blazor web assembly 服务对象引用未设置为对象的实例

Question

我正在 blazor wasm 中构建一些解决方案，我需要在初始加载任何页面之前从我的 api 获取一些用户信息。

所以我有这样的服务

public class UserService
{
    private readonly HttpClient http;
    private User u;

    public User User
    {
        get { return u; }
        set { u = value; }
    }

    public UserService(HttpClient http)
    {
        this.http = http;
        try
        {
            GetUser();
        }
        catch (Exception ex)
        {
            this.u = null;
            Console.WriteLine("UserService Ctor failed to get user from api except:" + ex.Message);
        }
    }

    private async void GetUser()
    {
        string endpoint = "user/getuser";
        this.u = await this.http.GetFromJsonAsync<User>(endpoint);
    }
}

}

在startup.cs

 builder.Services.AddSingleton<UserService>(new UserService(new HttpClient { BaseAddress = new Uri(builder.HostEnvironment.BaseAddress) }) );

在索引中

  @if (this.u != null && u.User.Login != null ){ @U.User.Login}


[Inject]
private UserService u { get; set; }

当我运行我得到

  Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
  Unhandled exception rendering component: Object reference not set to an instance of an object.
  System.NullReferenceException: Object reference not set to an instance of an object.
  ..... line 9 so this with null username

所以我假设这个渲染是在这个 userService 被实例化之前......我应该如何处理这个以等待任何渲染，直到 UserService ctor 中的这个初始加载完成？

Answer 1

好的谢谢大家我喜欢

        UserService u = new UserService(new HttpClient { BaseAddress = new Uri(builder.HostEnvironment.BaseAddress) });
        await u.GetUser();
        builder.Services.AddSingleton<UserService>(u);

这种方式很好，ctor也很简单

  public UserService(HttpClient http)
    {
        this.http = http;
    }

    public async Task GetUser()
    {
        string endpoint = "user/getuser";
        this.u = await this.http.GetFromJsonAsync<User>(endpoint);
    }

有没有更好的方法？我知道我可以使用 AuthenticationStateProvider 但我不想走这条路

blazor web assembly 服务对象引用未设置为对象的实例

blazor web assembly service object reference not set to an instance of an object

c#

dependency-injection

blazor

blazor-webassembly