"'<‘是值的无效开始。“ASP.NET-Core托管Blazor组件

Question

我以前搜索过，并试图执行答案，但没有成功。

进入邮递员的url /blogpage返回一个200 OK。

我被困住了，似乎把自己画在了一个角落里，希望有人指出显而易见的事情。

谢谢大家

更新了新的见解和工作代码.

铬中的误差

      Unhandled exception rendering component: '<' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0.
System.Text.Json.JsonException: '<' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0.
 ---> System.Text.Json.JsonReaderException: '<' is an invalid start of a value. LineNumber: 0 | BytePositionInLine: 0. ```



My Razor-Component code-block

@code {
    private List<BlogItem> BlogPosts = new List<BlogItem>();

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {

        BlogPosts = await Http.GetFromJsonAsync<List<BlogItem>>("GetAllBlogPosts");
    }
}
**EDIT - got a working code going** 

List<BlogItem> BlogPosts;
    protected override async Task OnInitializedAsync()
    {
        var response = await Http.GetAsync("/api/GetAllBlogPosts");
        if (response.IsSuccessStatusCode)
        {
            BlogPosts = await response.Content.ReadFromJsonAsync<List<BlogItem>>();
    enter code here

My BlogController

 [Route("api[controller]")]
    [ApiController]
    public class BlogController : ControllerBase
    {
        private readonly ApplicationDbContext _context;
        public BlogController(ApplicationDbContext context)
        {
            this._context = context;
        }
          public List<BlogItem> BlogPosts { get; set; } = new List<BlogItem>
          { 
        new BlogItem { Title = "My first blog", Summary = "My first blog ever"}
      };
    
        [HttpGet]
        [Route("GetAllBlogPosts")]
      public ActionResult<List<BlogItem>>Get()
        {
            return Ok(_context.BlogItem);
        }
**EDIT - changed the route to this - [Route("/api/GetAllBlogPosts")]**

Answer 1

我相信你的问题源于：

BlogPosts = await Http.GetFromJsonAsync<List<BlogItem>>("GetAllBlogPosts");

这一行假设Json结果来自符合List的API。如果调用API时出错，则结果将不一致。您的问题中报告的错误是试图反序列化这个未确认的结果。

要找到问题的真正根源，请尝试以下模式：

HttpResponseMessage res = await Http.GetAsync("GetAllBlogPosts");
if (res.IsSuccessStatusCode)
{
    // Only deserialize when we did not have an API failure
    BlogPosts = res.Content.ReadFromJsonAsync<List<BlogItem>>();
}
else
{
    // Otherwise treat the response as an error message
    string errMsg = await res.Content.ReadAsStringAsync();
    Console.WriteLine(errMsg);
    throw new Exception(errMsg);
}

这样，只有在响应成功时才会尝试反序列化。如果调用得到一个错误，它将将内容(错误消息)作为字符串读取。该字符串将告诉您问题的真正根源是什么。

Answer 2

固定动作

        [Route("GetAllBlogPosts")]
      public ActionResult<List<BlogItem>> Get()
        {
            return Ok(_context.Set<BlogItem>().ToList());
        }

并试图取代

if (response.IsSuccessStatusCode)
        {
  BlogPosts = await response.Content.ReadFromJsonAsync<List<BlogItem>>();

使用

if (response.IsSuccessStatusCode)
{
var stringData = await response.Content.ReadAsStringAsync();
BlogPosts = JsonConvert.DeserializeObject<List<BlogItem>>(stringData);
}
else{ ...error code}

Answer 3

它是否是一个BOM (字节顺序标记)-尝试打开类似于notepad++，选择UTF-8编码，保存，沙尝试重新启动。