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directory attribute update fails when value is String.Empty
Hope this will help someone else as well.
For example if you clear the address details of a user and update the context, DirectoryEntry CommitChanges will fail.
There are two work arounds for this issue - in your higher level classes (Entity or even higher) check if value is "" \ string.empty and change it to null.
2nd option: In DirectorySource class inside Update() method:
replace instances of i.GetValue(e.Key, null) with
(i.GetValue(e.Key, null)==String.Empty?null:i.GetValue(e.Key, null))
this should correct the issue from a lower level class.
Please note i have NOT tested this for attributes which as of ActiveDS type.