可以查找Json.net中不存在的Key

Question

我有几个不同的格式,但我无法弄清楚如何处理它们因为当我尝试通过键json.net崩溃找到.我希望它只会返回null.

foreach (var item in jsonObj)
{
    var msg = item.Value["Msg"];
    if (msg != null)
    {
       txtErrors.Text += msg + Environment.NewLine;
    }
}

//格式化一个

{[UserNotFound, {
  "SeverityType": 3,
  "ValidationType": 2,
  "Msg": "Email Not Found"
}]}

我的代码有效.

//格式2(因为我没有在服务器端捕获异常)

{
  "Message": "An error has occurred.",
  "ExceptionMessage": "Object reference not set to an instance of an object.",
  "ExceptionType": "System.NullReferenceException",
  "StackTrace": "  "
}

我当然可以修复此问题并捕获异常.但是,如果我再次忘记,我也不会让它在客户端崩溃.所以我很乐意打印出"消息",但我不知道如何做到这一点,所以它不会崩溃var msg = item.Value["Msg"];

我尝试做var时得到的错误 msg = item.Value["Msg"];

System.InvalidOperationException was unhandled
  Message=Cannot access child value on Newtonsoft.Json.Linq.JValue.
  StackTrace:
       at Newtonsoft.Json.Linq.JToken.get_Item(Object key)
       at Fitness.WindowsPhone7.UI.MainPage.<btnSignIn_Click>b__0(IRestResponse response)
       at RestSharp.RestClientExtensions.<>c__DisplayClass1.<ExecuteAsync>b__0(IRestResponse response, RestRequestAsyncHandle handle)
       at RestSharp.RestClient.ProcessResponse(IRestRequest request, HttpResponse httpResponse, RestRequestAsyncHandle asyncHandle, Action`2 callback)
       at RestSharp.RestClient.<>c__DisplayClass3.<ExecuteAsync>b__0(HttpResponse r)
       at RestSharp.RestClient.<>c__DisplayClass5.<>c__DisplayClass7.<ExecuteAsync>b__2(Object s)
       at System.Reflection.RuntimeMethodInfo.InternalInvoke(RuntimeMethodInfo rtmi, Object obj, BindingFlags invokeAttr, Binder binder, Object parameters, CultureInfo culture, Boolean isBinderDefault, Assembly caller, Boolean verifyAccess, StackCrawlMark& stackMark)
       at System.Reflection.RuntimeMethodInfo.InternalInvoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, StackCrawlMark& stackMark)
       at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
       at System.Delegate.DynamicInvokeOne(Object[] args)
       at System.MulticastDelegate.DynamicInvokeImpl(Object[] args)
       at System.Delegate.DynamicInvoke(Object[] args)
       at System.Windows.Threading.DispatcherOperation.Invoke()
       at System.Windows.Threading.Dispatcher.Dispatch(DispatcherPriority priority)
       at System.Windows.Threading.Dispatcher.OnInvoke(Object context)
       at System.Windows.Hosting.CallbackCookie.Invoke(Object[] args)
       at System.Windows.Hosting.DelegateWrapper.InternalInvoke(Object[] args)
       at System.Windows.RuntimeHost.ManagedHost.InvokeDelegate(IntPtr pHandle, Int32 nParamCount, ScriptParam[] pParams, ScriptParam& pResult)

Answer 1

假设您使用Newtonsoft.Json:

您可以使用JObject来测试是否存在属性:

JObject jObj; //initialized somewhere, perhaps in your foreach
var msgProperty = jObj.Property("msg");

//check if property exists
if (msgProperty != null) {
    var mag = msgProperty.Value;
} else {
    //there is no "msg" property, compensate somehow.
}

Answer 2

您可以使用TryGetValue，这是一种标准方法，可以精确地执行您需要的操作。我之所以说是标准的，是因为Try方法可以在.NET框架周围找到，并且通常总是具有相同的方法签名。

使用它可以获得这样的价值。

JObject json = new JObject();
JToken value;
if (json.TryGetValue("myProperty", out value))
{
    string finalValue = (string)value;
}

TryGetValue返回一个布尔值，指示是否找到该值，如果找到该值，则将作为第二个参数传递的值设置为属性值。否则设置为null。