Code syntax explaination help

Question

I am learning WPF and there's a piece of code which I don't quite understand the method declared with constraints:

public static T FindAncestor<T>(DependencyObject dependencyObject)
    where T : class // Need help to interpret this method declaration

I understand this is a shared method and T has be a class but what is what is 'static T FindAncestor'? Having troubles interpreting it as a whole. Thanks!

Code:

public static class VisualTreeHelperExtensions
{
    public static T FindAncestor<T>(DependencyObject dependencyObject)
        where T : class // Need help to interpret this method
    {
        DependencyObject target = dependencyObject;
        do
        {
            target = VisualTreeHelper.GetParent(target);
        }
        while (target != null && !(target is T));
        return target as T;
    }
}

Michael Hays · Accepted Answer · 2011-08-18 17:21:44Z

1

The static keyword in front means that you do not have to instantiate VisualTreeHelperExtensions in order to call the FindAncestor method. You can say:

VisualTreeHelperExtensions.FindAncestor<MyClass>(myObj);

Where myObj is a DependencyObject. The where, as you said, makes sure that T (MyClass in this case) is indeed a class

For convenience, Methods like this can be declared like this:

public static T FindAncestor<T>(this DependencyObject dependencyObject)
  where T : class // Need help to interpret this method declaration

Which would allow you to call the method like so:

myObj.FindAncestor<MyClass>();

Effectively adding a method to your DependencyObject after the fact.

edited Aug 18, 2011 at 17:21

answered Aug 18, 2011 at 16:23

Michael Hays

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1 Comment

k80sg Over a year ago

Thanks for the comprehensive explanation.

Daniel A. White · Accepted Answer · 2011-08-18 16:20:57Z

1

The T is a placeholder for a type - what is known as a generic. The where clause is a generic constraint that requires reference types.

answered Aug 18, 2011 at 16:20

Daniel A. White

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Tigran · Accepted Answer · 2011-08-18 16:22:38Z

1

Hope I understand your question.

It's a declaration of public static function.

If it's not you're asking for, please explain better.

answered Aug 18, 2011 at 16:22

Tigran

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Charles Boyung · Accepted Answer · 2011-08-18 16:24:27Z

1

It means that it is a static method (you can call it without an instance of the class created) that returns an object of type T. FindAncestor is the name of the method.

answered Aug 18, 2011 at 16:24

Charles Boyung

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