The Credentials property contains the authentication credentials used to access a resource on a host. In most client-side scenarios, you should use the DefaultCredentials, which are the credentials of the currently logged on user. To do this, set the UseDefaultCredentials property to true instead of setting this property.
If the WebClient class is being used in a middle tier application, such as an ASP.NET application, the DefaultCredentials belong to the account running the ASP page (the server-side credentials). Typically, you would set this property to the credentials of the client on whose behalf the request is made.