What you can do is create two services, the first will handle the Windows authentication and the second will handle the Basic authentication. If Windows authentication fails you can fallback to Basic authentication.
One service with BasicHttpBinding cannot support multiple authentication mechanisms, because the ClientCredentialType is a single object and not a collection and the HttpClientCredentialType enumeration does not contain mixed modes.
Multiple Authentication Support
will come with WCF 4.5 - until then you will have to use workarounds.
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