C#: Address review comment: Make dictionary type more specific.

Author: calumgrant

csharp/extractor/Semmle.Extraction/Context.cs

@@ -60,7 +60,9 @@ public Entity CreateEntity<Type, Entity>(ICachedEntityFactory<Type, Entity> fact
         /// <param name="factory">The entity factory.</param>
         /// <param name="init">The initializer for the entity.</param>
         /// <returns>The new/existing entity.</returns>
-        public Entity CreateEntityFromSymbol<Type, Entity>(ICachedEntityFactory<Type, Entity> factory, Type init) where Entity : ICachedEntity
+        public Entity CreateEntityFromSymbol<Type, Entity>(ICachedEntityFactory<Type, Entity> factory, Type init)
+            where Entity : ICachedEntity
+            where Type: ISymbol
         {
             return init == null ? CreateEntity2(factory, init) : CreateNonNullEntity(factory, init, symbolEntityCache);
         }
@@ -149,8 +151,9 @@ private void CheckEntityHasUniqueLabel(string id, ICachedEntity entity)
         private Entity CreateNonNullEntity<Type, Entity>(ICachedEntityFactory<Type, Entity> factory, Type init) where Entity : ICachedEntity
             => CreateNonNullEntity(factory, init, objectEntityCache);
 
-
-        private Entity CreateNonNullEntity<Type, Entity>(ICachedEntityFactory<Type, Entity> factory, Type init, IDictionary<object, ICachedEntity> dictionary) where Entity : ICachedEntity
+        private Entity CreateNonNullEntity<Type, Src, Entity>(ICachedEntityFactory<Type, Entity> factory, Type init, IDictionary<Src, ICachedEntity> dictionary)
+            where Entity : ICachedEntity
+            where Type : Src
         {
             if (dictionary.TryGetValue(init, out var cached))
                 return (Entity)cached;
@@ -215,17 +218,9 @@ public void AddFreshLabel(IEntity entity)
 #if DEBUG_LABELS
         readonly Dictionary<string, ICachedEntity> idLabelCache = new Dictionary<string, ICachedEntity>();
 #endif
-        class SymbolComparer : IEqualityComparer<object>
-        {
-            IEqualityComparer<ISymbol> comparer = SymbolEqualityComparer.IncludeNullability;
-
-            bool IEqualityComparer<object>.Equals(object x, object y) => comparer.Equals((ISymbol)x, (ISymbol)y);
-
-            int IEqualityComparer<object>.GetHashCode(object obj) => comparer.GetHashCode((ISymbol)obj);
-        }
 
         readonly IDictionary<object, ICachedEntity> objectEntityCache = new Dictionary<object, ICachedEntity>();
-        readonly IDictionary<object, ICachedEntity> symbolEntityCache = new Dictionary<object, ICachedEntity>(10000, new SymbolComparer());
+        readonly IDictionary<ISymbol, ICachedEntity> symbolEntityCache = new Dictionary<ISymbol, ICachedEntity>(10000, SymbolEqualityComparer.IncludeNullability);
         readonly Dictionary<ICachedEntity, Label> entityLabelCache = new Dictionary<ICachedEntity, Label>();
         readonly HashSet<Label> extractedGenerics = new HashSet<Label>();