Question 247309 in Json Question posted in
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How do I get this call to httpClient.GetFromJsonAsync() to work?

I have two similar calls on a Blazor page:

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.

        [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’.

Tags:

3

Answers


  1. Chosen as BEST ANSWER
    0

    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.


  2. 0

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

    The API for the second HttpClient call returns a string.

    You should use GetStringAsync to get the string value.

    String? txt = await httpClient.GetStringAsync("api/BgQuizAnswer/GetHello");
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  3. 0

    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:

            [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.

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