Check user connected under Active Directory authentication

Hello all,
I have to integrate my ASP.NET 4 Web Application with an Active Directory authentication, and know the user connected
without prompt him a login page (he does not have to insert any username/password).

Something like this (in my Default.aspx page):


protected void Page_Load(object sender, EventArgs e)
{

string username = //something that gives me the user connected
string role = //something that gives me the role of the user connected


if (role == "admin")
// goes to page1.apx
else
// goes to page2.aspx

}


How can make this authentication?

Thanks in advance.

Luigi


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