我正在尝试访问我创建的视图模型中的一个属性，TrialViewModel它可以帮助我实现我的试用应用程序。TrialViewModel指向以购买应用程序、获取试用许可模式等方法命名的类的链接TrialExperienceHelper.cs。当我尝试TrialViewModel从我的视图中访问时，我得到一个 NullReferenceException 但我不知道为什么？
试用视图模型
public class TrialViewModel : PropertyChangedNotifier
{
#region fields
private RelayCommand buyCommand;
#endregion fields
#region constructors
public TrialViewModel()
{
// Subscribe to the helper class's static LicenseChanged event so that we can re-query its LicenseMode property when it changes.
TrialExperienceHelper.LicenseChanged += TrialExperienceHelper_LicenseChanged;
}
#endregion constructors
#region properties
public RelayCommand BuyCommand
{
get
{
if (this.buyCommand == null)
{
// The RelayCommand is constructed with two parameters - the action to perform on invocation,
// and the condition under which the command can execute. It's important to call RaiseCanExecuteChanged
// on a command whenever its can-execute condition might have changed. Here, we do that in the TrialExperienceHelper_LicenseChanged
// event handler.
this.buyCommand = new RelayCommand(
param => TrialExperienceHelper.Buy(),
param => TrialExperienceHelper.LicenseMode == TrialExperienceHelper.LicenseModes.Trial);
}
return this.buyCommand;
}
}
public string LicenseModeString
{
get
{
return TrialExperienceHelper.LicenseMode.ToString()/* + ' ' + AppResources.ModeString*/;
}
}
#endregion properties
#region event handlers
// Handle TrialExperienceHelper's LicenseChanged event by raising property changed notifications on the
// properties and commands that
internal void TrialExperienceHelper_LicenseChanged()
{
this.RaisePropertyChanged("LicenseModeString");
this.BuyCommand.RaiseCanExecuteChanged();
}
#endregion event handlers
}
TrialExperienceHelper.cs
public static class TrialExperienceHelper
{
#region enums
/// <summary>
/// The LicenseModes enumeration describes the mode of a license.
/// </summary>
public enum LicenseModes
{
Full,
MissingOrRevoked,
Trial
}
#endregion enums
#region fields
#if DEBUG
// Determines how a debug build behaves on launch. This field is set to LicenseModes.Full after simulating a purchase.
// Calling the Buy method (or navigating away from the app and back) will simulate a purchase.
internal static LicenseModes simulatedLicMode = LicenseModes.Trial;
#endif // DEBUG
private static bool isActiveCache;
private static bool isTrialCache;
#endregion fields
#region constructors
// The static constructor effectively initializes the cache of the state of the license when the app is launched. It also attaches
// a handler so that we can refresh the cache whenever the license has (potentially) changed.
static TrialExperienceHelper()
{
TrialExperienceHelper.RefreshCache();
PhoneApplicationService.Current.Activated += (object sender, ActivatedEventArgs e) => TrialExperienceHelper.
#if DEBUG
// In debug configuration, when the user returns to the application we will simulate a purchase.
OnSimulatedPurchase();
#else // DEBUG
// In release configuration, when the user returns to the application we will refresh the cache.
RefreshCache();
#endif // DEBUG
}
#endregion constructors
#region properties
/// <summary>
/// The LicenseMode property combines the active and trial states of the license into a single
/// enumerated value. In debug configuration, the simulated value is returned. In release configuration,
/// if the license is active then it is either trial or full. If the license is not active then
/// it is either missing or revoked.
/// </summary>
public static LicenseModes LicenseMode
{
get
{
#if DEBUG
return simulatedLicMode;
#else // DEBUG
if (TrialExperienceHelper.isActiveCache)
{
return TrialExperienceHelper.isTrialCache ? LicenseModes.Trial : LicenseModes.Full;
}
else // License is inactive.
{
return LicenseModes.MissingOrRevoked;
}
#endif // DEBUG
}
}
/// <summary>
/// The IsFull property provides a convenient way of checking whether the license is full or not.
/// </summary>
public static bool IsFull
{
get
{
return (TrialExperienceHelper.LicenseMode == LicenseModes.Full);
}
}
#endregion properties
#region methods
/// <summary>
/// The Buy method can be called when the license state is trial. the user is given the opportunity
/// to buy the app after which, in all configurations, the Activated event is raised, which we handle.
/// </summary>
public static void Buy()
{
MarketplaceDetailTask marketplaceDetailTask = new MarketplaceDetailTask();
marketplaceDetailTask.ContentType = MarketplaceContentType.Applications;
marketplaceDetailTask.Show();
}
/// <summary>
/// This method can be called at any time to refresh the values stored in the cache. We re-query the application object
/// for the current state of the license and cache the fresh values. We also raise the LicenseChanged event.
/// </summary>
public static void RefreshCache()
{
TrialExperienceHelper.isActiveCache = CurrentApp.LicenseInformation.IsActive;
TrialExperienceHelper.isTrialCache = CurrentApp.LicenseInformation.IsTrial;
TrialExperienceHelper.RaiseLicenseChanged();
}
private static void RaiseLicenseChanged()
{
if (TrialExperienceHelper.LicenseChanged != null)
{
TrialExperienceHelper.LicenseChanged();
}
}
#if DEBUG
private static void OnSimulatedPurchase()
{
TrialExperienceHelper.simulatedLicMode = LicenseModes.Full;
TrialExperienceHelper.RaiseLicenseChanged();
}
#endif // DEBUG
#endregion methods
#region events
/// <summary>
/// The static LicenseChanged event is raised whenever the value of the LicenseMode property has (potentially) changed.
/// </summary>
public static event LicenseChangedEventHandler LicenseChanged;
#endregion events
}
看法
protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
modeTextBlock.Text = TrialViewModel.LicenseModeString; //NullReferenceException
}
private void popupButton_Click(object sender, RoutedEventArgs e)
{
var button = sender as Button;
string option = button.Name;
switch (option)
{
case "purchaseButton":
// A command takes a parameter and in this case we can pass null.
TrialViewModel.BuyCommand.Execute(null); //NullReferenceException
break;
case "cancelButton":
if (editPagePivotControl != null && editPagePivotControl.SelectedIndex != 0)
{
editPagePivotControl.SelectedIndex = 0;
}
break;
}
}
private TrialViewModel TrialViewModel
{
get
{
return this.DataContext as TrialViewModel;
}
}
有任何想法吗？我真的需要能够访问Buy命令以将用户发送到 Marketplace 购买页面，并获得当前的许可模式。