I am trying to write a ValidationRule that checks if a string is null:
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;
}
}
In my Window I am using it like this:
<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>
If I start the Designer I get this 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()
Why does this happen? If I dont activate ValidatesOnTargetUpdated it is working. But I have to validate when the window is loading.
Thank you for all answers and have a nice day.