Fix edge case regarding foreach item adding

Author: Vectorial1024

DictionaryList/DictionaryList.cs

@@ -80,6 +80,11 @@ public TValue this[int index]
             }
             set
             {
+                if (!_issetLookup[index])
+                {
+                    // adding items during iteration is not allowed!
+                    _version++;
+                }
                 _list[index] = value;
                 _issetLookup[index] = true;
             }

DictionaryListTest/Tests.cs

@@ -205,5 +205,15 @@ public void ListCannotAddItemsDuringForeach()
                 _dictList.Add(9);
             }
         });
+
+        Assert.Throws<InvalidOperationException>(() =>
+        {
+            _dictList.UnsetAt(1);
+            foreach (var _ in _dictList)
+            {
+                // it should somehow throw the exception during iteration
+                _dictList[1] = 4;
+            }
+        });
     }
 }

README.md

@@ -162,7 +162,7 @@ But still, here is a table outlining the performances when `N = 100000`:
 | Remove Many Items In-place (memory) | 0 B         | 48 B           | 0 B        | 312 B            |
 | Remove Many Items w/ LINQ (speed)   | 104 ms ⭐    | 793 ms 👌      | 885 ms     | 1693 ms          |
 | Remove Many Items w/ LINQ (memory)  | 198072 B ⭐  | 529280 B 👌    | 673168 B   | 1473296 B        |
-| Add Items During `foreach`          | ❌           | ❌              | ❌ new key  | ❌                |
+| Add Items During `foreach`          | ❌           | ❌ new key      | ❌ new key  | ❌                |
 | Emit Key/Index During `foreach`     | 🤷 [1]      | ✔️             | ✔️         | ✔️               |
 | Replace Items During `foreach`      | 🤷 [1]      | ✔️             | ✔️         | ❌ if key exists  |
 | Remove Items During `foreach`       | ❌           | ✔️             | ✔️         | ❌                |