how to use session variable in class files asp.net 4.0 C#

Try this code.

webpage.aspx.cs
===============
protected void Page_Load(object sender, EventArgs e)
{
Session["SessionName"] = "SessionVal";
Sample.testClass obj = new Sample.testClass();
Response.Write(obj.getSession());
}

testClass.cs
===============
public string getSession()
{
if (HttpContext.Current.Session["SessionName"] != null)
{
string sessionID = HttpContext.Current.Session["SessionName"].ToString();
return sessionID;
}
else
{
return "null";
}
}