02 July 2009

Pile on the layers

Here's a quote from MSDN:

"Most ASP.NET data source controls, such as the SqlDataSource, are used in a two-tier application architecture where the presentation layer (the ASP.NET Web page) communicates directly with the data tier (the database, an XML file, and so on)."

This is a cute way of saying that all those nice data-bound ASP.net controls - the ones that are supposed to take a data source and then do all the work of Selecting, Inserting, Updating, and Deleting (ie, CRUD) for you - will not work in a 3-layered structure.

That was the bad news, except that we knew it all along.

Here comes the good news:

"The ObjectDataSource works with a middle-tier business object to select, insert, update, delete, page, sort, cache, and filter data declaratively without extensive code."


So what do we need to do in order to get the data-sourced controls to work with an ObjectDataSource and, by extension, a BL and DAL?

Creating an ObjectDataSource Control Source Object has some answers. We need to define a stateless class (no non-static members) to provide CRUD logic for the data to populate the data-bound control on our form. Optionally, this class can also provide functions to filter the data and sort it. Then, using the (very handy) wizard provided by the ObjectDataSource, we select the class with the CRUD functions and identify the parameters it needs to preform Select.

Very well, but what about the parameters for Insert, Update, and Delete?

We can define another class to represent a record/row in our data schema. (The TypeName property of the ObjectDataSource must be set to this class.) All properties of the class will be polled (by reflection, one presumes) in order to fill the data-sourced control, and will be filled in return for the Update function. By setting the ConflictDetection property of the ObjectDataSource, we can even decide how updates should be done:

OverwriteChanges will simply fill an object with the new values and pass it to the Update function.
CompareAllValues will fill two objects: the first with the old values, the second with the new, and pass both to the Update function.

Two items of note:
1. The wizard will only work if your latest working build contains all the classes and functions you want to use! In other words, Build your project before running the wizard.

2. The CRUD functions I'm describing here are in my BL. They themselves do NOT do the actual persistence to the DB; rather they massage the data as appropriate and pass it in the appropriate format to the DAL, which does the real work.

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