I have successfully created a rest API to connected to a Microsoft SQL database. All get, post, pull & delete functions work successfully in the API. I then created a UI using the Blazor server app. The UI communicates with the API to retrieve & send data. All info in the database have been retrieved successfully but I encounter this error "Error: System.Text.Json.JsonException: 'N' is an invalid start of a value. Path: $ | LineNumber: 0 | " when trying to add new data (task) to the database. When I reload the page after the error has occurred, the data is added successfully even though it fails in the initial process.

Below is the Create click...

@code{
    public class TasksClass
    {
        public int TaskID { get; set; }
        public string TaskTitle { get; set; }
        public string TaskDetails { get; set; }
        public string DateAdded { get; set; }
    }

    private IEnumerable<TasksClass> tasks = Array.Empty<TasksClass>();

    private string modalTitle;
    private int TaskID;
    private string TaskTitle;
    private string TaskDetails;
    private DateTime DateAdded;

// Create click
    private async Task createClick()
    {
        try
        {
            var tskObj = new TasksClass()
            {
                TaskTitle = TaskTitle,
                TaskDetails = TaskDetails,
                DateAdded = DateAdded.ToString("yyyy-MM-dd")
            };

            var request = new HttpRequestMessage(HttpMethod.Post,
                config["API_URL"]   "Tasks/AddTasks");

            request.Content = new StringContent(JsonSerializer.Serialize(tskObj), null, "application/json-patch json");

            var client = ClientFactory.CreateClient();

            var response = await client.SendAsync(request);

            using var responseStream = await response.Content.ReadAsStreamAsync();

            string res = await JsonSerializer.DeserializeAsync<string>(responseStream);

            await JS.InvokeVoidAsync("alert", res);

            await refreshList();
        }
        catch(HttpRequestException)
        {
            throw new BadHttpRequestException("no");
        }
        

        }

I put a breakpoint in the createClick & did a walk through & realized the error occurs here

string res = await JsonSerializer.DeserializeAsync(responseStream);

        await JS.InvokeVoidAsync("alert", res);

I can't seem to figure out exactly what the problem is.

CodePudding user response：

That error is being thrown because the response body starts with an "N", but the JsonDeserializer is expecting it to start as a JSON object (e.g. with a curly brace). My assumption is that Tasks/AddTasks is just returning a string, so it's not in JSON format. Quick solution is to just not deserialize the response, because all you want is the raw string response anyway.

To do so, just replace these lines:

using var responseStream = await response.Content.ReadAsStreamAsync()    
string res = await JsonSerializer.DeserializeAsync<string>(responseStream);

with this:

string res = await response.Content.ReadAsStringAsync();