Support removing features from FeatureCollection.

src/Microsoft.AspNet.FeatureModel/FeatureCollection.cs

@@ -66,7 +66,12 @@ namespace Microsoft.AspNet.FeatureModel
         void SetInterface(Type type, object feature)
         {
             if (type == null) throw new ArgumentNullException("type");
-            if (feature == null) throw new ArgumentNullException("feature");
+
+            if (feature == null)
+            {
+                Remove(type);
+                return;
+            }
 
             lock (_containerSync)
             {
@@ -167,7 +172,20 @@ namespace Microsoft.AspNet.FeatureModel
 
         public bool Remove(Type key)
         {
-            throw new NotImplementedException();
+            if (key == null) throw new ArgumentNullException("key");
+
+            lock (_containerSync)
+            {
+                Type priorFeatureType;
+                if (_featureTypeByName.TryGetValue(key.FullName, out priorFeatureType))
+                {
+                    _featureTypeByName.Remove(key.FullName);
+                    _featureByFeatureType.Remove(priorFeatureType);
+                    Interlocked.Increment(ref _containerRevision);
+                    return true;
+                }
+                return false;
+            }
         }
 
         public bool TryGetValue(Type key, out object value)

test/Microsoft.AspNet.FeatureModel.Tests/InterfaceDictionaryTests.cs

@@ -48,5 +48,36 @@ namespace Microsoft.AspNet.FeatureModel.Tests
 
             Assert.Throws<ArgumentException>(() => interfaces.Add(typeof(IThing), thing));
         }
+
+        [Fact]
+        public void RemovedInterfaceIsRemoved()
+        {
+            var interfaces = new FeatureCollection();
+            var thing = new Thing();
+
+            interfaces.Add(typeof(IThing), thing);
+
+            Assert.Equal(interfaces[typeof(IThing)], thing);
+
+            Assert.True(interfaces.Remove(typeof(IThing)));
+
+            object thing2;
+            Assert.False(interfaces.TryGetValue(typeof(IThing), out thing2));
+        }
+
+        [Fact]
+        public void SetNullValueRemoves()
+        {
+            var interfaces = new FeatureCollection();
+            var thing = new Thing();
+
+            interfaces.Add(typeof(IThing), thing);
+            Assert.Equal(interfaces[typeof(IThing)], thing);
+
+            interfaces[typeof(IThing)] = null;
+
+            object thing2;
+            Assert.False(interfaces.TryGetValue(typeof(IThing), out thing2));
+        }
     }
 }