Declarative Dependency Property Definition with T4 + DTE

This blog post describes a technique for specifying WPF / Silverlight Dependency Properties declaritively via attributes as illustrated by the following example:

[DependencyPropertyDecl("Maximum", typeof(double), 0.0)]
[DependencyPropertyDecl("Minimum", typeof(double), 0.0)]
public partial class RangeControl : UserControl
{
    ...
}

At design-time, the declarations are read via a T4 template and the required code is generated. For more information, read on …

A few months ago, I wrote an article on CodeProject about a technique for generating dependency properties via T4 templates. In brief; with this technique, you provide an XML description of your classes and their dependency properties, run the T4 template, and partial classes are generated which contain these properties – no more writing DP boiler plate code! For those of you who are unfamiliar with T4, it is a code generation engine which is built into Visual Studio, for a primer, I refer you to my CodeProject article.

A few days back, Daniel Vaughan published a blog post about using T4 templates to generate project meta-data. He demonstrated how this could be used to solve the age-old problem of hard-coded property name strings when implementing INotifyPropertyChanged. His solution uses both T4 and DTE – an automation library which allows you to programmatically access the items within your projects and solutions. Daniel’s blog post inspired me to revisit my DP code generation solution in order to simplify it and remove the need for the XML file.

My first step was to define a custom attribute which can be used to indicate that a class requires the template to generate a DP:

[AttributeUsage(AttributeTargets.Class , AllowMultiple = true)]
public class DependencyPropertyDecl : Attribute
{
    public DependencyPropertyDecl(string name, Type type, object defaultValue)
    {
        this.name = name;
        this.type = type;
        this.defaultValue = defaultValue;
    }
 
    public string name;
    public Type type;
    public object defaultValue;
}

With DTE, it should be possible to inspect all the classes in the project, identifying those for which this attribute exists and generate the required DPs. However, looking at the DTE APIs, this is not such an easy task, whilst the API defines a ‘base’ interface CodeElement from which CodeClass, CodeNamespace, etc. derive, there is no unified concept of ‘Children’, also the Project and Solution are not CodeElements. In other words, in order to find all the classes in a solution, you must recurse over a heterogeneous collection of classes and properties.

In order to ease the task of locating the classes within the project that have the DependencyPropertyDecl attribute associated, I created a simple Linq-to-DTE implementation:

public IEnumerable<CodeElement> CodeElementsInProjectItems(ProjectItems projectItems)
{
    foreach (ProjectItem projectItem in projectItems)
    {
        foreach(CodeElement el in CodeElementsInProjectItem(projectItem))
        {
            yield return el;
        }
    }
}
 
public IEnumerable<CodeElement> CodeElementsInProjectItem(ProjectItem projectItem)
{
    FileCodeModel fileCodeModel = projectItem.FileCodeModel;
 
    if (fileCodeModel != null)
    {
        foreach (CodeElement codeElement in fileCodeModel.CodeElements)
        {
            //WalkElements(codeElement, null);
            foreach(CodeElement el in CodeElementDescendantsAndSelf(codeElement))
            {
                yield return el;
            }
        }
    }
 
    if (projectItem.ProjectItems != null)
    {
        foreach (ProjectItem childItem in projectItem.ProjectItems)
        {
            foreach (CodeElement el in CodeElementsInProjectItem(childItem))
            {
                yield return el;
            }
        }
    }        
} 
 
public IEnumerable<CodeElement> CodeElementsDescendants(CodeElements codeElements)
{
    foreach(CodeElement element in codeElements)
    {
        foreach (CodeElement descendant in CodeElementDescendantsAndSelf(element))
        {
            yield return descendant;                
        }
    }
}
 
public IEnumerable<CodeElement> CodeElementDescendantsAndSelf(CodeElement codeElement)
{
    yield return codeElement;
 
    CodeElements codeElements;
 
    switch(codeElement.Kind)
    {        
 
        /* namespaces */
        case vsCMElement.vsCMElementNamespace:
        {
            CodeNamespace codeNamespace = (CodeNamespace)codeElement;                                        
            codeElements = codeNamespace.Members;
            foreach(CodeElement descendant in CodeElementsDescendants(codeElements))
            {
                yield return descendant;                
            }
            break;
        }
 
        /* Process classes */
        case vsCMElement.vsCMElementClass:
        {            
            CodeClass codeClass = (CodeClass)codeElement;            
            codeElements = codeClass.Members;
            foreach(CodeElement descendant in CodeElementsDescendants(codeElements))
            {                
                yield return descendant;                
            }            
            break;    
        }        
    }    
}

There might be simpler way of achieving this, it is the first time I have created a recursive IEnumerable implementation, so please let me know if I have done something stupid! For a much more elegant solution to the problem of creating an IEnumerable implementation on a tree like structure via recursion, have a look at this blog post by David Jade. However, as I mentioned earlier, the heterogeneous nature of the DTE API onto the project structure prohibits this method.

Anyhow, with this nasty IEnumerable implementation out of the way, we can use the power of Linq to provide a concise implementation of ‘Find all the classes in the solution which have one or more DependencyPropertyDecl attributes’, and perform the required code-gen as follows:

// for details of how the DTE 'project' is located see Daniel's blog post 
// or the attached source of this blog.
var elements = CodeElementsInProjectItems(project.ProjectItems);
var classes = elements.Where(el => el.Kind == vsCMElement.vsCMElementClass)
                               .Cast<CodeClass>()
                               .Where(cl => Attributes(cl).Any(at => at.Name=="DependencyPropertyDecl"));
 
foreach(var clazz in classes)
{
    GenerateClass(clazz);
}

The GenerateClass function is a simple modification of the one I wrote for generating classes based on XML descriptions. In this implementation, we acquire the CodeAttribute.Value property for each of our custom attribute instances and from there obtain the DP name, type and default value:

<#+
/// <summary>
/// Generates a class along with its associated DPs
/// </summary>
private void GenerateClass(CodeClass clazz)
{
    string classNamespace = clazz.Namespace.Name;
    string className =  clazz.Name;

    bool classRaisesPropertyChanged = false;

#>

namespace <#= classNamespace #>
{
    public partial class <#= className #> 
    <#+ if(classRaisesPropertyChanged){ #>: INotifyPropertyChanged<#+ } #>
    {
<#+
    var attributes = Attributes(clazz).Where(att => att.Name=="DependencyPropertyDecl");
    foreach(CodeAttribute attribute in attributes)
    {
        string[] attributeValues = attribute.Value.Split(',');

        string propertyName = attributeValues[0].Trim().Replace("\"","");
        string propertyType = attributeValues[1].Trim().Substring(7, attributeValues[1].Length - 9);
        string summary = null;
        string metadata = null;
        string defaultValue = attributeValues[2].Trim();
        string typeConverter = null;
        bool propertyChangedCallback = true;
        bool isAttached = false;
        #>

        #region <#= propertyName #>
        <#+        

        GenerateCLRAccessor(typeConverter, propertyType, propertyName, summary);

        bool handleDPPropertyChanged = propertyChangedCallback || classRaisesPropertyChanged;

        GenerateDependencyProperty(className, propertyType, defaultValue, propertyName,
                                   handleDPPropertyChanged, isAttached, metadata, summary);        

        if (handleDPPropertyChanged)
        {
            GenerateChangeEventHandler
            (className, propertyName, propertyChangedCallback, classRaisesPropertyChanged);
        } 

        if (isAttached)
        {
            GenerateAttachedPropertyAccessor(propertyName, propertyType);
        }
        #>
        #endregion
    <#+
    } // end foreach dps

    if (classRaisesPropertyChanged)
    {
        GenerateINotifyPropertChangedImpl();
    }
    #>
    }
}

<#+
}
#>

(please forgive the lack of syntax highlighting wordpress/codeproject do not support T4 templates!)

The GenerateCLRAccessor, GenerateDependencyProperty, … functions are re-used from my previous XML-based approach to DP generation.

For a simple demonstration of this approach, here is the complete implementation for a simple range control which has a Maximum and Minimum property, where if the user enters a Minimum which is greater that the Maximum, the values are swapped:

<UserControl x:Class="WpfDeclarativeDPCodeGen.RangeControl"

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

    DataContext="{Binding RelativeSource={RelativeSource Self}}">
    <StackPanel x:Name="LayoutRoot" Background="White" Orientation="Horizontal">
        <TextBox  Width="50" Text="{Binding Minimum}"/>
        <TextBlock Text=" : " VerticalAlignment="Center"/>
        <TextBox  Width="50" Text="{Binding Maximum}"/>
    </StackPanel>
</UserControl>

[DependencyPropertyDecl("Maximum", typeof(double), 0.0)]
[DependencyPropertyDecl("Minimum", typeof(double), 0.0)]
[TypeConverter(typeof(string))]
public partial class RangeControl : UserControl
{
    public RangeControl()
    {
        InitializeComponent();
    }
 
    /// <summary>
    /// If max is less than min, swap their values
    /// </summary>
    private void Swap()
    {
        if (Maximum < Minimum)
        {
            double swap = Minimum;
            Minimum = Maximum;
            Maximum = swap;
        }
    }
 
    partial void OnMaximumPropertyChanged(DependencyPropertyChangedEventArgs e)
    {
        Swap();
    }
 
    partial void OnMinimumPropertyChanged(DependencyPropertyChangedEventArgs e)
    {
        Swap();
    }
}

When using this approach, the workflow is quite simple, create your class, add the DependencyPropertyDecl attributes, then execute the code generation template. The output is an accompanying partial class which contains the DP code itself, plus partial methods which can be implemented in order to perform logic when the property changes.

I have not-yet completely ported my XML-based DP generation templates to this new declarative approach, for example, the DependencyPropertyDecl attribute needs to also indicate property meta-data, whether a property is attached, etc … However, I thought I would share my ideas in case I didn’t get the time (or urge) to make a complete implementation.

I feel that this approach has great potential far beyond DP code generation, simple examples could include attributes that indicate that a T4 template should generate a ‘generic’ INotifyPropertyChanged or IEditableObject implementation, and of course, the more complex application-specific possibilities are endless.

You can download an example project containing the RangeControl above: WpfDeclarativeDpCodeGen.zip.

Try adding new DependencyPropertyDecl attributes, and re-run the code generation template (right click CodeGen/GeneratedObjects.tt -> Run Custom Tool), and inspect the generated output – CodeGen/GeneratedObjects.cs.

Finally, thanks to Daniel Vaughan for the inspiration.

Regards,
Colin E.