How to access session at Outside of Web Forms page class

HttpContext.Current.Session["user"].ToString();

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string firstName = "Jeff";
string lastName = "Smith";
string city = "Seattle";

// Save to session state in a Web Forms page class.
Session["FirstName"] = firstName;
Session["LastName"] = lastName;
Session["City"] = city;

// Read from session state in a Web Forms page class.
firstName = (string)(Session["FirstName"]);
lastName = (string)(Session["LastName"]);
city = (string)(Session["City"]);

// Outside of Web Forms page class, use HttpContext.Current.
HttpContext context = HttpContext.Current;
context.Session["FirstName"] = firstName;
firstName = (string)(context.Session["FirstName"]);