more component read tests

src/ComponentManager.cs

@@ -49,7 +49,9 @@ namespace Encompass
 
         internal IEnumerable<KeyValuePair<Guid, TComponent>> GetActiveComponentsByType<TComponent>() where TComponent : struct, IComponent
         {
-            return activeComponents[typeof(TComponent)].Select((id) => new KeyValuePair<Guid, TComponent>(id, (TComponent)IDToComponent[id]));
+            return activeComponents.ContainsKey(typeof(TComponent)) ?
+                activeComponents[typeof(TComponent)].Select((id) => new KeyValuePair<Guid, TComponent>(id, (TComponent)IDToComponent[id])) :
+                Enumerable.Empty<KeyValuePair<Guid, TComponent>>();
         }
 
         internal KeyValuePair<Guid, TComponent> GetActiveComponentByType<TComponent>() where TComponent : struct, IComponent

test/EngineTest.cs

@@ -295,5 +295,67 @@ namespace Tests
 
             Assert.Throws<IllegalMessageReadException>(() => world.Update(0.01f));
         }
+
+        static KeyValuePair<Guid, MockComponent> pairA;
+        static KeyValuePair<Guid, MockComponent> pairB;
+
+        class SameValueComponentReadEngine : Engine
+        {
+            public override void Update(float dt)
+            {
+                var components = this.ReadComponents<MockComponent>();
+
+                pairA = components.First();
+                pairB = components.Last();
+            }
+        }
+
+        // Tests that components with identical values should be distinguishable by ID
+        [Test]
+        public void SameValueComponents()
+        {
+            var worldBuilder = new WorldBuilder();
+            worldBuilder.AddEngine<SameValueComponentReadEngine>();
+
+            MockComponent componentA;
+            componentA.myInt = 20;
+            componentA.myString = "hello";
+
+            MockComponent componentB;
+            componentB.myInt = 20;
+            componentB.myString = "hello";
+
+            var entity = worldBuilder.CreateEntity();
+            entity.AddComponent(componentA);
+            entity.AddComponent(componentB);
+
+            var world = worldBuilder.Build();
+            world.Update(0.01f);
+
+            Assert.That(pairA, Is.Not.EqualTo(pairB));
+            Assert.That(pairA.Value, Is.EqualTo(pairB.Value));
+        }
+
+        static IEnumerable<KeyValuePair<Guid, MockComponent>> emptyComponentReadResult;
+
+        class ReadEmptyMockComponentsEngine : Engine
+        {
+            public override void Update(float dt)
+            {
+                emptyComponentReadResult = this.ReadComponents<MockComponent>();
+            }
+        }
+
+        [Test]
+        public void ReadComponentsOfTypeWhereNoneExist()
+        {
+            var worldBuilder = new WorldBuilder();
+            worldBuilder.AddEngine<ReadEmptyMockComponentsEngine>();
+
+            var world = worldBuilder.Build();
+            world.Update(0.01f);
+
+            Assert.That(emptyComponentReadResult, Is.Empty);
+        }
     }
 }