C# Share session web service

ASP.NET C# Share session web service? Home.aspx set session, but Service.asmx get session return NULL

Home.aspx
Home.aspx.cs
protected void Page_Load(object sender, EventArgs e)
{
    Session["UserId"] = "Guest";
}

Service.asmx
Service.asmx.cs
[WebMethod(EnableSession = true)]
public string EchoSession()
{
    String json = "{\"id\":\"" + System.Web.HttpContext.Current.Session["UserId"] + "\"}";
    return json;
}

Summary: Web services don't have Session by default. Add an attribute to the WebMethod.