Question

我正在尝试编写一个检查字符串是否为null的ValidationRule：

public class NotNullValidationRule : ValidationRule
{
  public override ValidationResult Validate(object value, CultureInfo cultureInfo)
  {
    string str = value as string;

    return string.IsNullOrEmpty(str) ? new ValidationResult(false, Application.Current.FindResource("EmptyStringNotAllowed")) : ValidationResult.ValidResult;
  }
}

在我的窗口中，我正在使用它：

<TextBox
    Name="TxtDescription"
    Width="Auto"
    controls:TextBoxHelper.Watermark="{DynamicResource Description}">
    <TextBox.Text>
        <Binding Path="MachineToEdit.Description">
            <Binding.ValidationRules>
                <validation:NotNullValidationRule ValidatesOnTargetUpdated="True"/>
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

如果我启动Designer，我会得到这个NullReferenceException：

   at System.Windows.Data.BindingExpression.RunValidationRule(ValidationRule validationRule, Object value, CultureInfo culture)
   at System.Windows.Data.BindingExpression.ValidateOnTargetUpdated()
   at System.Windows.Data.BindingExpression.TransferValue(Object newValue, Boolean isASubPropertyChange)
   at System.Windows.Data.BindingExpression.Activate(Object item)
   at System.Windows.Data.BindingExpression.AttachToContext(AttachAttempt attempt)
   at System.Windows.Data.BindingExpression.MS.Internal.Data.IDataBindEngineClient.AttachToContext(Boolean lastChance)
   at MS.Internal.Data.DataBindEngine.Task.Run(Boolean lastChance)
   at MS.Internal.Data.DataBindEngine.Run(Object arg)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.DispatcherOperation.InvokeImpl()
   at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at MS.Internal.CulturePreservingExecutionContext.Run(CulturePreservingExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Windows.Threading.DispatcherOperation.Invoke()
   at System.Windows.Threading.Dispatcher.ProcessQueue()
   at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   at System.Windows.Application.RunDispatcher(Object ignore)
   at System.Windows.Application.RunInternal(Window window)
   at System.Windows.Application.Run(Window window)
   at Microsoft.VisualStudio.DesignTools.DesignerContract.Isolation.DesignerProcess.RunApplication()
   at Microsoft.VisualStudio.DesignTools.DesignerContract.Isolation.DesignerProcess.<>c__DisplayClass5_0.<Main>b__0()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

为什么会这样？如果我没有激活ValidatesOnTargetUpdated它正在工作。但我必须在窗口加载时验证。

感谢您的所有答案，祝您度过愉快的一天。

Answer 1

警告未经过测试！

我猜测在设计时验证方法中的值为null。因此，您应检查当前是否处于设计时间，然后返回有效的内容，如ValidationResult.ValidResult。

public class NotNullValidationRule : ValidationRule
{
  public override ValidationResult Validate(object value, CultureInfo cultureInfo)
  {

   if ((bool)(DesignerProperties.IsInDesignModeProperty.GetMetadata(typeof(DependencyObject)).DefaultValue)) 
    {
        return ValidationResult.ValidResult;
    }

    string str = value as string;

    return string.IsNullOrEmpty(str) ? new ValidationResult(false, Application.Current.FindResource("EmptyStringNotAllowed")) : ValidationResult.ValidResult;
  }
}

Answer 2

编辑：比代码隐藏更好的答案

在VS2017中，在XAML编辑器中启用项目代码也为我修复了它。

---以前的答案---

我遇到了同样的问题，不幸的是托马斯V的答案没有用。我能够通过将ValidationRules添加到代码隐藏来解决问题。也许不是最理想的方法，但它确实纠正了这个问题。

你还可以看看在Thomas V的设计师检查中包装代码隐藏逻辑，但它对我没有用。

XAML：

<TextBox x:Name="FirstNameTextBox">
    <TextBox.Text>
        <Binding x:Name="FirstNameTextBoxBinding"
                 Path="TheNewUser.TheNewUser.GivenName"
                 UpdateSourceTrigger="PropertyChanged" 
                 Mode="TwoWay"
                 NotifyOnValidationError="True"
                 Delay="500" />
    </TextBox.Text>
</TextBox>

代码隐藏：

public NewUserWizard_Info_View()
    {
        InitializeComponent();

        Loaded += TriggerValidationOnLoaded;

        FirstNameTextBoxBinding.ValidationRules.Add(new ValidateEmptyOrNull()
        {
            ValidatesOnTargetUpdated = true
        });            
    }

    private void TriggerValidationOnLoaded(object obj, RoutedEventArgs e)
    {
     // This is needed to trigger the validation on first load
        FirstNameTextBox.GetBindingExpression(TextBox.TextProperty).UpdateSource();
    }

在设计时ValidationRule ValidatesOnTargetUpdated NullReferenceException

2 个答案: