Assembly: WindowsBase (in windowsbase.dll)
UnsetValue is a sentinel value that is used for scenarios where the WPF property system is unable to determine a requested DependencyProperty value. UnsetValue is used rather than a null reference (Nothing in Visual Basic), because a null reference (Nothing in Visual Basic) could be a valid property value, as well as a valid (and frequently used) DefaultValue.
A dependency property has a default value established in metadata and that value is returned. This value might come from DefaultMetadata.
Some other value was established by the property system, and the default value is no longer relevant.
Setting a DefaultValue of UnsetValue is specifically disallowed.
DependencyObject.ReadLocalValue returns UnsetValue when the requested property has not been locally set.
Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter EditionThe Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.