Added a shared/reusable implementation for multi-locking within strea…

…m operators, I.E. being able to process upstream notifications and downstream notifications at the same time, with different locks, while still preserving notification order. (#893)

src/DynamicData/Internal/SwappableLock.cs

@@ -0,0 +1,48 @@
+// Copyright (c) 2011-2023 Roland Pheasant. All rights reserved.
+// Roland Pheasant licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for full license information.
+
+namespace DynamicData;
+
+internal ref struct SwappableLock
+{
+    public static SwappableLock CreateAndEnter(object gate)
+    {
+        var result = new SwappableLock()
+        {
+            _gate = gate
+        };
+
+        Monitor.Enter(gate, ref result._hasLock);
+
+        return result;
+    }
+
+    public void SwapTo(object gate)
+    {
+        if (_gate is null)
+            throw new InvalidOperationException("Lock is not initialized");
+
+        var hasNewLock = false;
+        Monitor.Enter(gate, ref hasNewLock);
+
+        if (_hasLock)
+            Monitor.Exit(_gate);
+
+        _hasLock = hasNewLock;
+        _gate = gate;
+    }
+
+    public void Dispose()
+    {
+        if (_hasLock && (_gate is not null))
+        {
+            Monitor.Exit(_gate);
+            _hasLock = false;
+            _gate = null;
+        }
+    }
+
+    private bool _hasLock;
+    private object? _gate;
+}