From ddacc112acfc482bb35498bb63a0ce38db797ca0 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@users.noreply.github.com>
Date: Wed, 8 Mar 2023 15:35:22 +0100
Subject: [PATCH] Throw if setting ItemsSource when Items present.

---
 src/Avalonia.Controls/ItemCollection.cs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/Avalonia.Controls/ItemCollection.cs b/src/Avalonia.Controls/ItemCollection.cs
index 4daea44d5e..c9265558f0 100644
--- a/src/Avalonia.Controls/ItemCollection.cs
+++ b/src/Avalonia.Controls/ItemCollection.cs
@@ -121,6 +121,10 @@ namespace Avalonia.Controls
 
         internal void SetItemsSource(IEnumerable? value)
         {
+            if (_mode != Mode.ItemsSource && Count > 0)
+                throw new InvalidOperationException(
+                    "Items collection must be empty before using ItemsSource.");
+
             _mode = value is not null ? Mode.ItemsSource : Mode.Items;
             SetSource(value ?? CreateDefaultCollection());
         }