<asp:LoginView runat="server">
   <LoggedInTemplate>
      ......
      <asp:LoginName ID="LoginName1" runat="server" FormatString="Открыт сеанс, {0} !" />
      ......
   </LoggedInTemplate>
</asp:LoginView>

Session["UserName"] = Context.User.Identity.Name;

<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:cnnAspNetDB %>" 
   SelectCommand="SELECT [UserId] FROM [vw_aspnet_Users] WHERE ([UserName] = @UserName)">
   <SelectParameters>
      <asp:SessionParameter Name="UserName" SessionField="UserName" Type="String" />
   </SelectParameters>
</asp:SqlDataSource>

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
         ???????????????????????????????
         Session["UserId"] = ???????????? ;
    }
}

if (!IsPostBack && Context.User.Identity.Name.Length !=  0 )
{
   Session["UserName"] = Context.User.Identity.Name;

   ConnectionStringSettings settings = WebConfigurationManager.ConnectionStrings["XXX"];
   string connectionString = settings.ConnectionString;

   string queryString = 
           "SELECT [UserId] FROM [vw_aspnet_Users] WHERE [UserName] = '" + Session["UserName"] + "'";
   using (SqlConnection connection = new SqlConnection(connectionString))
   { 
      SqlCommand command = new SqlCommand(queryString, connection);

      connection.Open();
      object result = command.ExecuteScalar();
      Session["UserId"] = result.ToString();
   }
}