Adding custom attributes to an element in XAML?

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The closest you can get are attached properties. Basically, another class defines a known property (i.e. MyProperty), which can be set on other elements.

An example would be the Canvas.Left property, which is used by the Canvas to position a child element. But any class can define an attached property.

Attached properties are the key behind attached behaviors, which is a great feature of WPF/Silverlight.

EDIT:

Here is an example class:

namespace MyNamespace {
    public static class MyClass {

        public static readonly DependencyProperty MyPropertyProperty = DependencyProperty.RegisterAttached("MyProperty",
            typeof(string), typeof(MyClass), new FrameworkPropertyMetadata(null));

        public static string GetMyProperty(UIElement element) {
            if (element == null)
                throw new ArgumentNullException("element");
            return (string)element.GetValue(MyPropertyProperty);
        }
        public static void SetMyProperty(UIElement element, string value) {
            if (element == null)
                throw new ArgumentNullException("element");
            element.SetValue(MyPropertyProperty, value);
        }
    }
}

Then in XAML you can use it like so:

xmlns:local="clr-namespace:MyNamespace"

<Canvas local:MyClass.MyProperty="MyValue" ... />

You can get the property from code using MyClass.GetMyProperty and passing in the element on which the property is set.

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