Convert `Hashtable` usage to `Dictionary<TKey, TValue`

[elachlan]

Is your feature request related to a problem? Please describe

Hashtable has been used all over the winforms code base.

Describe the solution you'd like and alternatives you've considered

Refactoring to Dictionary<TKey, TValue is explicit in its typing and avoids boxing.

The changes should not affect Public APIs.

System.Windows.Forms: 31 references
System.Windows.Forms.Design: 126 references

Related: #8140, #2644

[elachlan]

HelpProvider has a few Hashtables. _boundControls a bit weird since the key and value are the same and of type Control.

winforms/src/System.Windows.Forms/src/System/Windows/Forms/HelpProvider.cs

Line 26 in ba551ea

private readonly Hashtable _boundControls = new Hashtable();

I thought this might be a good use of Hashset<T>, but this suggests the performance isn't as good as List<T> for objects until after 20 items. https://stackoverflow.com/a/10762995/908889

Does the team have a suggestion here?

[elachlan]

@Jericho, you might want to jump in on this issue as well.

[Jericho]

@elachlan sure. As soon as I'm done working on #8140, I'll inventory the remaining HashTable and create a list to track progress (like I did for the ArrayList).

[elachlan]

I attempted WeakHashtable. It is a complicated one. I am unsure how to proceed.

Maybe add a type parameter and implement IDictionary<object, TValue>? That would allow using a private Dictionary<EqualityWeakReference, TValue>, thus hiding the implementation and avoiding boxing.