Non-invocable member 'System.Web.HttpRequest.ServerVariables' cannot be used like a method

I want to retrive value as below :

    string server = Request.ServerVariables("SERVER_NAME");
    //Declare the form being accessed ex: Default.aspx
    string url = Request.ServerVariables("URL");
    // Declare the query string in the URL
    string querystring = Request.ServerVariables("QUERY_STRING");

But I got a error :

Non-invocable member 'System.Web.HttpRequest.ServerVariables' cannot be used like a method.

Please help to find my mistake.Thanks.