How to Debug the Design Time Errors in WPF XAML File?

[Reblogged from How to Debug the Design Time Errors in WPF XAML File? thanks to Manish Dubeyy]

Introduction

While developing WPF applications, design view plays an important role not only placing the controls but also we can see the run time view at design time. How does it look like? It becomes frustrating when we see some design time errors and we cannot put a break point in XAML file to diagnose the error, moreover due to this single error sometimes, whole designer fails rendering other controls. So this article enables us to debug the design view of XAML documents in WPF.

Background

It is a pre-requisite that one should be familiar with basic WPF and most importantly one should know how to set design time data context. You can refer to other articles on CodeProject to know how to set a design time data context like this one.

Problem

While designing WPF applications, we frequently see the following types of error in our design view (See pic). Since XAML code does not allow us to insert a break point and debug the stuff, I will share a small trick to trap this error. (References are already there on the internet, but still developers are not so habituated to using it. The reason is that most of them don’t know it.)

ErrorPreview

Steps To Debug the Design Time Errors

  1. First of all, close all opened XAML documents in Visual Studio.
  2. Open the new instance of the same application in Visual Studio (say app2).
  3. Again, close all opened XAML documents. (To be on the safe side, you may close all documents in app2).
  4. Now open Task Manager just to verify whether XDesProc.exe must not be running. (Basically XDesProc.exeis responsible for debugging XAML files, so if any XAML documents are opened, then it launches automatically). So, if you find that XDesProc.exe is running (in Processes Tab), just kill it (right click on process and click on End Process Tree option).
  5. Now switch to app1 (original instance of Visual Studio in which you want break point to be hit). Now, open the file containing the design data view model (MainWindowViewModelDesignData.cs) and place a break point in the first line of its constructor.
  6. Switch to app2 again and open the View (i.e. MainWindow.xaml in which design time error is raising). It will launch the XDesProc.exe in Task Manager.
  7. Switch to app1 again, and go to Debug -> Attach to Processes… context menu item in Menu bar of Visual Studio.
  8. Search the XDesProc.exe and click on attach button.
  9. Switch to app2 and close and reopen the same XAML document (MainWindow.xaml). Once you do it, break point will get hit!2-Solve
  10. Here, you will find that you have not instantiated the PersonList property which is causingNullReference Exception in WPF Designer. After fixing it, you will see that now the designer is showing the data as well.3-DesignView

Points of Interest

For Visual Studio 2010 users, you will find devenv.exe in place of XDesProc.exe. Also, you can tweak with the exception settings if your break point does not becomes active (i.e., Go to Debug -> Exceptions settings -> Common Language RunTime Exceptions).

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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WPF: Attached Property in XAML Markup (The object ‘…’ already has a child and cannot add ‘…’)

Let’s say you need an attached property to set an arbitrary header content to any DependencyObject, to let you use that value and populate the Header property of any HeaderedContentControl, creating the class directly in your WPF test application:

public static class HeaderManager
{
	public static readonly DependencyProperty HeaderProperty = DependencyProperty.RegisterAttached(
		"Header",
		typeof(object),
		typeof(HeaderManager),
		new PropertyMetadata(null));

	public static void SetHeader(DependencyObject element, object value)
	{
		element.SetValue(HeaderProperty, value);
	}

	public static object GetHeader(DependencyObject element)
	{
		return (object)element.GetValue(HeaderProperty);
	}
}

Then you want to attach that property to an UserControl that when injected in a TabItem can self declare it’s own header, and in first attempt you will end up trying to do this:

<UserControl x:Class="RadicalTabRegion.Presentation.FirstView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:s="clr-namespace:RadicalTabRegion.Presentation.Regions.Specialized"
             mc:Ignorable="d"
             d:DesignHeight="300" d:DesignWidth="300">

    <s:HeaderManager.Header>
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="First View"/>
            <CheckBox/>
        </StackPanel>
    </s:HeaderManager.Header>

    <Grid>
        <TextBlock Text="I'm the first view"/>
    </Grid>

</UserControl>

But if you try to compile this, you will get this error:

“The object ‘UserControl’ already has a child and cannot add ‘Grid’. ‘UserControl’ can accept only one child.”

This is because the compiler needs to know that HeaderManager.Header is an attached property, before compile xaml in baml, but because the HeaderManager class is declared in the same assembly of the xaml, it can’t. Actually I think this can be overcome by Microsoft, but never mind there’s a couple of solutions for that.

The first solution is to move the markup declaration after the first element in the UserControl content, and our case after the Grid:

<UserControl x:Class="RadicalTabRegion.Presentation.FirstView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:s="clr-namespace:RadicalTabRegion.Presentation.Regions.Specialized"
             mc:Ignorable="d"
             d:DesignHeight="300" d:DesignWidth="300">

    <Grid>
        <TextBlock Text="I'm the first view"/>
    </Grid>

    <s:HeaderManager.Header>
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="First View"/>
            <CheckBox/>
        </StackPanel>
    </s:HeaderManager.Header>

</UserControl>

The second solution is to move the HeaderManager class into a different class library, so in a different assembly, and this is the best approach because let you use the attached property more naturally, without incurring in that compile error even if you declare the property just before the UserControl.Content, that is more natural for an Header property. For instance if you add the class into a class library called “RadicalTabRegion.Windows.Presentation” you will end-up with this XAML that just works fine:

<UserControl x:Class="RadicalTabRegion.Presentation.FirstView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:s="clr-namespace:RadicalTabRegion.Windows.Presentation.Regions.Specialized;assembly=RadicalTabRegion.Windows.Presentation"
             mc:Ignorable="d"
             d:DesignHeight="300" d:DesignWidth="300">

    <s:HeaderManager.Header>
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="First View"/>
            <CheckBox/>
        </StackPanel>
    </s:HeaderManager.Header>

    <Grid>
        <TextBlock Text="I'm the first view"/>
    </Grid>

</UserControl>

This example is a part of an implementation I’m making for Radical to implement a TabControlRegion, that is able to inject a simple UserControl into a TabItem (generated at runtime) and let the developer deeply customize the TabItem.Header simply declaring the attached property directly into the UserControl. This is under development right now, and it is just one of the possible solutions, if I will like it, I will post the entire code, and pull a request for integrating the new region directly into Radical.