From 97bfbacd0c45821d525bab25dd2a8d17bf6e5a79 Mon Sep 17 00:00:00 2001
From: Marek Safar <marek.safar@gmail.com>
Date: Mon, 25 May 2015 14:55:29 +0200
Subject: [PATCH] [corlib] TimeZoneInfo.GetSystemTimeZones should work as
 singleton. Fixes #29906

---
 mcs/class/corlib/System/TimeZoneInfo.cs          | 11 ++++++-----
 mcs/class/corlib/Test/System/TimeZoneInfoTest.cs |  6 ++++++
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/mcs/class/corlib/System/TimeZoneInfo.cs b/mcs/class/corlib/System/TimeZoneInfo.cs
index 0457583e4c0..dc144d38de6 100644
--- a/mcs/class/corlib/System/TimeZoneInfo.cs
+++ b/mcs/class/corlib/System/TimeZoneInfo.cs
@@ -639,16 +639,17 @@ namespace System
 			info.AddValue ("SupportsDaylightSavingTime", SupportsDaylightSavingTime);
 		}
 
-		//FIXME: change this to a generic Dictionary and allow caching for FindSystemTimeZoneById
-		private static List<TimeZoneInfo> systemTimeZones;
+		static ReadOnlyCollection<TimeZoneInfo> systemTimeZones;
+
 		public static ReadOnlyCollection<TimeZoneInfo> GetSystemTimeZones ()
 		{
 			if (systemTimeZones == null) {
-				systemTimeZones = new List<TimeZoneInfo> ();
-				GetSystemTimeZones (systemTimeZones);
+				var tz = new List<TimeZoneInfo> ();
+				GetSystemTimeZones (tz);
+				Interlocked.CompareExchange (ref systemTimeZones, new ReadOnlyCollection<TimeZoneInfo> (tz), null);
 			}
 
-			return new ReadOnlyCollection<TimeZoneInfo> (systemTimeZones);
+			return systemTimeZones;
 		}
 
 		public TimeSpan GetUtcOffset (DateTime dateTime)
diff --git a/mcs/class/corlib/Test/System/TimeZoneInfoTest.cs b/mcs/class/corlib/Test/System/TimeZoneInfoTest.cs
index bdce606080e..7ed78fc26e3 100644
--- a/mcs/class/corlib/Test/System/TimeZoneInfoTest.cs
+++ b/mcs/class/corlib/Test/System/TimeZoneInfoTest.cs
@@ -664,6 +664,12 @@ namespace MonoTests.System
 		[TestFixture]
 		public class GetSystemTimeZonesTests
 		{
+			[Test]
+			public void Identity ()
+			{
+				Assert.AreSame (TimeZoneInfo.GetSystemTimeZones (), TimeZoneInfo.GetSystemTimeZones ());
+			}
+
 			[Test]
 			public void NotEmpty ()
 			{
-- 
2.25.1