It’s not often I need to use this operator. I’m posting this so I have the name of the operator and the documentation when I need it next time.

:: operator - the namespace alias operator

using Gen = System.Collections.Generic;

// Use :: the namespace alias operator to access member in aliased namespace.
Gen::List<int> l1 = new Gen::List<int>();

// error CS0576: Namespace '<global namespace>' contains a definition conflicting with alias 'Gen'
Gen.List<int> l2 = new Gen.List<int>();

class Gen {};