首先在Global中写如下代码:

protected void Session_Start(Object sender, EventArgs e)   
 ...{   
 ArrayList lstName=new ArrayList();   
 this.Application.Add("name",lstName);   
 }   
    
 //****************************************************************************************************8   
 protected void Session_End(Object sender, EventArgs e)   
 ...{   
 Application.Lock();   
 string str=Session["name"].ToString();   
 ArrayList lstName=(ArrayList)this.Application["name"];   
 Application.UnLock();   
 }  
protected void Session_Start(Object sender, EventArgs e)
...{
ArrayList lstName=new ArrayList();
this.Application.Add("name",lstName);
}
//****************************************************************************************************8
protected void Session_End(Object sender, EventArgs e)
...{
Application.Lock();
string str=Session["name"].ToString();
ArrayList lstName=(ArrayList)this.Application["name"];
Application.UnLock();
}

然后在登陆页面写代码

string username=this.txtName.Text.Trim();   
 ArrayList lstName=(ArrayList)this.Application["name"];   
 foreach(string strname in lstName)   
 ...{   
 if(username.Equals(strname))   
 ...{   
 Response.Redirect("User_Login.aspx");   
 }   
 }   
    
 lstName=(ArrayList)Application["name"];   
 lstName.Add(this.txtName.Text.Trim());   
 this.Application.Lock();   
 this.Application["name"]=lstName;   
 Session["name"]=username;   
 this.Application.UnLock();   
 this.Response.Redirect("main.aspx");  
string username=this.txtName.Text.Trim();
ArrayList lstName=(ArrayList)this.Application["name"];
foreach(string strname in lstName)
...{
if(username.Equals(strname))
...{
Response.Redirect("User_Login.aspx");
}
}
lstName=(ArrayList)Application["name"];
lstName.Add(this.txtName.Text.Trim());
this.Application.Lock();
this.Application["name"]=lstName;
Session["name"]=username;
this.Application.UnLock();
this.Response.Redirect("main.aspx");