diff --git a/samples/book-app-project-cs/Services/BookCollection.cs b/samples/book-app-project-cs/Services/BookCollection.cs
index b219ba53..87ddef03 100644
--- a/samples/book-app-project-cs/Services/BookCollection.cs
+++ b/samples/book-app-project-cs/Services/BookCollection.cs
@@ -54,7 +54,7 @@ public Book AddBook(string title, string author, int year)
         return book;
     }
 
-    public List<Book> ListBooks() => _books;
+    public List<Book> ListBooks() => _books.ToList();
 
     public Book? FindBookByTitle(string title)
     {
diff --git a/samples/book-app-project-cs/Tests/BookCollectionTests.cs b/samples/book-app-project-cs/Tests/BookCollectionTests.cs
index 2140e071..b9db43b8 100644
--- a/samples/book-app-project-cs/Tests/BookCollectionTests.cs
+++ b/samples/book-app-project-cs/Tests/BookCollectionTests.cs
@@ -68,6 +68,16 @@ public void RemoveBook_NonexistentBook_ShouldReturnFalse()
         Assert.False(result);
     }
 
+    [Fact]
+    public void ListBooks_ShouldReturnCopy_AndPreventExternalMutation()
+    {
+        _collection.AddBook("Dune", "Frank Herbert", 1965);
+        var books = _collection.ListBooks();
+        books.Clear();
+
+        Assert.Single(_collection.Books);
+    }
+
     // --- FindByAuthor ---
 
     [Fact]