我怀疑这是一个非常简单的解决方案。我用React创建了一个全新的ASP.NET Core 6项目（在VS 2022中）。默认项目有一个WeatherForecast组件。我复制了这个组件并试图创建一个新的控制器。不幸的是，它只返回了备用Index.html。
代码如下：

// React
async getSitemapData() {
        //axios.get("Sitemap")
        //    .then(resp => {
        //        console.log(resp.data);
        //        this.setState({ sitemap: resp.data, loading: false });
        //    });
        const response = await fetch('sitemap');
        const data = await response.json(); //execution does not reach this line
        console.log(data);
        this.setState({ sitemap: data, loading: false });
}

//program.cs controller mapping
app.UseStaticFiles();
app.UseRouting();

app.MapControllerRoute(
    name: "default",
    pattern: "{controller}/{action=Index}/{id?}");

app.MapFallbackToFile("index.html");

//SitemapController.cs
using Microsoft.AspNetCore.Mvc;

namespace Intranet.Controllers
{
    [ApiController]
    [Route("[controller]")]

    //public class DataController : ControllerBase
//Edit updated controller name
    public class SitemapController : ControllerBase
    {
        [HttpGet]
        public IActionResult Get()
        {
            try
            {
                // Read the data.json file and return its contents as JSON
                var path = Path.Combine(Directory.GetCurrentDirectory(), "/App_Data/sitemap.json");
                var data = System.IO.File.ReadAllText(path);
                return new JsonResult(data);
            }
            catch (Exception ex)
            {
                return StatusCode(StatusCodes.Status500InternalServerError, ex.Message);
            }
        }
    }
}

下面是WeatherForecastController的函数：

//WeatherForecastController.cs
using Microsoft.AspNetCore.Mvc;

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

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

除了返回的数据类型（sitemap读取JSON文件并返回内容而不是数组。JSON是有效的，因为我已经在另一个正在修改的网站中使用了它），我所看到的一切都告诉我它应该工作。谢谢。