Fix closure in HttpConnectionPoolManager.StartMonitoringNetworkChanges (#50487)

Author: stephentoub

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPoolManager.cs

@@ -163,17 +163,20 @@ public void StartMonitoringNetworkChanges()
 
             // Monitor network changes to invalidate Alt-Svc headers.
             // A weak reference is used to avoid NetworkChange.NetworkAddressChanged keeping a non-disposed connection pool alive.
-            var poolsRef = new WeakReference<ConcurrentDictionary<HttpConnectionKey, HttpConnectionPool>>(_pools);
-            NetworkAddressChangedEventHandler networkChangedDelegate = delegate
-            {
-                if (poolsRef.TryGetTarget(out ConcurrentDictionary<HttpConnectionKey, HttpConnectionPool>? pools))
+            NetworkAddressChangedEventHandler networkChangedDelegate;
+            { // scope to avoid closure if _networkChangeCleanup != null
+                var poolsRef = new WeakReference<ConcurrentDictionary<HttpConnectionKey, HttpConnectionPool>>(_pools);
+                networkChangedDelegate = delegate
                 {
-                    foreach (HttpConnectionPool pool in pools.Values)
+                    if (poolsRef.TryGetTarget(out ConcurrentDictionary<HttpConnectionKey, HttpConnectionPool>? pools))
                     {
-                        pool.OnNetworkChanged();
+                        foreach (HttpConnectionPool pool in pools.Values)
+                        {
+                            pool.OnNetworkChanged();
+                        }
                     }
-                }
-            };
+                };
+            }
 
             var cleanup = new NetworkChangeCleanup(networkChangedDelegate);