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by Paul Vick.
Original Post: Default Instances
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Fresh off of writer's block, I thought I'd dive straight back into the sea of controversy and talk about a feature called "default instances." Default instances are a "new" feature in VB 2005 that is really the return of a very old feature, one that's been around for a long time. Now, the return of default instances has stirred some very passionate debate, but what I'm going to do is address this question in three separate entries. In this entry, I'm going to describe what default instances are at a technical level. In my next entry, I'm going to talk about what I see as the positive case for default instances. Then, in the final entry, I will talk about some limitations of default instances and address the controversy about their reintroduction more directly. You can choose whether or not you want to wait for the last entry before you start throwing brickbats...
So, what are default instances? Well, they're exactly what their name suggests: a way to provide a default instance of a class that you can use without having to New the class. If you've used versions of VB prior to VB 2002 (i.e. pre-.NET), you'll probably have come across the most common default instance, the form default instance. For example, in VB 6.0, if you created a form named Test, you could say in code:
Test.Show()
And voila! an instance of the form would just magically appear. What was going on behind the scenes was a bit of magic. When you referred to "Test," the VB runtime would go check and see if it already had a default instance of Test lying around. If it did, then it would hand back that default instance when you called Show(). If there wasn't a default instance lying around, the runtime would create a new instance of Test and use that as the default instance going forward.
Default instances disappeared in VB 2002 and VB 2003, but are making a comeback in VB 2005. The high-level "why," I'll leave to my second entry, so let's talk about the "how" for the moment. Hopefully, all of you are familiar with the introduction of the My namespace in VB 2005. (If not, go read this MSDN article and then come back.) One part of the My namespace is the Forms object. If you bring up a VB 2005 project with a Form named Test in it, you'll notice that the My.Forms object has a property called Test. This property returns a default instance of Test. So you can say, sort of like you did in VB 6.0, "My.Forms.Test.Show()" and voila! up pops a new instance of Test. What happens behind the scenes is that the Test property of My.Forms checks to see if it's already returned an instance of Test and, if it has, it just returns that instance. If it hasn't returned an instance yet, it just creates a new instance. Pretty simple.
We also extended default instances to work in exactly the same way that VB 6.0 default instances did. Besides saying My.Forms.Test.Show(), you can also just say Test.Show() and it will also access the default instance. Essentially, Test.Show() is a shortcut to saying My.Forms.Test.Show().
There are, however, a couple of differences between the way default instances are implemented in VB6 and the way they are implemented in VB 2005:
In VB6, if you tried to use the Is operator to test the default instance to see if it was Nothing, the expression would always evaluate to False. This is because referring to the default instance caused it to be created if it didn't exist! In VB 2005, we recognize when you are testing the default instance using the Is (or IsNot) operator and won't auto-create the instance in that case. So you can see if the default instance exists or not.
A common source of default instance errors was using the default instance instead of Me within the default instance's type. For example, if Form1's load method has the statement "Form1.BackColor = Color.Red", this will only change the background color of the deafult instance's form. If you create a new instance of Form1 that is not the default instance, it's background color will not be red! To assist users in this case, we give an error when you refer to the default instance inside of the type, telling you to use Me instead.
Well, that's about it for the technical side of things. Now onto the good, the bad and the ugly...
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