调用GetPrivateHttpClient时出现GetFromJsonAsync不存在的C#错误

我创建了一个C#类库，其中包含GetHttpClient.cs类。使用GetPublicHttpClient()方法时无报错，但调用GetPrivateHttpClient()时出现错误：

“Task 不包含GetFromJsonAsync的定义，最佳扩展方法重载HttpClientJsonExtensions.GetFromJsonAsync<WeatherForecast[]>(HttpClient, string?, CancellationToken)需要System.Net.Http.HttpClient类型的接收者”

已安装Microsoft.Extensions.Http 8.0.0等所需包，附上相关代码，查阅文档后仍无法解决问题。

GetHttpClient.cs代码 #

using BaseLibrary.DTOs;

namespace ClientLibrary.Helpers
{
public class GetHttpClient(IHttpClientFactory httpClientFactory, LocalStorageService localStorageService)
{
private const string HeaderKey = "Authorization";

        public async Task<HttpClient> GetPrivateHttpClient()
        {
            var client = httpClientFactory.CreateClient("SystemApiClient");
            var stringToken = await localStorageService.GetToken();
            if (string.IsNullOrEmpty(stringToken)) return client;

            var deserializeToken = Serializations.DeserializeJsonString<UserSession>(stringToken);
            if (deserializeToken == null) return client;

            client.DefaultRequestHeaders.Authorization =
                new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", deserializeToken.Token);
            return client;
        }

        public HttpClient GetPublicHttpClient()
        {
            var client = httpClientFactory.CreateClient("SystemApiClient");
            client.DefaultRequestHeaders.Remove(HeaderKey);
            return client;
        }

    }

}

UserAccountService.cs代码 #

using System.Net.Http.Json;
using BaseLibrary.DTOs;
using BaseLibrary.Responses;
using ClientLibrary.Helpers;
using ClientLibrary.Services.Contracts;

namespace ClientLibrary.Services.Implementations
{
public class UserAccountService(GetHttpClient getHttpClient) : IUserAccountService
{
public const string AuthUrl = "api/authentication";
public async Task<GeneralResponse> CreateAsync(Register user)
{
var httpClient = getHttpClient.GetPublicHttpClient();
var result = await httpClient.PostAsJsonAsync($"{AuthUrl}/register", user);
if (!result.IsSuccessStatusCode) return new GeneralResponse(false, "Error occurred");

            return await result.Content.ReadFromJsonAsync<GeneralResponse>()!;
        }

        public async Task<LoginResponse> SignInAsync(Login user)
        {
            var httpClient = getHttpClient.GetPublicHttpClient();
            var result = await httpClient.PostAsJsonAsync($"{AuthUrl}/login", user);
            if (!result.IsSuccessStatusCode) return new LoginResponse(false, "Error occurred");

            return await result.Content.ReadFromJsonAsync<LoginResponse>()!;
        }

        public Task<LoginResponse> RefreshTokenAsync(RefreshToken token)
        {
            throw new NotImplementedException();
        }

        public async Task<WeatherForecast[]> GetWeatherForecast()
        {
            var httpClient = getHttpClient.GetPrivateHttpClient();
            var result = await httpClient.GetFromJsonAsync<WeatherForecast[]>("api/weatherforecast");
          
            return result!;
        }
    }

}

问题根源是GetPrivateHttpClient()是异步方法，返回的是Task，但调用时没有加await，导致httpClient变量类型为Task，而非实际需要的HttpClient实例，因此找不到GetFromJsonAsync扩展方法（该方法仅对HttpClient类型生效）。

修改UserAccountService.cs中的GetWeatherForecast方法，给GetPrivateHttpClient()调用添加await：

public async Task<WeatherForecast[]> GetWeatherForecast()
{
    var httpClient = await getHttpClient.GetPrivateHttpClient(); // 新增await关键字
    var result = await httpClient.GetFromJsonAsync<WeatherForecast[]>("api/weatherforecast");
  
    return result!;
}

补充说明：GetPublicHttpClient是同步方法，直接返回HttpClient实例，所以之前调用时无需await也能正常使用；而GetPrivateHttpClient因为需要异步获取token，所以设计为异步方法，必须通过await才能拿到内部的HttpClient对象。

内容的提问来源于stack exchange，提问作者Ramin Emami