For example, an instance of this class:

public class Point

{ public int X; public int Y; }

public class C

{ public ObjectId Id; public Dictionary<Point, int> Points; }

is serialized as:

> db.test.find()
{ "_id" : ObjectId("4de7fc1fe447ad107cce92da"), "Points" : [
[

{ "_t" : "Point", "X" : 1, "Y" : 1 }

,
1
],
[

{ "_t" : "Point", "X" : 2, "Y" : 2 }

,
2
]
] }
>

But the "_t" values are redundant because the type of the key is known to be Point.