I have two similar calls on a Blazor page:

我在Blazor的页面上有两个类似的电话：

private async void SelectQuiz()
    {
        List<String>? ls  = await httpClient.GetFromJsonAsync<List<String>>("api/BgQuizAnswer/GetBgQuizList");
        String?       txt = await httpClient.GetFromJsonAsync<String>      ("api/BgQuizAnswer/GetHello");

The first call works fine while the second one fails.

第一个调用运行正常，而第二个调用失败。

        [HttpGet]
        [Route("GetBgQuizList")]
        public ActionResult<List<String>> GetBgQuizList()
        {
            return _bgQuizAnswerService.GetBgQuizList();
        }
        [HttpGet]
        [Route("GetHello")]
        public ActionResult<String> GetHello()
        {
            return "Hello";
        }

Here's the start of the error messages I get when I inspect the page:


Unhandled Exception:

System.Text.Json.JsonException: 'H' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0.

---> System.Text.Json.JsonReaderException: 'H' is an invalid start of a value. LineNumber: 0 | BytePositionInLine: 0.


Note that the 'H' is the first letter of the return string. If I change 'Hello' to 'Jello', the error complains about 'J'.

下面是我在检查页面时收到的错误消息的开头：未处理的异常：System.Text.Json.JsonException：‘h’是无效的值开头。路径：$|线号：0|BytePositionInLine：0。->System.Text.Json.JsonReaderException：‘h’是无效的值开头。LineNumber：0|BytePositionInLine：0。请注意，‘H’是返回字符串的第一个字母。如果我将‘Hello’更改为‘Jello’，则错误提示‘J’。

You're trying to retrieve content as JSON, but your API doesn't return it as JSON. Thus you need to change your GetHello method to correct this:

您试图以JSON形式检索内容，但您的API没有将其作为JSON返回。因此，您需要更改GetHello方法以更正此错误：

        [HttpGet]
        [Route("GetHello")]
        public ActionResult<String> GetHello()
        {
            return Json("Hello");
        }

Why is this not necessary for GetBgQuizList? Because ASP.NET Core serialises complex types like lists to JSON as default.

为什么这对于GetBgQuizList不是必需的？因为ASP.NET Core默认将复杂类型(如列表)序列化为JSON。

A valid JSON should be enclosed with {...} (braces) which represents an object or [...] (square brackets) which represents an array.

有效的JSON应用表示对象或[...]的{...}(大括号)括起来(方括号)，表示数组。

The API for the second HttpClient call returns a string.

第二个HttpClient调用的API返回一个字符串。

You should use GetStringAsync to get the string value.

您应该使用GetStringAsync来获取字符串值。

String? txt = await httpClient.GetStringAsync("api/BgQuizAnswer/GetHello");

Replacing

return "Hello";

with

return JsonSerializer.Serialize("Hello");

works.


Replacing

String? txt = await httpClient.GetFromJsonAsync ("api/BgQuizAnswer/GetHello");

with

String? txt = await httpClient.GetStringAsync("api/BgQuizAnswer/GetHello");

also works.

将返回“Hello”；替换为Return JsonSerializer.Serialize(“Hello”)；起作用。替换字符串？Txt=等待httpClient.GetFromJsonAsync(“API/BgQuizAnswer/GetHello”)；带字符串？Txt=等待httpClient.GetStringAsync(“api/BgQuizAnswer/GetHello”)；也可以工作。