Blazor UserManager.GetUserAsync 抛出 NullReferenceException

Question

UserManager.GetUserAsync(authState.User)UserManager除非已经被调用，否则会抛出异常。

例如;

这引发NullReferenceException了await UserManager.GetUserAsync(authState.User);

@ page "/"
@inject UserManager<ApplicationUser> UserManager

<AuthorizeView>
  ...
</AuthorizeView>

@code{
    [CascadingParameter]
    private Task<AuthenticationState> authenticationStateTask { get; set; }

    protected async override Task OnInitializedAsync()
    {
      var authState = await authenticationStateTask;
      var currentUser = await UserManager.GetUserAsync(authState.User); // exception here
    }
}

但这工作正常；

@ page "/"
@inject UserManager<ApplicationUser> UserManager

<AuthorizeView>
  ...
</AuthorizeView>

@code{
    [CascadingParameter]
    private Task<AuthenticationState> authenticationStateTask { get; set; }

    protected async override Task OnInitializedAsync()
    {
      var allUsers = UserManager.Users.ToList(); // hack

      var authState = await authenticationStateTask;
      var currentUser = await UserManager.GetUserAsync(authState.User); // this is now working OK
    }
}

为什么我需要UserManager先触发才能使用该GetUsersAsync方法？

Answer 1

不支持它们的原因是它们仅在服务器上可用。你的问题的真正原因是你没有提供UserManager<>注射。

注：IdentityUser是用户的基类。该模板不提供继承此的ApplicationUser。

所以在Startup.cs

 services.AddTransient<UserManager<ApplicationUser>>();

@page "/"
@using Microsoft.AspNetCore.Identity
@inject UserManager<ApplicationUser> UserManager

@currentUser?.Email

@code{
    [CascadingParameter]
    private Task<AuthenticationState> authenticationStateTask { get; set; }

    ApplicationUser currentUser;

    protected async override Task OnInitializedAsync()
    {
      var authState = await authenticationStateTask;
        if (authState.User.Identity.IsAuthenticated)
        {
            currentUser = await UserManager.GetUserAsync(authState.User); // exception was here
        }
    }
}