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 Design Time Resources Dictionary

[Reblogged from WPF Design Time Support (Part 2) thanks to jbe2277]

The WPF Designer works best if there is just one WPF project in the solution which is the application (exe) and so contains the App.xaml file. Then it will find all XAML resources that are necessary to render the views flawlessly. But in larger applications the views are often separated into different WPF projects (e.g. per module). In such a scenario the WPF designer is not able to find the resources defined or referenced in App.xaml anymore when this file resides in another project.

Microsoft introduced the Design-time Resources Dictionary file to overcome this issue. Blend is able to detect that some resources could not be resolved and asks us if we want to add a resource dictionary to use for displaying resources at design time.

blend-designtimeresources

Visual Studio uses this file as well but it is not able to create the file. If you want to create this file in Visual Studio then you have to create a Resource Dictionary (WPF). Use DesignTimeResources.xaml as file name and after creation move the file into the Properties folder in your project tree.

Now unload the project (Context menu of the project node in Solution Explorer). Then select Edit YourProject.csproj in the context menu of the unloaded project node. Search for the XML element that contains DesignTimeResources.xaml and replace it with the following XML snippet:

<Page Include="Properties\DesignTimeResources.xaml" Condition="'$(DesignTime)'=='true' OR ('$(SolutionPath)'!='' AND Exists('$(SolutionPath)') AND '$(BuildingInsideVisualStudio)'!='true' AND '$(BuildingInsideExpressionBlend)'!='true')">
  <Generator>MSBuild:Compile</Generator>
  <SubType>Designer</SubType>
  <ContainsDesignTimeResources>true</ContainsDesignTimeResources>
</Page>

Reload the project. The DesignTimeResource dictionary can be filled with MergedDictionaries. In the following example the merged dictionaries reside in another assembly. Thus, the long Source path is used.

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="/Waf.InformationManager.Common.Presentation;Component/Resources/ImageResources.xaml"/>
        <ResourceDictionary Source="/Waf.InformationManager.Common.Presentation;Component/Resources/ConverterResources.xaml"/>

This file will be ignored by the compiler. It does not have any effects on the running application. It is just used to improve the design time experience.