為什么 JsonConvert.DeserializeObject 返回錯誤：異常：值不能為空。 （參數“來源”）

Question

我有一個public async Task GetCompaniesAsync()編碼如下：

public async Task GetCompaniesAsync()
{
    _getCompaniesSuccessful = false;

    var result = await _http.GetAsync($"https://somesite.com/api/companies");

    if (result.IsSuccessStatusCode)
    {
        // I split the code after the suggestion of @mjwills (thanks)
        var content = await result.Content.ReadAsStringAsync();
        var obj = JsonConvert.DeserializeObject<StockMarket>(content).Companies;

        // at this point, content.Length has a value (e.g. 24429)
        // but, obj is null (after JsonConvert)

        if (obj != null)
        {
            _companies = obj.ToList();
            _getCompaniesSuccessful = true;
        }
}

在其他一些課程中，我使用以下代碼：

await MyService.GetCompaniesAsync();
                
if (MyService.GetCompaniesSuccessful)
{
    foreach (var record in MyService.Companies)
    {
        await Context.Channel.SendMessageAsync($"{record.TickerSymbol,-10}\t{record.CompanyName,-10}");
    }
}

但是當我收到以下錯誤時：異常：值不能為空。 (參數'source')會不會是數據還沒取完？ 請幫忙...🙏🙏🙏

順便說一下，webapi url返回的是JSON數據，長這樣：

{
    "companies": [
        {
            "ticker": "HEY",
            "name": "Hey Corporation",
            "status": "open"
        },
        {
            "ticker": "PER",
            "name": "Pears Corporation",
            "status": "close"
        },
        {
            "ticker": "BRGR",
            "name": "Burger Inc.",
            "status": "open"
        },
    ]
}           

這些是 StockMarket 和 Company 類：

public class StockMarket
{
    public StockMarket()
    {
    }

    public ICollection<Company> Companies { get; set; }
}

public class Company
{
    public Company()
    {
        TickerSymbol = "";
        CompanyName = "";
        Status = "";
    }

    public string TickerSymbol { get; set; }
    public string CompanyName { get; set; }
    public string Status { get; set; }
}

Answer 1

查看示例，您的 JSON 有一個“name”鍵，但您的 Company 類的屬性稱為“CompanyName”。

嘗試將屬性更改為簡單的名稱，或者用它來裝飾它

[JsonProperty("name", NullValueHandling = NullValueHandling.Ignore)]

此外，同樣適用於 TickerSymbol 屬性。