From fde3cfe9e36b499afe31c04aaee953090df81ba0 Mon Sep 17 00:00:00 2001
From: Evan Hemsley <evan@moonside.games>
Date: Fri, 21 Jun 2019 17:23:52 -0700
Subject: [PATCH] fix null lookup bug in message manager

---
 encompass-cs/MessageManager.cs |  4 +++-
 test/EngineTest.cs             | 23 +++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/encompass-cs/MessageManager.cs b/encompass-cs/MessageManager.cs
index d22f2c0..b6a758d 100644
--- a/encompass-cs/MessageManager.cs
+++ b/encompass-cs/MessageManager.cs
@@ -28,7 +28,9 @@ namespace Encompass
 
         internal IEnumerable<TMessage> GetMessagesByType<TMessage>() where TMessage : struct, IMessage
         {
-            return messageTypeToMessages[typeof(TMessage)].Cast<TMessage>();
+            return messageTypeToMessages.ContainsKey(typeof(TMessage)) ?
+                messageTypeToMessages[typeof(TMessage)].Cast<TMessage>() :
+                Enumerable.Empty<TMessage>();
         }
     }
 }
diff --git a/test/EngineTest.cs b/test/EngineTest.cs
index 069c49f..c38a0ae 100644
--- a/test/EngineTest.cs
+++ b/test/EngineTest.cs
@@ -227,6 +227,29 @@ namespace Tests
             }
         }
 
+        static IEnumerable<MockMessage> emptyReadMessagesResult;
+        [Reads(typeof(MockMessage))]
+        class ReadMessagesWhenNoneExistEngine : Engine
+        {
+            public override void Update(float dt)
+            {
+                emptyReadMessagesResult = ReadMessages<MockMessage>();
+            }
+        }
+
+        [Test]
+        public void ReadMessagesWhenNoneHaveBeenEmitted()
+        {
+            var worldBuilder = new WorldBuilder();
+            worldBuilder.AddEngine<ReadMessagesWhenNoneExistEngine>();
+
+            var world = worldBuilder.Build();
+
+            world.Update(0.01f);
+
+            emptyReadMessagesResult.Should().BeEmpty();
+        }
+
         [Test]
         public void EmitUndeclaredMessage()
         {