They are two different Structures from two different namespaces (System.Collection and Autodesk.AutoCAD.DatabaseServices).
DictionaryEntry keys and values are both object type, a DBDictionaryEntry key have to be a string and its value an ObjectId.
But there're explicit cast operators in the DBDictionaryEntry struct definition:
public static explicit operator DictionaryEntry(DBDictionaryEntry entry); public static explicit operator DBDictionaryEntry(DictionaryEntry entry);
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