Avoid race condition in WebClient.IsBusy. (#42076)

diff --git a/src/libraries/System.Net.WebClient/src/System/Net/WebClient.cs b/src/libraries/System.Net.WebClient/src/System/Net/WebClient.cs
index 8849777..c02b169 100644 (file)
--- a/src/libraries/System.Net.WebClient/src/System/Net/WebClient.cs
+++ b/src/libraries/System.Net.WebClient/src/System/Net/WebClient.cs
@@ -213,7 +213,7 @@ namespace System.Net
 
         public RequestCachePolicy? CachePolicy { get; set; }
 
-        public bool IsBusy => _asyncOp != null;
+        public bool IsBusy => Volatile.Read(ref _callNesting) > 0;
 
         protected virtual WebRequest GetWebRequest(Uri address)
         {