Gets the lookup dictionary of the System.Collections.ObjectModel.KeyedCollection`2.
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By default, the System.Collections.ObjectModel.KeyedCollection`2 includes a lookup dictionary that is created when the first item is added. When an item is added to the System.Collections.ObjectModel.KeyedCollection`2, the item's key is extracted once and saved in the lookup dictionary for faster searches. This behavior can be overridden by using the KeyedCollection`2.#ctor(IEqualityComparer<`0>,System.Int32) constructor and specifying a dictionary creation threshold.
Retrieving the value of this property is an O(1) operation.