请教一个WPF动画问题。

[Griot]

各位大侠们，请教一个问题啊！
<Grid.Triggers>
<EventTrigger RoutedEvent="Loaded">
<BeginStoryboard>
<Storyboard x:Name="myStoryboard">
<StringAnimationUsingKeyFrames Storyboard.TargetName="txtSecond" Storyboard.TargetProperty="Text" Duration="0:0:30" RepeatBehavior="Forever">
<StringKeyFrameCollection>
<DiscreteStringKeyFrame Value="29" KeyTime="0:0:0" />
<DiscreteStringKeyFrame Value="28" KeyTime="0:0:1" />
<DiscreteStringKeyFrame Value="27" KeyTime="0:0:2" />
…………………………
其中Storyboard.TargetName="txtSecond"指向的是我页面上的一个TextBox，为什么Storyboard.TargetProperty="Text"的时候编译通过，但是运行的时候报错，错误信息是：


未处理 System.InvalidOperationException
Message=“Text”Storyboard.TargetProperty 路径包含非动画属性“Text”。
Source=PresentationFramework
StackTrace:
在 System.Windows.Media.Animation.Storyboard.VerifyPathIsAnimatable(PropertyPath path)
在 System.Windows.Media.Animation.Storyboard.ClockTreeWalkRecursive(Clock currentClock, DependencyObject containingObject, INameScope nameScope, DependencyObject parentObject, String parentObjectName, PropertyPath parentPropertyPath, HandoffBehavior handoffBehavior, HybridDictionary clockMappings, Int64 layer)
在 System.Windows.Media.Animation.Storyboard.ClockTreeWalkRecursive(Clock currentClock, DependencyObject containingObject, INameScope nameScope, DependencyObject parentObject, String parentObjectName, PropertyPath parentPropertyPath, HandoffBehavior handoffBehavior, HybridDictionary clockMappings, Int64 layer)
在 System.Windows.Media.Animation.Storyboard.BeginCommon(DependencyObject containingObject, INameScope nameScope, HandoffBehavior handoffBehavior, Boolean isControllable, Int64 layer)
在 System.Windows.Media.Animation.BeginStoryboard.Begin(DependencyObject targetObject, INameScope nameScope, Int64 layer)
在 System.Windows.Media.Animation.BeginStoryboard.Invoke(FrameworkElement fe)
在 System.Windows.EventTrigger.EventTriggerSourceListener.Handler(Object sender, RoutedEventArgs e)
在 System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
在 System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
在 System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
在 System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
在 System.Windows.BroadcastEventHelper.BroadcastEvent(DependencyObject root, RoutedEvent routedEvent)
在 System.Windows.BroadcastEventHelper.BroadcastLoadedEvent(Object root)
在 MS.Internal.LoadedOrUnloadedOperation.DoWork()
在 System.Windows.Media.MediaContext.FireLoadedPendingCallbacks()
在 System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
在 System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget)
在 System.Windows.Media.MediaContext.RenderMessageHandler(Object resizedCompositionTarget)
在 System.Windows.Media.MediaContext.Resize(ICompositionTarget resizedCompositionTarget)
在 System.Windows.Interop.HwndTarget.OnResize()
在 System.Windows.Interop.HwndTarget.HandleMessage(WindowMessage msg, IntPtr wparam, IntPtr lparam)
在 System.Windows.Interop.HwndSource.HwndTargetFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
在 MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
在 MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
在 System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
在 MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
在 System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
在 MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
在 MS.Win32.UnsafeNativeMethods.ShowWindow(HandleRef hWnd, Int32 nCmdShow)
在 System.Windows.Window.ShowHelper(Object booleanBox)
在 System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
在 MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
在 System.Windows.Threading.DispatcherOperation.InvokeImpl()
在 System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
在 System.Threading.ExecutionContext.runTryCode(Object userData)
在 System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
在 System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
在 System.Windows.Threading.DispatcherOperation.Invoke()
在 System.Windows.Threading.Dispatcher.ProcessQueue()
在 System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
在 MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
在 MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
在 System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
在 MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
在 System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
在 MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
在 MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
在 System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
在 System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
在 System.Windows.Threading.Dispatcher.Run()
在 System.Windows.Application.RunDispatcher(Object ignore)
在 System.Windows.Application.RunInternal(Window window)
在 System.Windows.Application.Run(Window window)
在 System.Windows.Application.Run()
在 分诊台WPF.App.Main() 位置 E:\工作夹\分诊导医\分诊台WPF\obj\x86\Debug\App.g.cs:行号 0
在 System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
在 System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
在 Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
在 System.Threading.ThreadHelper.ThreadStart_Context(Object state)
在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
在 System.Threading.ThreadHelper.ThreadStart()
InnerException: