Removed locks form Ref<T> (#19476)

src/Avalonia.Base/Utilities/Ref.cs

@@ -57,18 +57,18 @@ namespace Avalonia.Utilities
 
             public RefCounter(IDisposable item)
             {
-                _item = item;
+                _item = item ?? throw new ArgumentNullException();
                 _refs = 1;
             }
 
-            public void AddRef()
+            internal bool TryAddRef()
             {
                 var old = _refs;
                 while (true)
                 {
                     if (old == 0)
                     {
-                        throw new ObjectDisposedException("Cannot add a reference to a nonreferenced item");
+                        return false;
                     }
                     var current = Interlocked.CompareExchange(ref _refs, old + 1, old);
                     if (current == old)
@@ -77,6 +77,8 @@ namespace Avalonia.Utilities
                     }
                     old = current;
                 }
+
+                return true;
             }
 
             public void Release()
@@ -101,12 +103,11 @@ namespace Avalonia.Utilities
 
             internal int RefCount => _refs;
         }
-
+        
         class Ref<T> : CriticalFinalizerObject, IRef<T> where T : class
         {
-            private T? _item;
+            private volatile T? _item;
-            private readonly RefCounter _counter;
+            private volatile RefCounter? _counter;
-            private readonly object _lock = new object();
 
             public Ref(T item, RefCounter counter)
             {
@@ -114,65 +115,64 @@ namespace Avalonia.Utilities
                 _counter = counter;
             }
 
-            public void Dispose()
+            public void Dispose() => Dispose(true);
+            void Dispose(bool disposing)
             {
-                lock (_lock)
+                var item = Interlocked.Exchange(ref _item, null);
+
+                if (item != null)
                 {
-                    if (_item != null)
+                    var counter = _counter!;
-                    {
+                    _counter = null;
-                        _counter.Release();
+                    if (disposing)
-                        _item = null;
+                        GC.SuppressFinalize(this);
-                    }
+                    counter.Release();
-                    GC.SuppressFinalize(this);
                 }
             }
 
             ~Ref()
             {
-                Dispose();
+                Dispose(false);
             }
+            
 
-            public T Item
+            public T Item => _item ?? throw new ObjectDisposedException("Ref<" + typeof(T) + ">");
-            {
-                get
-                {
-                    lock (_lock)
-                    {
-                        return _item!;
-                    }
-                }
-            }
 
             public IRef<T> Clone()
             {
-                lock (_lock)
+                // Snapshot current ref state so we don't care if it's disposed in the meantime.
-                {
+                var counter = _counter;
-                    if (_item != null)
+                var item = _item;
-                    {
+                
-                        var newRef = new Ref<T>(_item, _counter);
+                // Check if ref was invalid
-                        _counter.AddRef();
+                if (item == null || counter == null)
-                        return newRef;
-                    }
                     throw new ObjectDisposedException("Ref<" + typeof(T) + ">");
-                }
+
+                // Try to add a reference to the counter, if it fails, the item is disposed.
+                if (!counter.TryAddRef())
+                    throw new ObjectDisposedException("Ref<" + typeof(T) + ">");
+
+                return new Ref<T>(item, counter);
             }
 
             public IRef<TResult> CloneAs<TResult>() where TResult : class
             {
-                lock (_lock)
+                // Snapshot current ref state so we don't care if it's disposed in the meantime.
-                {
+                var counter = _counter;
-                    if (_item != null)
+                var item = (TResult?)(object?)_item;
-                    {
+                
-                        var castRef = new Ref<TResult>((TResult)(object)_item, _counter);
+                // Check if ref was invalid
-                        Interlocked.MemoryBarrier();
+                if (item == null || counter == null)
-                        _counter.AddRef();
-                        return castRef;
-                    }
                     throw new ObjectDisposedException("Ref<" + typeof(T) + ">");
-                }
+
+                // Try to add a reference to the counter, if it fails, the item is disposed.
+                if (!counter.TryAddRef())
+                    throw new ObjectDisposedException("Ref<" + typeof(T) + ">");
+
+                return new Ref<TResult>(item, counter);
             }
 
-            public int RefCount => _counter.RefCount;
+            public int RefCount => _counter?.RefCount ?? throw new ObjectDisposedException("Ref<" + typeof(T) + ">");
         }
     }