#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.

About Sean
Software developer in the Twin Cities area, passionate about software development and sailing.

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