DotNetOpenAuth detect fetch request like google

With google, you can fetch the user's email like this:

var fetch = new FetchRequest();
fetch.Attributes.AddRequired(WellKnownAttributes.Contact.Email);
request.AddExtension(fetch);

and get it back like this:

var fetch = response.GetExtension<FetchResponse>();
string email = "";
if (fetch != null)
{
    email = fetch.GetAttributeValue(Well开发者_开发问答KnownAttributes.Contact.Email);
}

When writing a provider, how can I return the values asked for?

The OpenIdProviderWebForms sample that comes with DotNetOpenAuth includes returning user attributes. Have you checked it out?