NullReferenceException将记录添加到表

Question

当我尝试使用WP7上的LINQ向表中添加记录时，我收到NullReferenceException。我对C＃/ LINQ相对来说比较新，所以我已经复制了一个可以正常工作的现有方法，但是现在我无法使它适用于新记录。代码如下;

private ObservableCollection<DBControl.Categories> _category; 
    public ObservableCollection<DBControl.Categories> Category 
    { 
     get 
     { 
      return _category; 
     } 
     set 
     { 
      if (_category != value) 
      { 
       _category = value; 
       NotifyPropertyChanged("Category"); 
      } 
     } 
    } 


    private void button1_Click(object sender, RoutedEventArgs e) 
    { 

     string TestCategory = "Cars"; 

     // Create a new to-do item based on the text box. 
     DBControl.Categories newCat = new DBControl.Categories { CategoryDesc = TestCategory }; 
     //CategoryDesc 

     // Add a to-do item to the observable collection. 
     **Category.Add(newCat);** 

     // Add a to-do item to the local database. 
     BoughtItemDB.Category.InsertOnSubmit(newCat); 

     BoughtItemDB.SubmitChanges(); 
    } 

的这是给我的错误代码行是Category.Add（newCat）

至于我可以告诉一切正常这可能意味着我犯了一个愚蠢的错误（再次）。

任何帮助，非常感谢。

表格定义如下;

[Table(Name = "Categories")] 
    public class Categories : INotifyPropertyChanged, INotifyPropertyChanging 
    { 
     // Define ID: private field, public property and database column. 
     private int _categoryId; 

     [Column(IsPrimaryKey = true, IsDbGenerated = true, DbType = "INT NOT NULL Identity", CanBeNull = false, AutoSync = AutoSync.OnInsert)] 
     public int CategoryId 
     { 
      get 
      { 
       return _categoryId; 
      } 
      set 
      { 
       if (_categoryId != value) 
       { 
        NotifyPropertyChanging("CategoryId"); 
        _categoryId = value; 
        NotifyPropertyChanged("CategoryId"); 
       } 
      } 
     } 

     // Define item category: private field, public property and database column. 
     private string _categoryDesc; 

     [Column] 
     public string CategoryDesc 
     { 
      get 
      { 
       return _categoryDesc; 
      } 
      set 
      { 
       if (_categoryDesc != value) 
       { 
        NotifyPropertyChanging("CategoryDesc"); 
        _categoryDesc = value; 
        NotifyPropertyChanged("CategoryDesc"); 
       } 
      } 
     } 
     #region INotifyPropertyChanged Members 

     public event PropertyChangedEventHandler PropertyChanged; 

     // Used to notify the page that a data context property changed 
     private void NotifyPropertyChanged(string propertyName) 
     { 
      if (PropertyChanged != null) 
      { 
       PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); 
      } 
     } 

     #endregion 

     #region INotifyPropertyChanging Members 

     public event PropertyChangingEventHandler PropertyChanging; 

     // Used to notify the data context that a data context property is about to change 
     private void NotifyPropertyChanging(string propertyName) 
     { 
      if (PropertyChanging != null) 
      { 
       PropertyChanging(this, new PropertyChangingEventArgs(propertyName)); 
      } 
     } 

     #endregion 
    } 

Answer 1

您需要初始化Category或_category。它没有被设置为任何值，因此您正在尝试将Add（）添加到非初始化对象。