B-G Blog

How do you obtain a reference to an ancestor (within the visual tree) of a Silverlight FrameworkElement, given its name?  The question seems pretty straight forward.  FrameworkElement exposes a method, aptly named FindName, which appears to do just that.  Easy solution, right?  Well....maybe.

Using the FindName method works great, as long as the object you're looking for resides within the same XAML namescope as the FrameworkElement.  How do you know if your target object is in the same XAML namescope as your FrameworkElement?  Does the XAML for the FrameworkElement contain the XAML for the target?  If so, then it's in the same namescope.  Should the XAML for the target be nested somewhere along the way (such as within a few levels of UserControls), then you're out of luck.

So what happens if your target isn't in the same XAML namescope as your FrameworkElement?  It's still part of the visual tree, so there should be some way to walk the tree down.  FrameworkElement doesn't appear to expose any members to facilitate this type of action.  Still, there must be a way to do it.

I eventually discovered how to recurse through a visual tree, regardless of the XAML namescope.  The key to doing this lies within the VisualTreeHelper class.  I put together a simple class that would wrap the calls to VisualTreeHelper, and handle the object recursion for me.

This was my first implementation:

public static class VisualTreeWalker

{

    public static FrameworkElement FindName(string name, DependencyObject reference)

    {

        return FindNameInternal(name, reference);

    }

 

    private static FrameworkElement FindNameInternal(string name, DependencyObject reference)

    {

        foreach (DependencyObject obj in GetChildren(reference))

        {

            FrameworkElement elem = obj as FrameworkElement;

            if (elem != null && elem.Name == name)

            {

                return elem;

            }

 

            elem = FindNameInternal(name, obj);

            if (elem != null)

            {

                return elem;

            }

        }

 

        return null;

    }

 

    private static IEnumerable<DependencyObject> GetChildren(DependencyObject reference)

    {

        int childCount = VisualTreeHelper.GetChildrenCount(reference);

        for (int i = 0; i < childCount; i++)

        {

            yield return VisualTreeHelper.GetChild(reference, i);

        }

    }

}

Here is an example where we are attempting to find a nested TextBlock named ChildControl:

TextBlock test = (TextBlock)VisualTreeWalker.FindName("ChildControl", this);

The code performs a top-down search through the visual tree, exhausting each new branch before moving onto the next.  Even though I was pleased with the results, I starting thinking about ways to improve it.  What if I want to look for a specific type, rather than the base FrameworkElement?  It shouldn't be very difficult to add this functionality, and this would be a good excuse to work with Generics on the method level.  I couldn't resist.

This was my next implementation:

public static class VisualTreeWalker

{

    public static FrameworkElement FindName(string name, DependencyObject reference)

    {

        return FindName<FrameworkElement>(name, reference);

    }

 

    public static T FindName<T>(string name, DependencyObject reference) where T : FrameworkElement

    {

        if (string.IsNullOrEmpty(name))

        {

            throw new ArgumentNullException("name");

        }

 

        if (reference == null)

        {

            throw new ArgumentNullException("reference");

        }

 

        return FindNameInternal<T>(name, reference);

    }

 

    private static T FindNameInternal<T>(string name, DependencyObject reference) where T : FrameworkElement

    {

        foreach (DependencyObject obj in GetChildren(reference))

        {

            T elem = obj as T;

            if (elem != null && elem.Name == name)

            {

                return elem;

            }

 

            elem = FindNameInternal<T>(name, obj);

            if (elem != null)

            {

                return elem;

            }

        }

 

        return null;

    }

 

    private static IEnumerable<DependencyObject> GetChildren(DependencyObject reference)

    {

        int childCount = VisualTreeHelper.GetChildrenCount(reference);

        for (int i = 0; i < childCount; i++)

        {

            yield return VisualTreeHelper.GetChild(reference, i);

        }

    }

}

Here is a new example where we are attempting to find a nested TextBlock named ChildControl:

TextBlock test = VisualTreeWalker.FindName<TextBlock>("ChildControl", this);

Once I was satisfied with this new implementation, I couldn't help but think of ways to improve it even more.  Wouldn't it be cool to have some functionality that would return all instances of a specific type?  Ooh.  How cool would it be to impliment searching off of a name pattern in addition to an exact match?

Well, I'll leave those up to you.  Go ahead and give it a shot.  You're already thinking about how you would write the code, aren't you?