#124 – One Example of WPF’s Use of Dependency Property Callbacks

There are three callbacks related to dependency properties that a class can respond to:

• PropertyChangedCallback – react to a new value
• ValidateValueCallback – determine whether a value is valid
• CoerceValueCallback – coerce a requested value to some different value

The DataGrid.FrozenColumnCount property is an example of a dependency property in a WPF class that implements all three callbacks.

FrozenColumnCount is used to specified the leftmost n columns in a data grid that should not scroll horizontally.  These columns remain fixed while the other columns scroll.

DataGrid uses the dependency property callbacks for this property as follows:

• PropertyChangedCallback – notify constituent controls in the DataGrid of the new value so that they can render properly.  (E.g. the control that renders the column headers).
• ValidateValueCallback – validation fails if a negative value is used for FrozenColumnCount
• CoerceValueCallback – if value greater than the number of columns is specified, coerce FrozenColumnCount to be equal to the number of columns.