Verifies that the Freezable is not frozen and that it is being accessed from a valid threading context. Freezable inheritors should call this method at the beginning of any API that writes to data members that are not dependency properties.
This method calls VerifyAccess to verify the threading context is accessible, and it throws an exception if the Freezable instance is already frozen.
Notes to Inheritors
Classes that derive from Freezable should call WritePreamble before attempting to write to any members that are not dependency properties. If you call WritePreamble in an API, you can omit a call to ReadPreamble.