When the UseDefaultCredentials property on an instance of a client proxy class is set to true the client will send the default credentials to enable authentication at the service.

First, try without sending credentials to a service that requires them via Windows authentication. This should fail (if it works properly, see the Note below):

Now try with sending the default credentials:

Output:

NOTE: For this sample to work as intended you must configure the Application directory using the IIS Connection Manager:

Bring up the IIS Connection Manager by typing "inetmgr" in the Run command window from the Start menu.
Find the application directory corresonding to this sample by expanding the Web Sites folder and then Default Web Site. By default, the path from there will be QuickStart\webservices\samples\UseDefaultCredentials\vb.
Right click on the directory and select Properties.
Go to the Directory Security tab and click on Edit.
Unselect "Anonymous Access" and select "Integrated Windows authentication". (Nothing else should be selected.)
Click "Okay" and exit from the windows.