为什么跨域资源共享（CORS）在服务器端不起作用？

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我创建了2个项目：客户端和服务器。

客户端是一个Razor Web应用程序，在索引页面上包含用于调用api的javascript。

<div class="text-center">
    <h1 class="display-4">Welcome</h1>
    <p>Learn about <a href="https://docs.microsoft.com/aspnet/core">building Web apps with ASP.NET Core</a>.</p>

    <button id="sender">SEND</button>

    <div id="content"></div>
</div>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.min.js" integrity="sha256-9/aliU8dGd2tb6OSsuzixeV4y/faTqgFtohetphbbj0=" crossorigin="anonymous"></script>

<script>
    $(document).ready(() => {
        $('#sender').click(() => {
            $.get("http://localhost:9000/weatherforecast")
                .done(() => {
                    $('#content').text('done');
                })
                .fail(() => {
                    $('#content').text('fail');
                });
        });
    });
</script>

服务器是带有Weatherforecast模板的ASPNET Core（3.1）Web Api。我没有添加CORS中间件，我相信默认情况下不应允许跨源请求。

WeatherForecastController.cs

[ApiController]
[Route("[controller]")]
public class WeatherForecastController : ControllerBase
{
    private static readonly string[] Summaries = new[]
    {
        "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
    };

    private readonly ILogger<WeatherForecastController> _logger;

    public WeatherForecastController(ILogger<WeatherForecastController> logger)
    {
        _logger = logger;
    }

    [HttpGet]
    public IEnumerable<WeatherForecast> Get()
    {
        var rng = new Random();
        return Enumerable.Range(1, 5).Select(index => new WeatherForecast
        {
            Date = DateTime.Now.AddDays(index),
            TemperatureC = rng.Next(-20, 55),
            Summary = Summaries[rng.Next(Summaries.Length)]
        })
        .ToArray();
    }
}

启动文件

    public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddRazorPages();
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }
        else
        {
            app.UseExceptionHandler("/Error");
        }

        app.UseStaticFiles();

        app.UseRouting();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapRazorPages();
        });
    }
}

当我单击发送时，请求失败，这是正确的响应：

enter image description here

但是，如果在api控制器上放置一个断点，它仍然会命中Get（）方法：

enter image description here