无法评估 Blazor Webassembly API 调用子项 [英] Blazor Webassembly API Call Children could not be evaluated

问题描述

我正在尝试使用我的新 Blazor Webassembly 应用程序调用现有的 rest API，但是当我进行调用时，我总是收到错误无法评估儿童.API 遇到断点，当我单步执行时，它成功返回数据，我也可以查看是否在邮递员中调用它，但是 Blazor 调用失败.我认为这可能是类不匹配的问题，但是 TeamModel 的类结构是相同的，因为它现在是共享资源.我是 Blazor 的新手，我确定我遗漏了一些东西，但可以提供一些指导或帮助.

I'm trying to call an existing rest API I have with my new Blazor Webassembly app, however when I make the call I always get the error Children could not be evaluated. The API hits the breakpoint and when I step through it returns the data successfully which I can also see if I call it in postman, however the Blazor call fails. I thought perhaps it was an issue with mismatching classes, however the class structure for TeamModel is identical as it is now a shared resource. I'm new with Blazor and I'm sure I'm missing something but could do with some direction or help.

private async Task FetchTeamData()
{
        try
        {
            List<TeamModel> teams;
            teams = await Http.GetFromJsonAsync<List<TeamModel>>("http://localhost:50663/api/team");
        }
        catch (Exception ex)
        {
            //Always blowing up with Children could not be evaluated here.
        }
}

[HttpGet]
public IActionResult GetAllTeams()
{
    List<TeamModel> teams = _teamRepository.GetAllTeams();

    return Ok(teams);
}

好吧，我已经将范围缩小到项目读取 API 的方式有问题.我可以很好地调用其他托管 API.我不确定是因为它的 localhost 还是其他一些问题，但是没有一个 localhost API 方法会正确调用我总是从它那里得到相同的响应.

Well I've narrowed it down to being something wrong with the way the project is reading that API. I can call other hosted APIs fine. I'm not sure if its because its localhost or some other issue, but none of the localhost API methods will call properly I always get the same response from it.

推荐答案

我刚刚遇到了同样的问题，这是一个 CORS 问题.只需在您的 API 服务器的 startup.cs 中添加类似这样的内容

I just ran into the same problem, it's a CORS issue. Just add in startup.cs of your API server something like this

app.UseCors(policy => 
    policy.WithOrigins("http://localhost:5000", "https://localhost:5001")
    .AllowAnyMethod()
    .WithHeaders(HeaderNames.ContentType));

来源:https://github.com/dotnet/AspNetCore.Docs/blob/master/aspnetcore/blazor/common/samples/3.x/BlazorWebAssemblySample/Pages/CallWebAPI.razor

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