Fix IReadOnlyDictionary DeepClone throwing InvalidCastException

DeepClone.Test/SpecializedCollectionTests.cs

@@ -39,6 +39,62 @@ public void HashSet_DeepClone_ShouldCreateIndependentCopy()
 		Assert.AreEqual("Item1", originalItem.Name);
 	}
 
+	/// <summary>
+	/// Tests deep cloning an IReadOnlyDictionary.
+	/// </summary>
+	[TestMethod]
+	public void ReadOnlyDictionary_DeepClone_ShouldCreateIndependentCopy()
+	{
+		// Arrange
+		Dictionary<string, SimpleObject> original = new()
+		{
+			["a"] = new() { Id = 1, Name = "Item1" },
+			["b"] = new() { Id = 2, Name = "Item2" }
+		};
+
+		// Act
+		IReadOnlyDictionary<string, SimpleObject> clone = ((IReadOnlyDictionary<string, SimpleObject>)original).DeepClone();
+
+		// Assert
+		Assert.IsNotNull(clone);
+		Assert.HasCount(original.Count, clone);
+		Assert.AreEqual(1, clone["a"].Id);
+		Assert.AreEqual("Item2", clone["b"].Name);
+		Assert.AreNotSame(original["a"], clone["a"]);
+
+		// Verify independence
+		clone["a"].Name = "Modified";
+		Assert.AreEqual("Item1", original["a"].Name);
+	}
+
+	/// <summary>
+	/// Tests deep cloning an empty IReadOnlyDictionary.
+	/// </summary>
+	[TestMethod]
+	public void ReadOnlyDictionary_DeepClone_Empty_ShouldReturnEmptyDictionary()
+	{
+		// Arrange
+		Dictionary<string, int> original = [];
+
+		// Act
+		IReadOnlyDictionary<string, int> clone = ((IReadOnlyDictionary<string, int>)original).DeepClone();
+
+		// Assert
+		Assert.IsNotNull(clone);
+		Assert.IsEmpty(clone);
+	}
+
+	/// <summary>
+	/// Tests that deep cloning a null IReadOnlyDictionary throws.
+	/// </summary>
+	[TestMethod]
+	public void ReadOnlyDictionary_DeepClone_Null_ShouldThrow()
+	{
+		IReadOnlyDictionary<string, int> original = null!;
+
+		Assert.ThrowsExactly<ArgumentNullException>(() => original.DeepClone());
+	}
+
 	/// <summary>
 	/// Tests deep cloning a SortedSet.
 	/// </summary>

DeepClone/DeepCloneContainerExtensions.cs

@@ -94,7 +94,7 @@
 	/// of individual elements. It checks if the object implements IDeepCloneable and calls
 	/// DeepClone() if it does, otherwise it returns the original object.
 	/// </remarks>
 	private static T DeepClone<T>(T source) => source == null ? default! : source is IDeepCloneable cloneable ? (T)cloneable.DeepClone() : source;
 
 	/// <summary>
 	/// Deep clones a collection of objects.
@@ -160,9 +160,16 @@
 	/// var clonedReadOnlyDict = originalReadOnlyDict.DeepClone();
 	/// </code>
 	/// </remarks>
+	/// <exception cref="ArgumentNullException">Thrown if source is null.</exception>
 	public static IReadOnlyDictionary<TKey, TValue> DeepClone<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> source)
-		where TKey : notnull =>
-		(IReadOnlyDictionary<TKey, TValue>)source.ToDictionary(DeepClone, DeepClone);
+		where TKey : notnull
+	{
+		Ensure.NotNull(source);
+
+		return source.ToDictionary(
+			pair => DeepClone(pair.Key),
+			pair => DeepClone(pair.Value));
+	}
 
 	/// <summary>
 	/// Deep clones a hash set.