Use SortedDictionary in System.Json.JsonObject

Currently JsonObject uses a Dictionary and JsonValue does not sort the keys
in SaveInternal(). As iterating over a Dictionary returns the values in
random order, this causes the JSON string to be non-deterministic.
Switching to SortedDictionary makes sure that the same object always will
produce the same JSON string.
The keys are sorted using StringComparer.Ordinal to produce the same order
in all locales.

2 files changed, 24 insertions, 3 deletions

diff --git a/mcs/class/System.Json/System.Json/JsonObject.cs b/mcs/class/System.Json/System.Json/JsonObject.cs
index 33177b1c5f2..91366a2243a 100644
--- a/mcs/class/System.Json/System.Json/JsonObject.cs
+++ b/mcs/class/System.Json/System.Json/JsonObject.cs
@@ -12,11 +12,12 @@ namespace System.Json
 {
 	public class JsonObject : JsonValue, IDictionary<string, JsonValue>, ICollection<JsonPair>
 	{
-		Dictionary<string, JsonValue> map;
+		// Use SortedDictionary to make result of ToString() deterministic
+		SortedDictionary<string, JsonValue> map;
 
 		public JsonObject (params JsonPair [] items)
 		{
-			map = new Dictionary<string, JsonValue> ();
+			map = new SortedDictionary<string, JsonValue> (StringComparer.Ordinal);
 
 			if (items != null)
 				AddRange (items);
@@ -27,7 +28,7 @@ namespace System.Json
 			if (items == null)
 				throw new ArgumentNullException ("items");
 
-			map = new Dictionary<string, JsonValue> ();
+			map = new SortedDictionary<string, JsonValue> (StringComparer.Ordinal);
 			AddRange (items);
 		}
 
diff --git a/mcs/class/System.Json/Test/System.Json/JsonValueTest.cs b/mcs/class/System.Json/Test/System.Json/JsonValueTest.cs
index bc04e2d403f..15d5bd82b38 100644
--- a/mcs/class/System.Json/Test/System.Json/JsonValueTest.cs
+++ b/mcs/class/System.Json/Test/System.Json/JsonValueTest.cs
@@ -46,6 +46,26 @@ namespace MonoTests.System
 			Assert.AreEqual (str, "[1, 2, 3, null]");
 		}
 
+		// Test that we correctly serialize JsonObject with null elements.
+		[Test]
+		public void ToStringOnJsonObjectWithNulls () {
+			var j = JsonValue.Load (new StringReader ("{\"a\":null,\"b\":2}"));
+			Assert.AreEqual (2, j.Count, "itemcount");
+			Assert.AreEqual (JsonType.Object, j.JsonType, "type");
+			var str = j.ToString ();
+			Assert.AreEqual (str, "{\"a\": null, \"b\": 2}");
+		}
+
+		[Test]
+		public void JsonObjectOrder () {
+			var obj = new JsonObject ();
+			obj["a"] = 1;
+			obj["c"] = 3;
+			obj["b"] = 2;
+			var str = obj.ToString ();
+			Assert.AreEqual (str, "{\"a\": 1, \"b\": 2, \"c\": 3}");
+		}
+
 		[Test]
 		public void QuoteEscapeBug_20869 () 
 		{