Custom Property Type and SaveData Method
In out project we use PropertyPageReferenceCollection to store references to local pages. It has PageReferenceCollection under the hood and calls its Parse and ToString methods to load and save the data. Evetything was fine until we decided to make some property (let’s call it Links) of this type required. The application started to crash while loading the page with empty Links property.
I’ve spend a lot of time looking how to prevent LoadData from crash while the error was in SaveData method. When LocalPageProvider loads a page it goes through all the properties from DB calls PropertyData.LoadData method for each property which value is not DBNull.Value. Since our SaveData implementation used PageReferenceCollection.ToString it returned string.Empty for empty collection and that value was stored in the DB. Then, after we set Links property as required, LoadData started to crash since string.Empty caused PropertyData.Clear to be called for required property.
Moral: always implement SaveData property so it returns null if PropertyData.IsNull is true.