From fe8bc60155556e58771481602cb109e54fc9447c Mon Sep 17 00:00:00 2001
From: Kester Maddock <kmaddock@contractor.ea.com>
Date: Thu, 12 Mar 2026 10:41:33 +1300
Subject: [PATCH] Fix an InvalidOperationException when WeakEventTable.Purge is
 called with EnableWeakEventMemoryImprovements enabled

---
 .../src/WindowsBase/System/Windows/WeakEventManager.cs     | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/WeakEventManager.cs b/src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/WeakEventManager.cs
index 12764610e41..4a53d910f73 100644
--- a/src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/WeakEventManager.cs
+++ b/src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/WeakEventManager.cs
@@ -404,6 +404,13 @@ protected virtual bool Purge(object source, object data, bool purgeAll)
 
                 if (ListenerList.PrepareForWriting(ref list) && source != null)
                 {
+                    // WeakEventTable.Purge is iterating through Table, Table[this, source] = list will invalidate the enumerator.
+                    // Purge the list next time.
+                    if (BaseAppContextSwitches.EnableWeakEventMemoryImprovements)
+                    {
+                        return true;
+                    }
+
                     Table[this, source] = list;
                 }