Improves thread safety of lazy initializations

The Interlocked.Exchange calls will ensure that the same object is
returned from each call even under a race.  That's not necessarily
required, but it should also make sure that no write re-ordering issues
occur on platforms that allow it.  And it also makes the possibility
of thread safety issues explicit.

1 files changed, 9 insertions, 1 deletions

diff --git a/Mono.Collections.Generic/ReadOnlyCollection.cs b/Mono.Collections.Generic/ReadOnlyCollection.cs
index fe78414..73ef974 100644
--- a/Mono.Collections.Generic/ReadOnlyCollection.cs
+++ b/Mono.Collections.Generic/ReadOnlyCollection.cs
@@ -11,6 +11,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Threading;
 
 namespace Mono.Collections.Generic {
 
@@ -19,7 +20,14 @@ namespace Mono.Collections.Generic {
 		static ReadOnlyCollection<T> empty;
 
 		public static ReadOnlyCollection<T> Empty {
-			get { return empty ?? (empty = new ReadOnlyCollection<T> ()); }
+			get
+			{
+				if (empty != null)
+					return empty;
+
+				Interlocked.CompareExchange (ref empty, new ReadOnlyCollection<T> (), null);
+				return empty;
+			}
 		}
 
 		bool ICollection<T>.IsReadOnly {