Missed new file in last commit.

src/libraries/System.Private.Xml/src/System/Xml/Serialization/ContextAwareTables.cs

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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Xml.Serialization
+{
+    using System;
+    using System.Collections;
+    using System.Runtime.CompilerServices;
+    using System.Runtime.Loader;
+
+    internal class ContextAwareTables<T> where T : class?
+    {
+        private Hashtable _defaultTable;
+        private ConditionalWeakTable<Type, T> _collectibleTable;
+
+        public ContextAwareTables()
+        {
+            _defaultTable = new Hashtable();
+            _collectibleTable = new ConditionalWeakTable<Type, T>();
+        }
+
+        internal T GetOrCreateValue(Type t, Func<T> f)
+        {
+            T? ret;
+            AssemblyLoadContext? alc = AssemblyLoadContext.GetLoadContext(t.Assembly);
+
+            // Null and non-collectible load contexts use the default table
+            if (alc == null || !alc.IsCollectible)
+            {
+                if ((ret = (T?)_defaultTable[t]) == null)
+                {
+                    lock (_defaultTable)
+                    {
+                        if ((ret = (T?)_defaultTable[t]) == null)
+                        {
+                            ret = f();
+                            _defaultTable[t] = ret;
+                        }
+                    }
+                }
+            }
+
+            // Collectible load contexts should use the ConditionalWeakTable so they can be unloaded
+            else
+            {
+                if (!_collectibleTable.TryGetValue(t, out ret))
+                {
+                    lock (_collectibleTable)
+                    {
+                        if (!_collectibleTable.TryGetValue(t, out ret))
+                        {
+                            ret = f();
+                            _collectibleTable.AddOrUpdate(t, ret);
+                        }
+                    }
+                }
+            }
+
+            return ret;
+        }
+    }
+}