Fix issue 12495: Infinite loop in ToolStripItemCollection.AddRange

src/System.Windows.Forms/src/System/Windows/Forms/Controls/ToolStrips/ToolStripItemCollection.cs

@@ -141,16 +141,23 @@ public void AddRange(ToolStripItemCollection toolStripItems)
             throw new NotSupportedException(SR.ToolStripItemCollectionIsReadOnly);
         }
 
+        // Return early if the collection is empty.
+        if (toolStripItems.Count == 0)
+        {
+            return;
+        }
+
         // ToolStripDropDown will look for PropertyNames.Items to determine if it needs
         // to resize itself.
         using (new LayoutTransaction(_owner, _owner!, PropertyNames.Items))
         {
-            for (int i = 0; i < toolStripItems.Count; i++)
+            // Create a temporary list to avoid modifying the original collection during iteration.
+            // This ensures that the collection is not altered while it is being iterated over,
+            // preventing issues such as InvalidOperationException.
+            var itemsToAdd = toolStripItems.InnerList.ToArray();
+            foreach (ToolStripItem item in itemsToAdd)
             {
-                // Items are removed from their origin when added to a different owner.
-                // Decrement the index to always add the items from index 0 which will preserve
-                // the original order and avoid a pesky ArgumentOutOfRangeException.
-                Add(toolStripItems[i--]);
+                Add(item);
             }
         }
     }

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/ToolStripItemCollectionTests.cs

@@ -125,6 +125,29 @@ public void ToolStripItemCollection_AddRange_ToolStripItemCollection_Success()
         Assert.Equal("c", contextMenuStrip.Items[2].Text);
     }
 
+    [WinFormsFact]
+    public void ToolStripItemCollection_AddRange_ToolStripItemCollection_SameOwner_Success()
+    {
+        using ToolStrip toolStrip = new();
+
+        // Create a ToolStripItemCollection with 2 items
+        ToolStripItemCollection itemCollection = new(toolStrip,
+            [
+            new ToolStripButton("Button 1"),
+            new ToolStripButton("Button 2")
+            ]);
+
+        toolStrip.Items.Count.Should().Be(0);
+
+        toolStrip.Items.AddRange(itemCollection);
+
+        itemCollection.Count.Should().Be(2);
+        toolStrip.Items.Count.Should().Be(2);
+
+        toolStrip.Items[0].Text.Should().Be("Button 1");
+        toolStrip.Items[1].Text.Should().Be("Button 2");
+    }
+
     private ToolStripItem[] CreateToolStripItems(params (string Key, string Name)[] items) =>
         items.Select(item => new ToolStripButton(item.Name) { Name = item.Key }).ToArray();