更新3.1-preview3

在3.1-preview3中，我们无法对每条消息使用fetch选项，该选项是全局的

WebAssemblyHttpMessageHandlerOptions.DefaultCredentials = FetchCredentialsOption.Include;

WebAssemblyHttpMessageHandler已被删除。使用的HttpMessageHanlder是WebAssembly.Net.Http.HttpClient.WasmHttpMessageHandler中的WebAssembly.Net.Http，但在您的依赖关系中不要包含WebAssembly.Net.Http，否则应用程序将无法启动。

如果要使用HttpClientFactory，可以这样实现：

public class CustomDelegationHandler : DelegatingHandler
{
    private readonly IUserStore _userStore;
    private readonly HttpMessageHandler _innerHanler;
    private readonly MethodInfo _method;

   public CustomDelegationHandler(IUserStore userStore,HttpMessageHandler innerHanler)
   {
       _userStore = userStore ?? throw new ArgumentNullException(nameof(userStore));
       _innerHanler = innerHanler ?? throw new ArgumentNullException(nameof(innerHanler));
       var type = innerHanler.GetType();
       _method = type.GetMethod("SendAsync",BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.InvokeMethod) ?? throw new InvalidOperationException("Cannot get SendAsync method");
       WebAssemblyHttpMessageHandlerOptions.DefaultCredentials = FetchCredentialsOption.Include;
   }
   protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,CancellationToken cancellationToken)
   {
        request.Headers.Authorization = new AuthenticationHeaderValue(_userStore.AuthenticationScheme,_userStore.AccessToken);            
        return _method.Invoke(_innerHanler,new object[] { request,cancellationToken }) as Task<HttpResponseMessage>;
   }
}

public void ConfigureServices(IServiceCollection services)
{
    services.AddTransient(p =>
    {
        var wasmHttpMessageHandlerType =  Assembly.Load("WebAssembly.Net.Http")
                        .GetType("WebAssembly.Net.Http.HttpClient.WasmHttpMessageHandler");
        var constructor = wasmHttpMessageHandlerType.GetConstructor(Array.Empty<Type>());
        return constructor.Invoke(Array.Empty<object>()) as HttpMessageHandler;
    })
    .AddTransient<CustomDelegationHandler>()
    .AddHttpClient("MyApiHttpClientName")
    .AddHttpMessageHandler<CustonDelegationHandler>();
}

3.0-> 3.1-preview2

在Blazor客户端，您需要告诉Fetch API发送凭据（cookie和授权标头）。

它在 Blazor文档 Cross-origin resource sharing (CORS)

        requestMessage.Properties[WebAssemblyHttpMessageHandler.FetchArgs] = new
        { 
            credentials = FetchCredentialsOption.Include
        };

例如：

@using System.Net.Http
@using System.Net.Http.Headers
@inject HttpClient Http

@code {
    private async Task PostRequest()
    {
        Http.DefaultRequestHeaders.Authorization =
            new AuthenticationHeaderValue("Bearer","{OAUTH TOKEN}");

        var requestMessage = new HttpRequestMessage()
        {
            Method = new HttpMethod("POST"),RequestUri = new Uri("https://localhost:10000/api/TodoItems"),Content = 
                new StringContent(
                    @"{""name"":""A New Todo Item"",""isComplete"":false}")
        };

        requestMessage.Content.Headers.ContentType = 
            new System.Net.Http.Headers.MediaTypeHeaderValue(
                "application/json");

        requestMessage.Content.Headers.TryAddWithoutValidation(
            "x-custom-header","value");

        requestMessage.Properties[WebAssemblyHttpMessageHandler.FetchArgs] = new
        { 
            credentials = FetchCredentialsOption.Include
        };

        var response = await Http.SendAsync(requestMessage);
        var responseStatusCode = response.StatusCode;
        var responseBody = await response.Content.ReadAsStringAsync();
    }
}

您可以使用WebAssemblyHttpMessageHandlerOptions.DefaultCredentials静态属性来全局设置此选项。

或者您可以实现DelegatingHandler并使用HttpClientFactory在DI中进行设置：

    public class CustomWebAssemblyHttpMessageHandler : WebAssemblyHttpMessageHandler
    {
        internal new Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,CancellationToken cancellationToken)
        {
            return base.SendAsync(request,cancellationToken);
        }
    }

    public class CustomDelegationHandler : DelegatingHandler
    {
        private readonly CustomWebAssemblyHttpMessageHandler _innerHandler;

        public CustomDelegationHandler(CustomWebAssemblyHttpMessageHandler innerHandler)
        {
            _innerHandler = innerHandler ?? throw new ArgumentNullException(nameof(innerHandler));
        }
        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,CancellationToken cancellationToken)
        {
            request.Properties[WebAssemblyHttpMessageHandler.FetchArgs] = new
            {
                credentials = "include"
            };
            return _innerHandler.SendAsync(request,cancellationToken);
        }
    }

在Setup.ConfigureServices

services.AddTransient<CustomWebAssemblyHttpMessageHandler>()
    .AddTransient<WebAssemblyHttpMessageHandler>()
    .AddTransient<CustomDelegationHandler>()
    .AddHttpClient(httpClientName)
    .AddHttpMessageHandler<CustomDelegationHandler>();

然后，您可以使用HttpClient为您的API创建一个IHttpClientFactory.CreateClient(httpClientName)

要使用IHttpClientFactory，您需要安装Microsoft.Extensions.Http软件包。

3.0-preview3 => 3.0-preview9

将WebAssemblyHttpMessageHandler替换为BlazorHttpMessageHandler