Hi,
Like you said, if you have an object you want to be expandable, you need to decorate it with the ExpandableObject Attribute. An example is shown in the LiveExplorer of the toolkit available on this page : https://wpftoolkit.codeplex.com/, for this sample : PropertyGrid/Using SelectedObjects/UsingAttributes.
In the "Plus" version of the toolkit, you can use CustomPropertyItems to create the content of the propertyGrid. In that mode, you can tell which propertyItem will be Expandable with the Property : IsExpanded. Even if this "IsExpanded" property isn't shown in the sample PropertyGrid/Using a CustomList/CustomProperties of the LiveExplorer, it can be used (Plus only version). Here's a discussion on the same subject : https://wpftoolkit.codeplex.com/discussions/463364
Like you said, if you have an object you want to be expandable, you need to decorate it with the ExpandableObject Attribute. An example is shown in the LiveExplorer of the toolkit available on this page : https://wpftoolkit.codeplex.com/, for this sample : PropertyGrid/Using SelectedObjects/UsingAttributes.
In the "Plus" version of the toolkit, you can use CustomPropertyItems to create the content of the propertyGrid. In that mode, you can tell which propertyItem will be Expandable with the Property : IsExpanded. Even if this "IsExpanded" property isn't shown in the sample PropertyGrid/Using a CustomList/CustomProperties of the LiveExplorer, it can be used (Plus only version). Here's a discussion on the same subject : https://wpftoolkit.codeplex.com/discussions/463364