Fix UnixDomainSocketBinding default security issue on Windows. (#5627)

Author: imcarolwang

src/System.Private.ServiceModel/tests/Scenarios/Binding/UDS/UDSBindingTests.cs

@@ -75,13 +75,9 @@ public void WindowsAuth()
                 System.ServiceModel.UnixDomainSocketBinding binding = new UnixDomainSocketBinding(System.ServiceModel.UnixDomainSocketSecurityMode.Transport);
                 binding.Security.Transport.ClientCredentialType = System.ServiceModel.UnixDomainSocketClientCredentialType.Windows;
 
-                var uriBuilder = new UriBuilder()
-                {
-                    Scheme = "net.uds",
-                    Path = UDS.GetUDSFilePath()
-                };
+                var uri = new Uri("net.uds://" + UDS.GetUDSFilePath());
                 factory = new System.ServiceModel.ChannelFactory<IEchoService>(binding,
-                    new System.ServiceModel.EndpointAddress(uriBuilder.ToString()));
+                    new System.ServiceModel.EndpointAddress(uri));
                 channel = factory.CreateChannel();
                 ((IChannel)channel).Open();
                 string result = channel.Echo(testString);

src/System.ServiceModel.UnixDomainSocket/src/System/ServiceModel/Channels/UnixDomainSocketChannelFactory.cs

@@ -40,7 +40,7 @@ protected override TChannel OnCreateChannel(EndpointAddress address, Uri via)
         {
             if(address.Identity == null)
             {
-                var hostIdentity = new DnsEndpointIdentity(address.Uri.Host ?? "localhost");
+                var hostIdentity = new DnsEndpointIdentity(string.IsNullOrEmpty(address.Uri.Host) ? "localhost" : address.Uri.Host);
                 var uriBuilder = new UriBuilder(address.Uri);
                 uriBuilder.Host = null;
                 address = new EndpointAddress(uriBuilder.Uri, hostIdentity,address.Headers.ToArray());