Question

在我的Windows Phone 8应用程序中的一个页面上，我正在尝试将特定记录加载到DataContext中。因此，它搜索图像以查找其列（ImageName）等于strVal1的记录。问题是我在存储DataContext的行中不断收到以下错误：

'System.ArgumentNullException'

strVal1不为null，所以我很确定我的DataContext是null，我不明白为什么。

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
    base.OnNavigatedTo(e);

    string strVal1 = this.NavigationContext.QueryString["value1"];
    DataContext = App.ViewModel.Images.Where(b => b.ImageName == strVal1);

}

在另一个页面上，我做了类似的事情，它运作得很好：

protected override void OnNavigatedTo(NavigationEventArgs e)
{

    if (DataContext == null)
    {
        string selectedIndex = "";
        if (NavigationContext.QueryString.TryGetValue("selectedItem", out selectedIndex))
        {
            int index = int.Parse(selectedIndex);
            DataContext = App.ViewModel.Images[index];
        }
    }
}

我想知道这是否是我搜索特定记录的方式。

如果你需要我的MainViewModel.cs，那就是：

public class MainViewModel : INotifyPropertyChanged
{
           // This is the URI of the public, read-only Northwind data service. 
    // To make updates and save changes, replace this URI 
    // with your own Northwind service implementation.
    private static readonly Uri _rootUri = 
        new Uri("http://localhost:49198/ImageDataServ.svc/");

    // Define the typed DataServiceContext.
    private ImageDBEntities _context;

    // Define the binding collection for Customers.
    private DataServiceCollection<Image> _images;

    // Gets and sets the collection of Customer objects from the feed.
    // This collection is used to bind to the UI (View).
    public DataServiceCollection<Image> Images
    {
        get { return _images; }

        private set
        {
            // Set the Titles collection.
            _images = value;

            // Register a handler for the LoadCompleted callback.
            _images.LoadCompleted += OnImagesLoaded;

            // Raise the PropertyChanged events.
            NotifyPropertyChanged("Images");
        }
    }

    // Used to determine whether the data is loaded.
    public bool IsDataLoaded { get; private set; }

    // Loads data when the application is initialized.
    public void LoadData()
    {
        // Instantiate the context and binding collection.
        _context = new ImageDBEntities(_rootUri);
        Images = new DataServiceCollection<Image>(_context);

        // Specify an OData query that returns all customers.
        var query = from ImageData in _context.Images
                    select ImageData;

        // Load the customer data.
        Images.LoadAsync(query);

    }

    // Displays data from the stored data context and binding collection 
    public void LoadData(ImageDBEntities context,
        DataServiceCollection<Image> _images)
    {
        _context = context;
        Images = _images;

        IsDataLoaded = true;
    }

    // Handles the DataServiceCollection<T>.LoadCompleted event.
    private void OnImagesLoaded(object sender, LoadCompletedEventArgs e)
    {
        // Make sure that we load all pages of the Customers feed.
        if (Images.Continuation != null)
        {
            Images.LoadNextPartialSetAsync();
        }
        IsDataLoaded = true;
    }

    // Declare a PropertyChanged for the UI to register 
    // to get updates from the ViewModel.
    public event PropertyChangedEventHandler PropertyChanged;

    // Notifies the binding about a changed property value.
    private void NotifyPropertyChanged(string propertyName)
    {
        var propertyChanged = PropertyChanged;
        if (propertyChanged != null)
        {
            // Raise the PropertyChanged event.
            propertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

Answer 1

应始终检查null并处理错误情况;但很可能比较失败并且没有结果，请确保它不是案例问题。

更改为

if (( App.ViewModel.Images != null ) &&
    ( App.ViewModel.Images.Any()))
{
   var images = App.ViewModel
                   .Images.Where(b => b.ImageName.ToLower() == strVal1.ToLower())
                          .ToList();

   if (images.Any())
      DataContext = images;
}

在数据库中查找特定记录时，DataContext获取Null Reference

1 个答案: