Building Our First Component

Earlier, we visited our first example of using an ASP component—the BrickCalc component—and we promised that we'd return to see how to build the component. The Brick Calculator component is a Windows Script Component (WSC), written using the Windows Script Component Wizard and Notepad. WSCs are an ideal way to prototype components, allowing us to write the components using VBScript or JScript, which means that they can be written in a matter of minutes.

 

If this is the first time you've heard of WSCs, then you might be asking yourself the same sorts of questions I asked myself when I first heard about them. For example, we've already said that a COM object is a binary file—so how does WSC create a COM object using VBSscript or JScript? Moreover, how will we deal with registration and getting a CLSID? How do we get an interface to work with?

A Similar Case

WSCs work in a very similar way to the OLE DB simple providers in Visual Basic. An OLE DB simple provider is a two-tiered component made up of your simple Visual Basic (or  C++ or J++ , for example) code and a DLL. The DLL processes the code you've written, and exposes a regular OLE DB programming interface to its consumers.

 

WSCs (previously known as Scriptlets) work in much the same way. Whenever we register a script-based COM object, the registry entry points to a COM-compliant executable called scrobj.dll. Furthermore, any script-based COM object has an extra key in the registry, which points to a script file. This file contains metadata, which defines both the executable code and the interfaces to be made externally available.

 

Without further ado, let's build our first component for use in ASP. In order to build the component, you'll need to download the WSC kit from http://msdn.microsoft.com/scripting. You'll need to download both the Windows Script Components (Windows Script Version 5) and the Windows Script Component Wizard—both of which are available at this site.

Planning the Brick Calculator Component

The Brick Calculator component has just one task to perform—calculating the number of bricks needed to build a wall. So, the component will implement one method called HowManyBricks(). Knowing that we only need one method to perform the required task, we need think about what information we should to supply the method, in the form of parameters, to perform the calculation.

 

If we were approaching this methodically in the real world, we could work out the size of the wall, check the size of the brick, and then divide the size of the wall by the size of the brick to find out how many bricks we need. To perform this calculation we'd need to know four things

 

q        The length of the wall

q        The height of the wall

q        The length of the bricks used

q        The height of the bricks used

 

We are defining one property of the component, BrickType, which will tell the component the type of brick we are using—breezeblock or house brick. This covers the length and the height of the bricks, which are hard coded into the component. We are then left with the length and height of the wall, which we will pass to our component as parameters to the method. Our method now looks like this:

 

   HowManyBricks(WallHeight, WallLength)

 

And that's all the information we need to create the shell of the component using the Windows Script Component Wizard—to which we will later add the functionality using script.

Using The Windows Script Component Wizard

This Wizard is made available from the Programs | Microsoft Windows Script option on the Start menu. When you start it up, you