UnsetValue is a sentinel value that is used for scenarios where the property system is unable to determine a requested DependencyProperty value. UnsetValue is used rather than nullNothingnullptra null reference (Nothing in Visual Basic), because nullNothingnullptra null reference (Nothing in Visual Basic) could be a valid property value, as well as a valid (and frequently used) default value. UnsetValue is never returned out of GetValue. When you call GetValue on a dependency property on a DependencyObject, one of the following applies:

• A dependency property has a default value established, and that value is returned.

• Some other value was established by the property system, and the default value is no longer relevant. For details, see Dependency Property Value Precedence (Silverlight 2).

Establishing a default value of UnsetValue is specifically disallowed.

ReadLocalValue returns UnsetValue when the requested property has not been locally set.

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