Saturday, 10 October 2009

Practical LINQ #4: Finding a descendant in a tree

Trees are everywhere. I don’t mean the green, woody variety. I mean the peculiar ones invented by programmers that have their root somewhere in the clouds and branch downwards. As inevitably as apples fall from trees on the heads of meditating physicists, so coders find themselves writing code to traverse tree structures.

In an earlier post we explored the tree created by the UI Automation API representing all the widgets on the Windows desktop. One very common tree-traversal operation is finding a particular child several nodes down from a root starting point. I gave the example of locating the Paint.Net drawing canvas within the main window of the application:

// find the Paint.Net drawing Canvas
 var canvas = mainWindow.FindDescendentByIdPath(new[] {
             "surfaceBox" });

What we have here is an extension method, FindDescendentByIdPath, that starts from a parent element and works its way down through the tree, at each level picking out the child with the given Automation Id. In my last post, I skipped over my implementation of this method, but it deserves a closer look because of the way it uses Functional techniques and LINQ to traverse the hierarchy.

So here it is:

public static AutomationElement FindDescendentByIdPath(this AutomationElement element, IEnumerable<string> idPath)
    var conditionPath = CreateConditionPathForPropertyValues(AutomationElement.AutomationIdProperty, idPath.Cast<object>());

    return FindDescendentByConditionPath(element, conditionPath);

public static IEnumerable<Condition> CreateConditionPathForPropertyValues(AutomationProperty property, IEnumerable<object> values)
    var conditions = values.Select(value => new PropertyCondition(property, value));

    return conditions.Cast<Condition>();

public static AutomationElement FindDescendentByConditionPath(this AutomationElement element, IEnumerable<Condition> conditionPath)
    if (!conditionPath.Any())
        return element;

    var result = conditionPath.Aggregate(
        (parentElement, nextCondition) => parentElement == null
                                              ? null
                                              : parentElement.FindChildByCondition(nextCondition));

    return result;

The first thing we do is convert the sequence of Automation Id strings into a sequence of Conditions (actually PropertyConditions) that the UI Automation API can use to pick out children of parent elements. This is handled for us by the CreateConditionPathForPropertyValues method in line 8.

The actual method of interest is FindDescendentByConditionPath, starting in line 15. Here we put the Enumerable.Aggregate method to a slightly unconventional use. Considered in the abstract, the Aggregate method takes elements from a sequence one by one and performs an function (of your choice) to combine the element with the previous result; the very first element is combined with a seed value.

In this case the seed value is the parent element at the top of tree, the sequence of elements is the list of conditions that we use to pick out the child at each level of the tree. And we provide a lambda function that, each time it is called, takes the element it found in the previous iteration, together with the next condition from the list and uses an extension method that I demonstrated in the earlier blog post, FindChildByCondition, to find the appropriate child.

I’ve found this method a great help when monkeying around in UI Automation trees. If you think you might, look for it in the the source code from last time.


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