From 4054f13313f4047b2c73174aa73e57f1baba0840 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tom=C3=A1=C5=A1=20Filip?=
 <34779382+tomasfil@users.noreply.github.com>
Date: Sat, 25 Jun 2022 17:17:26 +0200
Subject: [PATCH] Vanquish the curse of transform async, but for List now
 (#612)
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Co-authored-by: Tomáš Filip <tomas.filip@meac.cz>
---
 src/DynamicData/List/Internal/TransformAsync.cs | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/src/DynamicData/List/Internal/TransformAsync.cs b/src/DynamicData/List/Internal/TransformAsync.cs
index ab539f471..776b90a85 100644
--- a/src/DynamicData/List/Internal/TransformAsync.cs
+++ b/src/DynamicData/List/Internal/TransformAsync.cs
@@ -38,12 +38,21 @@ public IObservable<IChangeSet<TDestination>> Run()
             observer =>
             {
                 var state = new ChangeAwareList<TransformedItemContainer>();
+                var asyncLock = new SemaphoreSlim(1, 1);
 
                 return _source.Select(
                     async changes =>
                     {
-                        await Transform(state, changes).ConfigureAwait(false);
-                        return state;
+                        try
+                        {
+                            await asyncLock.WaitAsync().ConfigureAwait(false);
+                            await Transform(state, changes).ConfigureAwait(false);
+                            return state;
+                        }
+                        finally
+                        {
+                            asyncLock.Release();
+                        }
                     }).Select(tasks => tasks.ToObservable()).SelectMany(items => items).Select(
                     transformed =>
                     {