Don't discard the system proxy setting (Azure#16995)

Fixes Azure#16990

Author: pakrym

sdk/core/Azure.Core/CHANGELOG.md

@@ -5,6 +5,9 @@
 ### Added
 - `System.Text.Json.JsonConverter` implementation for the `ETag`
 
+### Fixes
+- System proxy settings are correctly applied on .NET Framework
+
 ## 1.6.0 (2020-10-28)
 
 ### Added

sdk/core/Azure.Core/src/Pipeline/HttpWebRequestTransport.cs

@@ -20,7 +20,7 @@ namespace Azure.Core.Pipeline
     internal class HttpWebRequestTransport : HttpPipelineTransport
     {
         public static readonly HttpWebRequestTransport Shared = new HttpWebRequestTransport();
-        private readonly IWebProxy? _proxy;
+        private readonly IWebProxy? _environmentProxy;
 
         /// <summary>
         /// Creates a new instance of <see cref="HttpWebRequestTransport"/>
@@ -29,7 +29,7 @@ public HttpWebRequestTransport()
         {
             if (HttpEnvironmentProxy.TryCreate(out IWebProxy webProxy))
             {
-                _proxy = webProxy;
+                _environmentProxy = webProxy;
             }
         }
 
@@ -105,7 +105,11 @@ private HttpWebRequest CreateRequest(Request messageRequest)
         {
             var request = WebRequest.CreateHttp(messageRequest.Uri.ToUri());
             request.Method = messageRequest.Method.Method;
-            request.Proxy = _proxy;
+            // Don't disable the default proxy when there is no environment proxy configured
+            if (_environmentProxy != null)
+            {
+                request.Proxy = _environmentProxy;
+            }
 
             foreach (var messageRequestHeader in messageRequest.Headers)
             {

sdk/core/Azure.Core/tests/TransportFunctionalTests.cs

@@ -6,6 +6,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using System.Net;
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
@@ -698,6 +699,44 @@ public async Task ProxySettingsAreReadFromEnvironment(string envVar, string url)
             }
         }
 
+#if NET461 // GlobalProxySelection.Select not supported on netcoreapp
+        [NonParallelizable]
+        [Test]
+        public async Task DefaultProxySettingsArePreserved()
+        {
+#pragma warning disable 618 // Use of obsolete symbol
+            var oldGlobalProxySelection = GlobalProxySelection.Select;
+#pragma warning restore 618
+            try
+            {
+                using (TestServer testServer = new TestServer(async context =>
+                {
+                    context.Response.Headers["Via"] = "Test-Proxy";
+                    byte[] buffer = Encoding.UTF8.GetBytes("Hello");
+                    await context.Response.Body.WriteAsync(buffer, 0, buffer.Length);
+                }))
+                {
+#pragma warning disable 618 // Use of obsolete symbol
+                    GlobalProxySelection.Select = new WebProxy(testServer.Address.ToString());
+#pragma warning restore 618
+
+                    var transport = GetTransport();
+                    Request request = transport.CreateRequest();
+                    request.Uri.Reset(new Uri("http://microsoft.com"));
+                    Response response = await ExecuteRequest(request, transport);
+                    Assert.True(response.Headers.TryGetValue("Via", out var via));
+                    Assert.AreEqual("Test-Proxy", via);
+                }
+            }
+            finally
+            {
+#pragma warning disable 618 // Use of obsolete symbol
+                GlobalProxySelection.Select = oldGlobalProxySelection;
+#pragma warning restore 618
+            }
+        }
+#endif
+
         [Test]
         public async Task ResponseHeadersAreSplit()
         {