Why is the view builder called twice?

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I have a wpf desktop application where several of my views are getting instantiated twice when there should only be one instance. The reason I know this is because I am using Visual Studio 2015 Diagnostic Tools to look at the heap. After I saw that there were two instances of my views, I placed a breakpoint in the constructor of one of the views to see if, in fact, the constructor was getting called twice. It is. Each time the breakpoint is hit, I look at the call stack to see which of my existing objects might be constructing the view twice, but the stack only shows external code in each of the frames in the stack. This is the stack the first time in the constructor:

MyApp.exe!MyApp.Views.LineItemView.LineItemView() Line 26   C#
[Native to Managed Transition]
[Managed to Native Transition]
mscorlib.dll!System.RuntimeType.CreateInstanceSlow(bool publicOnly, bool skipCheckThis, bool fillCache, ref System.Threading.StackCrawlMark stackMark)  Unknown
mscorlib.dll!System.Activator.CreateInstance(System.Type type, bool nonPublic)  Unknown
mscorlib.dll!System.RuntimeType.CreateInstanceImpl(System.Reflection.BindingFlags bindingAttr, System.Reflection.Binder binder, object[] args, System.Globalization.CultureInfo culture, object[] activationAttributes, ref System.Threading.StackCrawlMark stackMark)  Unknown
mscorlib.dll!System.Activator.CreateInstance(System.Type type, System.Reflection.BindingFlags bindingAttr, System.Reflection.Binder binder, object[] args, System.Globalization.CultureInfo culture, object[] activationAttributes) Unknown
mscorlib.dll!System.Activator.CreateInstance(System.Type type, object[] args)   Unknown
System.Xaml.dll!System.Xaml.Schema.SafeReflectionInvoker.CreateInstanceCritical(System.Type type, object[] arguments)   Unknown
System.Xaml.dll!System.Xaml.Schema.SafeReflectionInvoker.CreateInstance(System.Type type, object[] arguments)   Unknown
System.Xaml.dll!System.Xaml.Schema.XamlTypeInvoker.CreateInstance(object[] arguments)   Unknown
System.Xaml.dll!MS.Internal.Xaml.Runtime.ClrObjectRuntime.CreateInstanceWithCtor(System.Xaml.XamlType xamlType, object[] args)  Unknown
System.Xaml.dll!MS.Internal.Xaml.Runtime.ClrObjectRuntime.CreateInstance(System.Xaml.XamlType xamlType, object[] args)  Unknown
System.Xaml.dll!MS.Internal.Xaml.Runtime.PartialTrustTolerantRuntime.CreateInstance(System.Xaml.XamlType xamlType, object[] args)   Unknown
System.Xaml.dll!System.Xaml.XamlObjectWriter.Logic_CreateAndAssignToParentStart(MS.Internal.Xaml.Context.ObjectWriterContext ctx)   Unknown
System.Xaml.dll!System.Xaml.XamlObjectWriter.WriteEndObject()   Unknown
System.Xaml.dll!System.Xaml.XamlWriter.WriteNode(System.Xaml.XamlReader reader) Unknown
PresentationFramework.dll!System.Windows.FrameworkTemplate.LoadTemplateXaml(System.Xaml.XamlReader templateReader, System.Xaml.XamlObjectWriter currentWriter) Line 1024    C#
PresentationFramework.dll!System.Windows.FrameworkTemplate.LoadTemplateXaml(System.Xaml.XamlObjectWriter objectWriter) Line 982 C#
PresentationFramework.dll!System.Windows.FrameworkTemplate.LoadOptimizedTemplateContent(System.Windows.DependencyObject container, System.Windows.Markup.IComponentConnector componentConnector, System.Windows.Markup.IStyleConnector styleConnector, System.Collections.Generic.List<System.Windows.DependencyObject> affectedChildren, System.Windows.UncommonField<System.Collections.Hashtable> templatedNonFeChildrenField) Line 955  C#
PresentationFramework.dll!System.Windows.FrameworkTemplate.LoadContent(System.Windows.DependencyObject container, System.Collections.Generic.List<System.Windows.DependencyObject> affectedChildren) Line 582   C#
PresentationFramework.dll!System.Windows.StyleHelper.ApplyTemplateContent(System.Windows.UncommonField<System.Collections.Specialized.HybridDictionary[]> dataField, System.Windows.DependencyObject container, System.Windows.FrameworkElementFactory templateRoot, int lastChildIndex, System.Collections.Specialized.HybridDictionary childIndexFromChildID, System.Windows.FrameworkTemplate frameworkTemplate) Line 1869   C#
PresentationFramework.dll!System.Windows.FrameworkTemplate.ApplyTemplateContent(System.Windows.UncommonField<System.Collections.Specialized.HybridDictionary[]> templateDataField, System.Windows.FrameworkElement container) Line 502  C#
PresentationFramework.dll!System.Windows.FrameworkElement.ApplyTemplate() Line 365  C#
PresentationFramework.dll!System.Windows.FrameworkElement.MeasureCore(System.Windows.Size availableSize) Line 4280  C#
PresentationCore.dll!System.Windows.UIElement.Measure(System.Windows.Size availableSize) Line 664   C#
PresentationCore.dll!System.Windows.ContextLayoutManager.UpdateLayout() Line 317    C#
PresentationCore.dll!System.Windows.ContextLayoutManager.UpdateLayoutCallback(object arg) Line 589  C#
PresentationCore.dll!System.Windows.Media.MediaContext.InvokeOnRenderCallback.DoWork() Line 1562    C#
PresentationCore.dll!System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks() Line 1949  C#
PresentationCore.dll!System.Windows.Media.MediaContext.RenderMessageHandlerCore(object resizedCompositionTarget) Line 1826  C#
PresentationCore.dll!System.Windows.Media.MediaContext.RenderMessageHandler(object resizedCompositionTarget) Line 1721  C#
WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate callback, object args, int numArgs)  Unknown
WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.TryCatchWhen(object source, System.Delegate callback, object args, int numArgs, System.Delegate catchHandler) Unknown
WindowsBase.dll!System.Windows.Threading.DispatcherOperation.InvokeImpl()   Unknown
WindowsBase.dll!System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(object state)  Unknown
mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx)   Unknown
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx)   Unknown
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) Unknown
WindowsBase.dll!System.Windows.Threading.DispatcherOperation.Invoke()   Unknown
WindowsBase.dll!System.Windows.Threading.Dispatcher.ProcessQueue()  Unknown
WindowsBase.dll!System.Windows.Threading.Dispatcher.WndProcHook(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam, ref bool handled)  Unknown
WindowsBase.dll!MS.Win32.HwndWrapper.WndProc(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam, ref bool handled) Unknown
WindowsBase.dll!MS.Win32.HwndSubclass.DispatcherCallbackOperation(object o) Unknown
WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate callback, object args, int numArgs)  Unknown
WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.TryCatchWhen(object source, System.Delegate callback, object args, int numArgs, System.Delegate catchHandler) Unknown
WindowsBase.dll!System.Windows.Threading.Dispatcher.LegacyInvokeImpl(System.Windows.Threading.DispatcherPriority priority, System.TimeSpan timeout, System.Delegate method, object args, int numArgs)   Unknown
WindowsBase.dll!MS.Win32.HwndSubclass.SubclassWndProc(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam)  Unknown
[Native to Managed Transition]
[Managed to Native Transition]
WindowsBase.dll!System.Windows.Threading.Dispatcher.PushFrameImpl(System.Windows.Threading.DispatcherFrame frame)   Unknown
WindowsBase.dll!System.Windows.Threading.Dispatcher.PushFrame(System.Windows.Threading.DispatcherFrame frame)   Unknown
PresentationFramework.dll!System.Windows.Application.RunDispatcher(object ignore) Line 2756 C#
PresentationFramework.dll!System.Windows.Application.RunInternal(System.Windows.Window window) Line 1845    C#
PresentationFramework.dll!System.Windows.Application.Run(System.Windows.Window window) Line 261 C#
MyApp.exe!MyApp.Program.Main() Line 75  C#

And the second time in the constructor:

MyApp.exe!MyApp.Views.LineItemView.LineItemView() Line 26   C#
    mscorlib.dll!System.Activator.CreateInstance(System.Type type, bool nonPublic)  Unknown
    mscorlib.dll!System.RuntimeType.CreateInstanceImpl(System.Reflection.BindingFlags bindingAttr, System.Reflection.Binder binder, object[] args, System.Globalization.CultureInfo culture, object[] activationAttributes, ref System.Threading.StackCrawlMark stackMark)  Unknown
    mscorlib.dll!System.Activator.CreateInstance(System.Type type, System.Reflection.BindingFlags bindingAttr, System.Reflection.Binder binder, object[] args, System.Globalization.CultureInfo culture, object[] activationAttributes) Unknown
    mscorlib.dll!System.Activator.CreateInstance(System.Type type, object[] args)   Unknown
    System.Xaml.dll!System.Xaml.Schema.SafeReflectionInvoker.CreateInstanceCritical(System.Type type, object[] arguments)   Unknown
    System.Xaml.dll!System.Xaml.Schema.SafeReflectionInvoker.CreateInstance(System.Type type, object[] arguments)   Unknown
    System.Xaml.dll!System.Xaml.Schema.XamlTypeInvoker.CreateInstance(object[] arguments)   Unknown
    System.Xaml.dll!MS.Internal.Xaml.Runtime.ClrObjectRuntime.CreateInstanceWithCtor(System.Xaml.XamlType xamlType, object[] args)  Unknown
    System.Xaml.dll!MS.Internal.Xaml.Runtime.ClrObjectRuntime.CreateInstance(System.Xaml.XamlType xamlType, object[] args)  Unknown
    System.Xaml.dll!MS.Internal.Xaml.Runtime.PartialTrustTolerantRuntime.CreateInstance(System.Xaml.XamlType xamlType, object[] args)   Unknown
    System.Xaml.dll!System.Xaml.XamlObjectWriter.Logic_CreateAndAssignToParentStart(MS.Internal.Xaml.Context.ObjectWriterContext ctx)   Unknown
    System.Xaml.dll!System.Xaml.XamlObjectWriter.WriteEndObject()   Unknown
    System.Xaml.dll!System.Xaml.XamlWriter.WriteNode(System.Xaml.XamlReader reader) Unknown
    PresentationFramework.dll!System.Windows.FrameworkTemplate.LoadTemplateXaml(System.Xaml.XamlReader templateReader, System.Xaml.XamlObjectWriter currentWriter) Line 1024    C#
    PresentationFramework.dll!System.Windows.FrameworkTemplate.LoadTemplateXaml(System.Xaml.XamlObjectWriter objectWriter) Line 982 C#
    PresentationFramework.dll!System.Windows.FrameworkTemplate.LoadOptimizedTemplateContent(System.Windows.DependencyObject container, System.Windows.Markup.IComponentConnector componentConnector, System.Windows.Markup.IStyleConnector styleConnector, System.Collections.Generic.List<System.Windows.DependencyObject> affectedChildren, System.Windows.UncommonField<System.Collections.Hashtable> templatedNonFeChildrenField) Line 955  C#
    PresentationFramework.dll!System.Windows.FrameworkTemplate.LoadContent(System.Windows.DependencyObject container, System.Collections.Generic.List<System.Windows.DependencyObject> affectedChildren) Line 582   C#
    PresentationFramework.dll!System.Windows.StyleHelper.ApplyTemplateContent(System.Windows.UncommonField<System.Collections.Specialized.HybridDictionary[]> dataField, System.Windows.DependencyObject container, System.Windows.FrameworkElementFactory templateRoot, int lastChildIndex, System.Collections.Specialized.HybridDictionary childIndexFromChildID, System.Windows.FrameworkTemplate frameworkTemplate) Line 1869   C#
    PresentationFramework.dll!System.Windows.FrameworkTemplate.ApplyTemplateContent(System.Windows.UncommonField<System.Collections.Specialized.HybridDictionary[]> templateDataField, System.Windows.FrameworkElement container) Line 502  C#
    PresentationFramework.dll!System.Windows.FrameworkElement.ApplyTemplate() Line 365  C#
    PresentationFramework.dll!System.Windows.FrameworkElement.MeasureCore(System.Windows.Size availableSize) Line 4280  C#
    PresentationCore.dll!System.Windows.UIElement.Measure(System.Windows.Size availableSize) Line 664   C#
    PresentationCore.dll!System.Windows.ContextLayoutManager.UpdateLayout() Line 317    C#
    PresentationCore.dll!System.Windows.ContextLayoutManager.UpdateLayoutCallback(object arg) Line 589  C#
    PresentationCore.dll!System.Windows.Media.MediaContext.InvokeOnRenderCallback.DoWork() Line 1562    C#
    PresentationCore.dll!System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks() Line 1949  C#
    PresentationCore.dll!System.Windows.Media.MediaContext.RenderMessageHandlerCore(object resizedCompositionTarget) Line 1826  C#
    PresentationCore.dll!System.Windows.Media.MediaContext.RenderMessageHandler(object resizedCompositionTarget) Line 1721  C#
    WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate callback, object args, int numArgs)  Unknown
    WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.TryCatchWhen(object source, System.Delegate callback, object args, int numArgs, System.Delegate catchHandler) Unknown
    WindowsBase.dll!System.Windows.Threading.DispatcherOperation.InvokeImpl()   Unknown
    WindowsBase.dll!System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(object state)  Unknown
    mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx)   Unknown
    mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx)   Unknown
    mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) Unknown
    WindowsBase.dll!System.Windows.Threading.DispatcherOperation.Invoke()   Unknown
    WindowsBase.dll!System.Windows.Threading.Dispatcher.ProcessQueue()  Unknown
    WindowsBase.dll!System.Windows.Threading.Dispatcher.WndProcHook(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam, ref bool handled)  Unknown
    WindowsBase.dll!MS.Win32.HwndWrapper.WndProc(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam, ref bool handled) Unknown
    WindowsBase.dll!MS.Win32.HwndSubclass.DispatcherCallbackOperation(object o) Unknown
    WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate callback, object args, int numArgs)  Unknown
    WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.TryCatchWhen(object source, System.Delegate callback, object args, int numArgs, System.Delegate catchHandler) Unknown
    WindowsBase.dll!System.Windows.Threading.Dispatcher.LegacyInvokeImpl(System.Windows.Threading.DispatcherPriority priority, System.TimeSpan timeout, System.Delegate method, object args, int numArgs)   Unknown
    WindowsBase.dll!MS.Win32.HwndSubclass.SubclassWndProc(System.IntPtr hwnd, int msg, System.IntPtr wParam, System.IntPtr lParam)  Unknown
    [Native to Managed Transition]
    [Managed to Native Transition]
    WindowsBase.dll!System.Windows.Threading.Dispatcher.PushFrameImpl(System.Windows.Threading.DispatcherFrame frame)   Unknown
    WindowsBase.dll!System.Windows.Threading.Dispatcher.PushFrame(System.Windows.Threading.DispatcherFrame frame)   Unknown
    PresentationFramework.dll!System.Windows.Application.RunDispatcher(object ignore) Line 2756 C#
    PresentationFramework.dll!System.Windows.Application.RunInternal(System.Windows.Window window) Line 1845    C#
    PresentationFramework.dll!System.Windows.Application.Run(System.Windows.Window window) Line 261 C#
    MyApp.exe!MyApp.Program.Main() Line 75  C#

Any ideas on how to debug this?

From the name of your class LineItemView, it sounds like you have more items that you expected. You will need to provide more context to this, as there's not enough information available in the above. To debug it, check the DataContext of your items (not available in the constructor though) to see what is causing the construction. You could also check the Parent property. On both counts, you'll need to wire up the OnVisualParentChanged or OnInitialized methods.

In times like this it can also be useful to create a new, separate project with only the relevant code and see if the problem persists.

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