解析JSON時的NullReferenceException

Question

我試圖解析一個JSON文件，當有人點擊一個按鈕，從JSON數據替換按鈕的內容。

目前我遇到了數據保持空白的問題。代碼如下：

private void Button1_Tap(object sender, System.Windows.Input.GestureEventArgs e) 
    { 
     Button1.FontSize = 15; 
     Button1.Content = "Fetching..."; 
     var client = new WebClient(); 
     client.OpenReadCompleted += 
      (s, eargs) => 
      { 
       var serializer = new DataContractJsonSerializer(typeof(RadioRootObject)); 
       if (eargs.Error != null) 
       { 
        if (eargs.Error.Message.Contains("NotFound")) 
        { 
         MessageBox.Show("Could not retrieve playlist", "Error", MessageBoxButton.OK); 
         Button1.Content = "Could not retrieve playlist"; 
        } 
        else 
        { 
         MessageBox.Show("Could not retrieve playlist", "Error", MessageBoxButton.OK); 
         Button1.Content = "Could not retrieve playlist"; 
        } 
       } 
       else 
       { 
        var root = (RadioRootObject)serializer.ReadObject(eargs.Result); 
        var songHistory = root.station3; 
        Button1.Content = songHistory.text; 
       } 
      }; 
     var uri = new Uri("http://www.reignofcomputer.com/tmpsend/nowplaying.json"); 
     client.OpenReadAsync(uri); 
    } 

    public class station1 
    { 
     public string station { get; set; } 
     public string title { get; set; } 
     public string artist { get; set; } 
     public string text { get; set; } 
    } 

    public class station2 
    { 
     public string station { get; set; } 
     public int listeners { get; set; } 
     public string title { get; set; } 
     public string artist { get; set; } 
     public string text { get; set; } 
    } 

    public class station3 
    { 
     public string station { get; set; } 
     public int listeners { get; set; } 
     public string title { get; set; } 
     public string artist { get; set; } 
     public string text { get; set; } 
    } 

    public class RadioRootObject 
    { 
     public station1 station1 { get; set; } 
     public station2 station2 { get; set; } 
     public station3 station3 { get; set; } 
    } 

root和songHistory保持空，從而拋出一個NullReferenceException。

station1和station2在Button2_Tap和Button3_Tap使用，而不是在上面的代碼中，這類似於上面Button1_Tap所示。

我被告知DataContractJsonSerializer無法將json對象的屬性「1」與RadioRootObject類的屬性station1匹配，但我不確定如何使它匹配。

我無法修改JSON本身的數據。有任何想法嗎？

Answer 1

入住此博客張貼如何Parsing JSON in a Windows Phone Application

因此，試試這個

private void Button_Click(object sender, RoutedEventArgs e) 
    { 
     Button1.FontSize = 15; 
     Button1.Content = "Fetching...";var client = new WebClient(); 
     var uri = new Uri("http://www.reignofcomputer.com/tmpsend/nowplaying.json"); 
     client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(client_DownloadStringCompleted); 
     client.DownloadStringAsync(uri); 
    } 

void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e) 
    { 
     var jobj = JObject.Parse(e.Result); 
     var station3 = jobj["3"]; 
     Button1.Content = station3["text"]; 
    } 

Answer 2

由於JSON中的屬性爲「1」，RadioRootObject的成員名稱爲「station1」，因此這些值不匹配。你可以告訴串行什麼JSON的名字是使用DataMemberAttribute，像

public class RadioRootObject 
{ 
    [DataMember(Name="1")] 
    public station1 station1 { get; set; } 
    [DataMember(Name="2")] 
    public station2 station2 { get; set; } 
    [DataMember(Name="3")] 
    public station3 station3 { get; set; } 
} 

老實說，我認爲你必須有[DataContract]和你的類和成員[的DataMember]屬性呢（見the example for DataContractJsonSerializer ），但我可能是錯了:-)