ASP.NET 内核 — 上传.json 文件并转换为列表<t></t>

Question

So I managed to export a.json file by serializing my List. Now i'm trying to upload a.json file and desearialize it and add it back to my list. The idea is to be able to click on import, choose a file, then convert it so i can add it to my list. Here is the function in my controller that doesn't work:

** Update ** So now I managed to convert the.json file back to my List but if the list is empty and I want to add the new list to it, then I get this error:

[HttpPost]
    public async Task<IActionResult> Import(IFormFile file)
    {
        if (file == null || file.Length == 0)
            return Content("file not selected");

        var path = Path.Combine(Directory.GetCurrentDirectory(), "Import", file.FileName);

        using (var stream = new FileStream(path, FileMode.Create))
        {
            await file.CopyToAsync(stream);
        }

        String jsonImport = System.IO.File.ReadAllText(path);
        Console.WriteLine(jsonImport);
        List<Booking> bookings;
        _cache.TryGetValue("key", out bookings);
        List<Booking> newList = new List<Booking>();

        newList = JsonConvert.DeserializeObject<List<Booking>>(jsonImport);

        foreach (var item in newList)
        {
            bookings.Add(item);
            _cache.Set<List<Booking>>("key", bookings);
        }
        return RedirectToAction("Index");
    }

So I want to choose a file (user should only be allowed to upload.json format nothing else but idk how to do that yet) and then from that.json I want to add every entry to my bookings List. Here is the Booking Model:

public class Booking
{
    [Required]
    [Display(Name = "Ladestand")]
    [Range(0, 100)]
    public double chargeState { get; set; }

    [Required]
    [Display(Name = "Benötigte Fahrstrecke")]
    [Range(1, 1000)]
    public double distance { get; set; }

    [Required]
    [Display(Name = "Beginn")]

    public DateTime startTime { get; set; }

    [Required]
    [Display(Name = "Ende")]

    public DateTime endTime { get; set; }

    [Required]
    [Display(Name = "Anschlusstyp")]
    [EnumDataType(typeof(ConnectorType))]
    public ConnectorType connectorType { get; set; }
}

And here is the export function:

public ActionResult Export()
    {
        string fileName = "BookingsExport.json";
        List<Booking> bookingsList;
        _cache.TryGetValue("key", out bookingsList);

        string json = JsonConvert.SerializeObject(bookingsList);
        FileInfo file = new FileInfo(fileName);
        using (StreamWriter writer = file.CreateText())
        {
            writer.Write(json);
        }

        byte[] fileBytes = System.IO.File.ReadAllBytes(fileName);

        return File(fileBytes, System.Net.Mime.MediaTypeNames.Application.Json, fileName);
    }

Answer 1

You need to initialize the list, before you are trying to add new items to it.

var bookings = new List<Booking>();

Answer 2

That is because if you do not has value in the cache,it would set the value null to the bookings .You need to change like below:

List<Booking> bookings;
var flag = _cache.TryGetValue("key", out bookings);
if(!flag)
{
    bookings = new List<Booking>();
}