Prerequisites

The code in this article uses Sessions in ASP.NET, so you'll need to have them enabled on your server, by configuring the <sessionState> element in the Web.config file. Refer to the References section at the end of this article for more details.
You'll also need to have access to a file called Global.asax in the root of your site. If you run your own Web server, this is not a problem; you can simply create the file yourself. If you are using an ISP, you'll need to check with them if they support the use of the Global.asax file as, unfortunately, not all ISPs allow this.
This article also assumes you're using Visual Studio .NET 2002 or 2003, as it shows code and ASPX pages using the Code Behind model. Don't worry if you don't have Visual Studio .NET; it should be relatively easy to use the code in your own Code Editor like Macromedia Dreamweaver MX or Notepad.

Counting Users

One of the easiest ways to count individual users is in the Session_Start event that you can define in the Global.asax file. This event is fired whenever a user requests the first page in your Web site. This way, you have the ability to count each unique visitor only once during their visit. As long as the Session remains active on the server, the user won't be counted again. After the Session has timed out (it will automatically time out after a certain interval when no new pages have been requested) or has been explicitly ended, a request to a page will create a new Session, and the user will be counted again.

To keep track of the total number of users that have visited your site since you started the Web server, you can increase a counter for each request a user makes. Let's call this counter TotalNumberOfUsers. You can store that counter in a static variable in the Global class. This Global class is defined in the file Global.asax.cs, the Code Behind file for Global.asax. By creating static variables, you can be sure there is always just one instance of your hit counter variable present. Because the class defined in the Global.asax file is accessible throughout the entire application, you can retrieve and display the value for the counter on other pages in your site.

You can also create a second counter, called CurrentNumberOfUsers for example, that counts the number of active Sessions on your server. Just as with TotalNumberOfUsers, you increase the value of this counter whenever a new Session is started. However, you should decrease its value again when the Session ends. so you can keep track of the number of users that are currently visiting your site.

Let's take a look at how you can accomplish this:

You should start by making sure you have a file called Global.asax (note that the extension is different from ordinary aspx pages) in the root of your Web site. Usually, when you create a new Visual Studio ASP.NET Web Application, the Global.asax file is already there, and the skeleton for important events like Session_Start and Session_End are already present.
If you don't have the file, open the Visual Studio .NET Solution Explorer (Ctrl+Alt+L), right-click your Web project and choose Add | Add New Item... from the context menu. Scroll down the list with Web Project Items until you see Global Application Class. Alternatively, expand Web Project Items and then click Utility to limit the list of Web items. Select the Global Application Class and click Open to add the Global.asax file to your Web site project.

The page will open in Design View so you'll need to click the link "click here to switch to code view" to view the Code Behind file for the Global.asax file. You'll see some default using statements, followed by the definition for the Global class. You'll expand this class by adding a few private variables for the two hit counters. These private variables will then be made accessible through public properties. Using properties instead of public fields helps keeping your code cleaner and more stable. Calling code is not allowed to just arbitrarily change the field's value; it has to change the value through a public Set method. In this example, you'll make the code even a bit more safe by removing the Set method altogether. This makes it impossible for calling code to change the value of the counter; all it can do is read its value.

Modifying Global.asax

1. Locate the code that starts with public class Global : System.Web.HttpApplication and add the following shaded lines of code:

Whenever the Web application starts, the Global class is constructed and the two hit counters will be initialized to 0.

2. The next step is to add code to the Session_Start event. This event will fire once for each user when they request the first page in your Web application, so this place is perfect for your hit counter. Inside this event, the values of the two hit counters are increased; one for the total number of users and one for the current number of users.
   Locate the skeleton for the Session_Start event and add the following code:

3. Just as with the Session_Start event, you'll need to write some code for the Session_End event. However, instead of increasing the counters, you should decrease the counter for the current number of users only. This means that whenever a user Session times out (usually 20 minutes after they requested their last page), the counter will be decreased, so it accurately holds the number of current users on your site. You should leave the counter for the total number of users untouched.
   Locate the Session_End event, and add this code:

Making Your Counters Accessible by Other Pages

Since the hit counters are stored in the Global class, you need some way to get them out of there, so you can display them on a management page for example. The easiest way to do this, is to create two public properties. Because the Global class is in many respects just an ordinary class, it is easy to add public properties to it.

To add the properties, locate the skeleton for the Application_End event, and add the following code right below it:

With these two read-only properties in place, your calling code is now able to access the values of your hit counters. For example, to retrieve the number of users browsing your site right now, you can use this code: HitCounters.Global.CurrentNumberOfUsers where HitCounters is the default namespace for your Web application as defined on the Property Pages for the Web project in Visual Studio .NET.

Testing it Out

To test out your hit counters, create a new Web form and call it HitCounter.aspx. You can save the form anywhere in your site. In Design View, add two labels to the page and call them lblTotalNumberOfUsers and lblCurrentNumberOfUsers respectively. Add some descriptive text before the labels, so it's easy to see what value each label will display. You should end up with something similar to this in Code View:

Press F7 to view the Code Behind for the hit counter page, and add the following code to the Page_Load event:

The first two lines of code retrieve the total number of visitors and the current number of visitors from the Global class. The next two lines simply display the values for the counters on the appropriate labels on the page.

To test it out, save the page and view it in your browser. You'll see there is one current user. Also, note that the total number of users is 1. Open another browser (don't use Ctrl+N, but start a fresh instance or use an entirely different brand of browser) and open the counter page. You'll see there are two current users, and two users in total. Wait until both the Sessions have timed out (the default timeout defined in Web.config is 20 minutes) and open the hit counter page again. You'll see there is one current user, but the total number of users has been maintained and should be 3.

Summary

This article demonstrated how to create a hit counter that keeps track of the current and total number of users to your site. It stores these counters in static variables in the Global class so they are available in each page in your Web site. A big disadvantage of storing these variables in static variables is that their values get lost when the Web server is restarted.

Two other articles on this site will demonstrate how you can save the counter value for the total number of users in a text file or in a database. By saving the counter to a text file or database, its value can be maintained, even when you restart or reboot the Web server.

• Howto Create a Hit Counter Using a Text File in ASP.NET (http://Imar.Spaanjaars.Com/227/howto-create-a-hit-counter-using-a-text-file-in-aspnet-1x)
• Howto Create a Hit Counter Using a Database in ASP.NET (http://Imar.Spaanjaars.Com/238/howto-create-a-hit-counter-using-a-database-in-aspnet-1x-with-c-sharp)

Related Articles

• When Sessions End - Once And For All! at www.asp101.com (http://www.asp101.com/articles/john/sessionsend/default.asp)

References

• Configuring ASP.NET Applications to Use the Appropriate Session State (http://technet2.microsoft.com/WindowsServer/en/Library/51aa77d2-4485-4cb9-a75f-9186dc5d775f1033.mspx)
• <sessionState> Element (http://msdn.microsoft.com/library/en-us/cpgenref/html/gngrfsessionstatesection.asp)

Download Files

• Source Code for this Article (http://Imar.Spaanjaars.Com/Downloads/Articles/HitCounterInStaticVariables.zip)