ISourceCache.KeySelector (reactivemarbles#374)

* ISourceCache.KeySelector

Adds a getter to retrieve the KeySelector func that a cache uses for comparison.  Will help users more accurately know what a cache is using to determine its key, and utilize it in logic that needs to match in key selection

* Merged KeySelector initialization with null check

src/DynamicData/Cache/ISourceCache.cs

@@ -21,5 +21,10 @@ public interface ISourceCache<TObject, TKey> : IObservableCache<TObject, TKey>
         /// </summary>
         /// <param name="updateAction">The update action.</param>
         void Edit(Action<ISourceUpdater<TObject, TKey>> updateAction);
+
+        /// <summary>
+        /// Key selector used by the cache to retrieve keys from objects
+        /// </summary>
+        Func<TObject, TKey> KeySelector { get; }
     }
 }

src/DynamicData/Cache/SourceCache.cs

@@ -26,11 +26,7 @@ public class SourceCache<TObject, TKey> : ISourceCache<TObject, TKey>
         /// <exception cref="System.ArgumentNullException">keySelector</exception>
         public SourceCache(Func<TObject, TKey> keySelector)
         {
-            if (keySelector == null)
-            {
-                throw new ArgumentNullException(nameof(keySelector));
-            }
-
+            KeySelector = keySelector ?? throw new ArgumentNullException(nameof(keySelector));
             _innerCache = new ObservableCache<TObject, TKey>(keySelector);
         }
 
@@ -62,6 +58,8 @@ public SourceCache(Func<TObject, TKey> keySelector)
         /// <inheritdoc />
         public IEnumerable<TKey> Keys => _innerCache.Keys;
 
+        public Func<TObject, TKey> KeySelector { get; }
+
         /// <inheritdoc />
         public Optional<TObject> Lookup(TKey key) => _innerCache.Lookup(key);