Count user online incorrect

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C#

ASP.NET

Hi all,
I cannot count user online correctly when using Frame, the number of user online is increase very large amount. Here is my code:

void Session_Start(object sender, EventArgs e)
{
    Application.Lock();
    Application["CurrentUsers"] = (int)Application["CurrentUsers"] + 1;
    Application.UnLock();
}

void Session_End(object sender, EventArgs e)
{
    Application.Lock();
    if ((int)Application["CurrentUsers"] > 0)
    {
        Application["CurrentUsers"] = (int)Application["CurrentUsers"] - 1;
    }
    Application.UnLock();
}

Although it's correct when I use master page.

P/S; My page need to use frame!