using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.Services.Description;
using System.Web.SessionState;
namespace WebApplication
{
  public class Global : System.Web.HttpApplication
  {
    protected void Application_Start(object sender, EventArgs e)
    {
      Application.Lock();
      Application["count"] = 0; //Application.Set("count",0) /Application.Add("count",0)  初始化变量，这个作用等同，都是将count设置为0。
      Application["online"] = 0;
      Application.UnLock();
    }
    protected void Session_start(object sender, EventArgs e)
    {
      Application.Lock();
      Application["count"] = (int)Application["count"] + 1;
      Application["online"] = (int)Application["online"] + 1;
      Application.UnLock();
    }
    protected void Session_end(object sender, EventArgs e)
    {
      Application.Lock();
      Session.Abandon();//当以一个会话结束后，注销该会话
      Application["online"] = (int)Application["online"] - 1;
      Application.UnLock();
    }
  }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace WebApplication
{
  public partial class Index : System.Web.UI.Page
  {
    protected void Page_Load(object sender, EventArgs e)
    {
      Response.Write("<br />总访问人数是：" + Application["count"]);
      Response.Write("<br />当前在线人数是：" + Application["online"]);
    }
  }
}

<?xml version="1.0" encoding="utf-8"?>
<!--
 有关如何配置 ASP.NET 应用程序的详细信息，请访问
 https://go.microsoft.com/fwlink/?LinkId=169433
-->
<configuration>
 <system.web>
  <compilation debug="true" targetFramework="4.6.1"/>
  <httpRuntime targetFramework="4.6.1"/>
  <sessionState mode="InProc" timeout="1" cookieless="false"/> <!--设置一个会话的作用时间为一分钟，即一分钟内不做任何操作的话，该会话就会失效。-->
 </system.web>
 <system.codedom>
  <compilers>
   <compiler language="c#;cs;csharp" extension=".cs"
    type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.5.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
    warningLevel="4" compilerOptions="/langversion:default /nowarn:1659;1699;1701"/>
   <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb"
    type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.5.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
    warningLevel="4" compilerOptions="/langversion:default /nowarn:41008 /define:_MYTYPE=\"Web\" /optionInfer+"/>
  </compilers>
 </system.codedom>
</configuration>