Avoid boxing in EventMap (#4731)

It's currently using a Hashtable with Int32 keys, which means every operation on the Hashtable boxes the int.  Switching to a Dictionary removes those costs.  It also lets use use TryGetValue to perform a single lookup to get a value rather than first calling ContainsKey and then using the indexer, and avoiding multiple virtual calls (most APIs on Hashtable are virtual) when we're only using the base class.

src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/Automation/EventMap.cs

@@ -12,8 +12,7 @@
 //
 
 using System;
-using System.Collections;
-using System.Security;
+using System.Collections.Generic;
 using System.Windows;
 using System.Windows.Automation;
 using System.Windows.Automation.Peers;
@@ -145,13 +144,12 @@ namespace MS.Internal.Automation
                 {
                     if (_eventsTable == null)
                     {
-                        _eventsTable = new Hashtable(20, .1f);
+                        _eventsTable = new Dictionary<int, EventInfo>(20);
                         firstEvent = true;
                     }
 
-                    if (_eventsTable.ContainsKey(idEvent))
+                    if (_eventsTable.TryGetValue(idEvent, out EventInfo info))
                     {
-                        EventInfo info = (EventInfo)_eventsTable[idEvent];
                         info.NumberOfListeners++;
                     }
                     else
@@ -176,10 +174,8 @@ namespace MS.Internal.Automation
                 if (_eventsTable != null)
                 {
                     // Decrement the count of listeners for this event
-                    if (_eventsTable.ContainsKey(idEvent))
+                    if (_eventsTable.TryGetValue(idEvent, out EventInfo info))
                     {
-                        EventInfo info = (EventInfo)_eventsTable[idEvent];
-
                         // Update or remove the entry based on remaining listeners
                         info.NumberOfListeners--;
                         if (info.NumberOfListeners <= 0)
@@ -293,7 +289,7 @@ namespace MS.Internal.Automation
             return null;
         }
 
-        private static Hashtable _eventsTable;        // key=event id, data=listener count
+        private static Dictionary<int, EventInfo> _eventsTable;        // key=event id, data=listener count
         private readonly static object _lock = new object();
     }
 }