown>:0 
  at System.Net.Configuration.DefaultProxySectionInternal.GetDefaultProxy_UsingOldMonoCode () <0x3555ed8 + 0x00000> in <filename unknown>:0 
  at System.Net.Configuration.DefaultProxySectionInternal.GetSection () <0x3555cd8 + 0x0002a> in <filename unknown>:0 
  at System.Net.WebRequest.get_InternalDefaultWebProxy () <0x3555a20 + 0x00034> in <filename unknown>:0 
  at System.Net.HttpWebRequest..ctor (System.Uri uri) <0x3555460 + 0x000b6> in <filename unknown>:0 
  at System.Net.HttpRequestCreator.Create (System.Uri uri) <0x3554aa0 + 0x00004> in <filename unknown>:0 
  at System.Net.WebRequest.Create (System.Uri requestUri, System.Boolean useUriBase) <0x354cfd8 + 0x00116> in <filename unknown>:0 
  at System.Net.WebRequest.Create (System.Uri requestUri) <0x354cc00 + 0x00024> in <filename unknown>:0 
  at Microsoft.OData.Client.HttpWebRequestMessage.CreateRequest (System.String method, System.Uri requestUrl, Microsoft.OData.Client.DataServiceClientRequestMessageArgs args) <0x353fd18 + 0x00006> in <filename unknown>:0 
  at Microsoft.OData.Client.HttpWebRequestMessage..ctor (Microsoft.OData.Client.DataServiceClientRequestMessageArgs args) <0x353f750 + 0x0006e> in <filename unknown>:0 
  at Microsoft.OData.Client.RequestInfo.CreateRequestMessage (Microsoft.OData.Client.BuildingRequestEventArgs requestMessageArgs) <0x353a6d0 + 0x0016a> in <filename unknown>:0 
  at Microsoft.OData.Client.ODataRequestMessageWrapper.CreateRequestMessageWrapper (Microsoft.OData.Client.BuildingRequestEventArgs requestMessageArgs, Microsoft.OData.Client.RequestInfo requestInfo) <0x353a0b0 + 0x0000e> in <filename unknown>:0 
  at Microsoft.OData.Client.ODataMessageWritingHelper.CreateRequestMessage (Microsoft.OData.Client.BuildingRequestEventArgs requestMessageArgs) <0x3539d68 + 0x0000a> in <filename unknown>:0 
  at Microsoft.OData.Client.DataServiceRequest.CreateExecuteResult (System.Object source, Microsoft.OData.Client.DataServiceContext context, System.AsyncCallback callback, System.Object state, System.String method) <0x3511280 + 0x00192> in <filename unknown>:0 
  at Microsoft.OData.Client.DataServiceRequest.BeginExecute (System.Object source, Microsoft.OData.Client.DataServiceContext context, System.AsyncCallback callback, System.Object state, System.String method) <0x3507818 + 0x00018> in <filename unknown>:0 
  at Microsoft.OData.Client.DataServiceQuery`1[TElement].BeginExecute (System.AsyncCallback callback, System.Object state) <0x3507610 + 0x0005a> in <filename unknown>:0 
  at System.Threading.Tasks.TaskFactory`1[TResult].FromAsyncImpl (System.Func`3[T1,T2,TResult] beginMethod, System.Func`2[T,TResult] endFunction, System.Action`1[T] endAction, System.Object state, System.Threading.Tasks.TaskCreationOptions creationOptions) <0x3506d10 + 0x00188> in <filename unknown>:0 
  at System.Threading.Tasks.TaskFactory`1[TResult].FromAsync (System.Func`3[T1,T2,TResult] beginMethod, System.Func`2[T,TResult] endMethod, System.Object state) <0x34fd5f8 + 0x00014> in <filename unknown>:0 
  at Microsoft.OData.Client.DataServiceQuery`1[TElement].ExecuteAsync () <0x34fce38 + 0x00028> in <filename unknown>:0 
  at BlazorClientApp2.Pages.Index.OnInitializedAsync () [0x00094] in ..\Pages\Index.razor:12 
  at Microsoft.AspNetCore.Components.ComponentBase.RunInitAndSetParametersAsync () <0x2ec1cd8 + 0x0013a> in <filename unknown>:0

如何克服这个问题？ 使用Odata Connected Service的主要原因是利用它的财产跟踪功能。

微软.OData.Client 7.8.1:

工作样本 https://github.com/tomasfabian/Joker/tree/master/Samples/Blazor.WebAssembly

//Program.cs Main
var httpClient = new HttpClient {BaseAddress = new Uri(builder.HostEnvironment.BaseAddress)};

builder.Services.AddScoped(sp => httpClient);

Set Http请求传输模式 to HttpRequestTransportMode.HttpClient在 ODataServiceContext 中：

  public class ODataServiceContext : DataServiceContext
  {
    public ODataServiceContext(Uri serviceRoot, HttpClient httpClient)
      : base(serviceRoot, ODataProtocolVersion.V4)
    {
      if(httpClient != null)
        HttpRequestTransportMode = HttpRequestTransportMode.HttpClient; 
      
      if (EdmModel == null)
      {
        switch (HttpRequestTransportMode)
        {
          case HttpRequestTransportMode.HttpClient: 
            Format.LoadServiceModel = () => GetServiceModelAsync(httpClient).Result;
            break;
          case HttpRequestTransportMode.HttpWebRequest: 
            Format.LoadServiceModel = () => GetServiceModel(GetMetadataUri());
            break;
        }
        
        Format.UseJson();
      }
      else
      {
        Format.UseJson(EdmModel);
      }
    }
    
    public static IEdmModel EdmModel { get; set; }

    //WASM support
    public static async Task<IEdmModel> GetServiceModelAsync(HttpClient httpClient)
    {
      using (var stream = await httpClient.GetStreamAsync("$metadata"))
      using (var reader = XmlReader.Create(stream))
      {
        return EdmModel = CsdlReader.Parse(reader);
      }
    }

    public IEdmModel GetServiceModel(Uri metadataUri)
    {
      var request = WebRequest.CreateHttp(metadataUri);

      using (var response = request.GetResponse())
      using (var stream = response.GetResponseStream())
      using (var reader = XmlReader.Create(stream))
      {
        return CsdlReader.Parse(reader);
      }
    }

    private DataServiceQuery<Student> student;

    public DataServiceQuery<Student> Students => student = student ?? CreateQuery<Student>("Students");
  }

//Blazor Wasm Component
using System.Net.HttpClient;

[Inject]
public HttpClient HttpClient { get; set; }

[Inject]
public NavigationManager NavigationManager { get; set; }

    protected override async Task OnInitializedAsync()
    {
      await base.OnInitializedAsync();

      Uri serviceRoot = NavigationManager.ToAbsoluteUri("/odata"); //new Uri("http://localhost/odata")

      var dataServiceContext =
        new ODataServiceContext(serviceRoot, HttpClient);

      var result = await dataServiceContext.Students.ExecuteAsync();
    }

Microsoft.OData.Client 7.6.4（

有了这个包Microsoft.OData.Extensions.Client你将能够拦截dataServiceContext.Configurations.RequestPipeline.OnMessageCreating :

//Program.cs Main
var httpClient = new HttpClient {BaseAddress = new Uri(builder.HostEnvironment.BaseAddress)};

builder.Services.AddTransient(sp => httpClient);

builder.Services.AddODataClient().AddHttpClient(httpClient);

//Blazor Wasm Component

using Microsoft.OData.Extensions.Client;

[Inject]
public IODataClientFactory ClientFactory { get; set; }
    
[Inject]
public NavigationManager NavigationManager { get; set; }

    protected override async Task OnInitializedAsync()
    {
      await base.OnInitializedAsync();

      Uri serviceRoot = NavigationManager.ToAbsoluteUri("/odata"); //new Uri("http://localhost/odata")

      var dataServiceContext =
        ClientFactory.CreateClient<ODataServiceContext>(serviceRoot);

      var result = await dataServiceContext.Students.ExecuteAsync();
    }