Using a Singleton In an ASP.NET Application
This was just a bad idea to write, please ignore this post. I was tire, I was in a bad mood and my mind was no where near where it should have been to write this. This was one of those bad ideas that I thought up while driving home from a whirlwind trip to Atlanta. So please never ever reference this post again. :)
Recently I was asked how to use a Singleton in an ASP.NET application. To be more specific I was asked how would I use a singleton and make sure it was applied at the application level. If you have no idea what a Singleton is I will give you my simple definition. It is a pattern used to ensure that an object is only instantiated once. The classic example I offer is ensuring you only have one instance of a desktop application running.
I honestly was not sure about what I would do at the time, I felt I knew, but I was not sure about my thoughts. I was correct in what I was thinking at the time, but it was one of those moments where I just could not think of object names, etc. So it bothered me and as soon as I could get back to my laptop I did a quick and dirty example of what I was thinking.
Now I am not going to get into the details of how to implement a class as a singleton, there are lots of examples all over the web. What want to do is show how you might apply the principle at the application level of an ASP.NET application. In this example I will just store a the current time when an application in first kicked off. I will use a quasi formal singleton, meaning it is a property pattern I commonly use to create one instance of a value.
What I decided to do was modify the ASPPipeline HttpModule from my eBook. In ASP.NET there is an Application property on the current HttpContext. The Application property refers to the HttpApplication object for the web application. It defines many members at the application level, it is also allows you to add objects the same way you can to Session or ViewState. So what we are going to do here is just that, store a time value in the application object. Each time a page is requested my quasi singleton pattern will check to see if we have stored that value (object) and if not then store it.
Public ReadOnly Property StartTime() As String
Get
If IsNothing(HttpContext.Current.Application("StartTime")) Then
HttpContext.Current.Application("StartTime") = Now.ToString
End If
Return HttpContext.Current.Application("StartTime").ToString
End Get
End Property
This is a simple ReadOnly property called StartTime that returns a String. The first thing I do is to check and see if there is an object named “StartTime” established by using the IsNothing method. In C# the if statement would have used the != null check.
If the value has not been set then I set the value to the current time using the Now() method; HttpContext.Current.Application("StartTime") = Now.ToString.
Private Sub Application_BeginRequest(ByVal source As Object, _
ByVal e As EventArgs)
' Create HttpApplication and HttpContext objects to access
' request and response properties.
Dim application As HttpApplication = CType(source, HttpApplication)
sb.AppendFormat("BeginRequest - {0}{1}", Now.ToLongTimeString, vbCrLf)
sb.AppendFormat("Start Time - {0}{1}", StartTime, vbCrLf)
End Sub
I will not go into the details of the HttpModule, there are other posts about custom HttpModules and my eBook, where I go into details about this module. But basically in the BeginRequest event handler I am going to set the value in the application object by calling the StartTime property. Now each time a request is made to an ASP.NET application the initial value will be logged to the log file, while the other times for each event in the ASP.NET life cycle will get stored at the time they are called.