錯誤：System.NullReferenceException：對象引用未設置到對象

Question

工作與Windows的手機應用程序C＃的一個實例：

時添加我的信息我得到這個錯誤「System.NullReferenceException：對象引用未設置爲一個對象的實例。「tại「Data.Add（newInfo）」我使用MVVM模式。

在MainViewModel.cs，我添加功能是

public class MainViewModel : INotifyPropertyChanged 
{ 
    private MemberDataContext MemberDB; 

    public MainViewModel(string MemberDBConnectionString) 
    { 
     MemberDB = new MemberDataContext(MemberDBConnectionString); 
    } 


    private ObservableCollection<Member> _data; 
    public ObservableCollection<Member> Data 
    { 
     get { return _data; } 
     set 
     { 
      _data = value; 
      NotifyPropertyChanged("Data"); 
     } 
    } 

    public void LoadCollectionsFromDatabase() 
    { 
     var infoInDB = from Member ss in MemberDB.Members 
         select ss; 
     Data = new ObservableCollection<Member>(infoInDB); 
    } 


    public void Addinfo(Member newInfo) 
    { 
     MemberDB.Members.InsertOnSubmit(newInfo); 

     MemberDB.SubmitChanges(); 
     Data.Add(newInfo); **<~~~~~~~ Error in this line** 

    } 

    public void Update(Member currentmember, string fullname, string address) 
    { 
     currentmember.Address = address; 
     currentmember.FullName = fullname; 
    } 

    public void Delete(Member currentmember) 
    { 
     MemberDB.Members.DeleteOnSubmit(currentmember); 
     Data.Remove(currentmember); 
    } 

    public void SaveData() 
    { 
     MemberDB.SubmitChanges(); 
    } 

    public void LoadData() 
    { 
     if (Data == null) 
     { 
      Data = new ObservableCollection<Member>(MemberDB.Members); 
      var oderedFullName = from Member b in MemberDB.Members 
           orderby b.FullName 
           select b; 
     } 
    } 

    public MainViewModel() 
    { 
     MemberDB = new MemberDataContext("Data source=isostore:/Member.sdf"); 
     if (!MemberDB.DatabaseExists()) 
     { 
      MemberDB.CreateDatabase(); 
     } 
    } 



    #region INotifyPropertyChanged Members 

    public event PropertyChangedEventHandler PropertyChanged; 

    // Used to notify the app that a property has changed. 
    private void NotifyPropertyChanged(string propertyName) 
    { 
     if (PropertyChanged != null) 
     { 
      PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); 
     } 
    } 
    #endregion 

} 
public class ViewDetails : INotifyPropertyChanged 
{ 
    private string _fullname; 

    public string FullName 
    { 
     get 
     { 
      return _fullname; 
     } 
     set 
     { 
      _fullname = value; 

      if (PropertyChanged != null) 
      { 
       PropertyChanged(this, new PropertyChangedEventArgs("Fullname")); 
      } 
     } 
    } 

    private string _address; 

    public string Address 
    { 
     get 
     { 
      return _address; 
     } 
     set 
     { 
      _address = value; 

      if (PropertyChanged != null) 
      { 
       PropertyChanged(this, new PropertyChangedEventArgs("Address")); 
      } 
     } 
    } 
    public void LoadDetails(Member abc) 
    { 
     FullName = abc.FullName; 
     Address = abc.Address; 
    } 
    public void UpdateDetails(Member abc) 
    { 
     abc.FullName = FullName; 
     abc.Address = Address; 
    } 
    #region INotifyPropertyChanged Members 

    public event PropertyChangedEventHandler PropertyChanged; 

    // Used to notify the app that a property has changed. 
    private void NotifyPropertyChanged(string propertyName) 
    { 
     if (PropertyChanged != null) 
     { 
      PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); 
     } 
    } 
    #endregion 

我Add.xaml：

<StackPanel x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0"> 

     <TextBlock Text="FullName"/> 
     <TextBox x:Name="addfullname"/> 
     <TextBlock Text="Address"/> 
     <TextBox x:Name="addaddress" /> 


    </StackPanel> 

我添加按鈕：

private void Add_Click(object sender, EventArgs e) 
    { 
     if ((addaddress.Text.Length > 0) && (addfullname.Text.Length > 0)) 
     { 
      Member newInfo = new Member 
      { 
       FullName = addfullname.Text, 
       Address = addaddress.Text 
      }; 
      App.MainViewModel.Add(newInfo); 

      if (NavigationService.CanGoBack) 
      { 
       NavigationService.GoBack(); 
      } 
     } 
     //else MessageBox.Show("Data cannot be null!"); 

    } 

在我的App.xaml

public partial class App : Application 
{ 
    private static object _lockObject = new object(); 
    private static MainViewModel mainviewmodel; 
    public static MainViewModel MainViewModel 
    { 
     get 
     { 


      if (mainviewmodel == null) 
      { 
       lock (_lockObject) 
       { 
        mainviewmodel = new MainViewModel(); 
       } 
      } 
      return mainviewmodel; 
     } 


    } 

Answer 1

你不與new關鍵字實例化您的MainViewModel

private static MainViewModel mainviewmodel; 
     public static MainViewModel MainViewModel 
     { 
      get 
      { 
      mainviewmodel = new MainViewModel(); 
       return mainviewmodel; 
      } 
     } 

更新

此行應該是這樣的

private ObservableCollection<Member> _data = new ObservableCollection<Member>();