No Hashset<T, G> for .Net?

I'm getting back into using MonoTouch, and I tried usin开发者_Python百科g

HashSet<int, List<MyClass>> 

I have a using for System.Collections.Generic, however I'm getting an error on the HashSet declaration, specifically I'm getting

The type or namespace name 'HashSet`2' could not be found. Are you missing a using directive or an assembly reference?

My code is having no issues with List, but adding in the HashSet I get this issue. Anyone have an idea as to the issue?

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HashSet<> is a collection class supporting individual items, hence it only has one generic type parameter. The dual parameter mapping is a Dictionary<TKey, TValue>.