@@ -375,8 +375,7 @@ local Dictionary = (function ()
     end,
     TryAdd = function (this, key, value)
       if key == nil then throw(ArgumentNullException("key")) end
-      local exists = this:TryGetValue(key)
-      if exists then
+      if this:ContainsKey(key) then
         return false
       end
       this:set(key, value)
@@ -708,8 +707,7 @@ local ArrayDictionary = (function ()
     end,
     TryAdd = function (this, key, value)
       if key == nil then throw(ArgumentNullException("key")) end
-      local exists = this:TryGetValue(key)
-      if exists then
+      if this:ContainsKey(key) then
         return false
       end
       this:set(key, value)
 
@@ -70,19 +70,11 @@ end, {
 }, 1)
 
 local HashSet = {
-  __ctor__ = function (this, ...)
-    local n = select("#", ...)
-    local collection, comparer
-    if n == 1 then
-      local c = ...
-      if type(c) == "table" then
-        if c.GetEnumerator then
-          collection = c
-        end
-          comparer = c
-      end
-    elseif n == 2 then
-      collection, comparer = ...
+  __ctor__ = function (this, collection, comparer)
+    if type(collection) ~= "table" then
+      collection = nil
+    elseif not collection.GetEnumerator then
+      collection, comparer = nil, collection
     end
     this.dict = System.Dictionary(this.__genericT__, System.Boolean)(comparer)
     if collection then
@@ -236,7 +228,35 @@ local HashSet = {
       return match(unWrap(k))
     end)
   end,
-  TrimExcess = System.emptyFn
+  TrimExcess = System.emptyFn,
+  CopyTo = function (this, array, arrayIndex, count)
+    arrayIndex = arrayIndex or 0
+    count = count or this:getCount()
+    if array == nil then
+      throw(ArgumentNullException("array"))
+    end
+    if arrayIndex < 0 then
+      throw(System.ArgumentOutOfRangeException("arrayIndex"))
+    end
+    if count < 0 then
+      throw(System.ArgumentOutOfRangeException("count"))
+    end
+    local arrayCount = array:getCount()
+    if arrayIndex >= arrayCount then
+      throw(System.ArgumentException("arrayIndex"))
+    end
+    if arrayIndex + count > arrayCount then
+      throw(System.ArgumentException("count is greater than the available space from arrayIndex to the end of the destination array"))
+    end
+    count = count + arrayIndex
+    for _, kv in each(this.dict) do
+      if arrayIndex >= count then
+        return
+      end
+      array:set(arrayIndex, kv[1])
+      arrayIndex = arrayIndex + 1
+    end
+  end
 }
 
 function System.hashSetFromTable(t, T)
@@ -245,7 +265,7 @@ end
 
 System.HashSet = System.define("System.Collections.Generic.HashSet", function(T)
   return {
-    base = { System.ICollection_1(T), System.ISet_1(T) },
+    base = { System.ISet_1(T), System.IReadOnlySet_1(T) },
     __genericT__ = T,
     __genericTKey__ = T,
   }
 
@@ -28,6 +28,7 @@ local isArrayLike = System.isArrayLike
 local isDictLike = System.isDictLike
 local Array = System.Array
 local arrayEnumerator = Array.GetEnumerator
+local null = System.null
 
 local NullReferenceException = System.NullReferenceException
 local ArgumentNullException = System.ArgumentNullException
@@ -436,10 +437,11 @@ local function orderBy(source, keySelector, comparer, TKey, descending)
   if comparer == nil then comparer = Comparer_1(TKey).getDefault() end
   local keys = {}
   local function getKey(t)
-    local k = keys[t]
+    local key = t or t == nil and null
+    local k = keys[key]
     if k == nil then
       k = keySelector(t)
-      keys[t] = k
+      keys[key] = k
     end
     return k
   end
@@ -485,10 +487,11 @@ local function thenBy(source, keySelector, comparer, TKey, descending)
   if comparer == nil then comparer = Comparer_1(TKey).getDefault() end
   local keys = {}
   local function getKey(t)
-    local k = keys[t]
+    local key = t or t == nil and null
+    local k = keys[key]
     if k == nil then
       k = keySelector(t)
-      keys[t] = k
+      keys[key] = k
     end
     return k
   end
 
@@ -48,7 +48,7 @@ local SortedSet = {
 
 local SortedSetFn = System.define("System.Collections.Generic.SortedSet", function(T) 
   return { 
-    base = { System.ICollection_1(T), System.IReadOnlyCollection_1(T), System.ISet_1(T) }, 
+    base = { System.ISet_1(T), System.IReadOnlySet_1(T) }, 
     __genericT__ = T,
   }
 end, SortedSet, 1)
 
@@ -104,6 +104,12 @@ defInf("System.ISet_1", function(T)
   }
 end)
 
+defInf("System.IReadOnlySet_1", function(T)
+  return {
+    base = { IReadOnlyCollection_1(T) }
+  }
+end)
+
 defInf("System.IComparer")
 defInf("System.IComparer_1", emptyFn)
 defInf("System.IEqualityComparer")
Original file line number	Diff line number	Diff line change
`@@ -48,7 +48,7 @@ local SortedSet = {`
`48`	`48`
`49`	`49`	`local SortedSetFn = System.define("System.Collections.Generic.SortedSet", function(T)`
`50`	`50`	`return {`
`51`		`- base = { System.ICollection_1(T), System.IReadOnlyCollection_1(T), System.ISet_1(T) },`
	`51`	`+ base = { System.ISet_1(T), System.IReadOnlySet_1(T) },`
`52`	`52`	`__genericT__ = T,`
`53`	`53`	`}`
`54`	`54`	`end, SortedSet, 1)`