HTTP Request Header Value in C#, User Authentication

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C#

ASP.NET

Visual-Studio

SSO

HTTP

I have a an application where users are authenticated using single sign on and enter our system requesting our webpage.
I get/read the http request(to my server) header values(including username and password) in to my application using c#.
Following is the code snippet:

bool bauthenticate =false;
string strSSO = string.Empty;
NameValueCollection headers = base.Request.Headers;
for (int i = 0; i < headers.Count; i++)
        {
            string key = headers.GetKey(i);
            string value = headers.Get(i);
	if (key=="uid")
            strSSO = value.ToString();
            bauthenticate = true;
        }

Problem I am unable to make authentication of user false at logoff time. Please lemme know what I can do to authenticate the user false and redirect to sso login url and enter its credential again to login i.e Logoff the User.

Thanks and Regards in advance.