By default if a user now uses our GridView UI to enter a non-valid UnitsOnOrder/Discontinued combination, our LINQ to SQL data model classes will raise an exception.  The <asp:LinqDataSource> will in turn catch this error and provides an event that users can use to handle it.  If no one handles the event then the GridView (or other) control bound to the <asp:LinqDataSource> will catch the error and provide an event for users to handle it.  If no one handles the error there then it will be passed up to the Page to handle, and if not there to the global Application_Error() event handler in the Global.asax file.  Developers can choose any place along this path to insert appropriate error handling logic to provide the right end-user experience.

For the application we defined above, probably the best place to handle any update errors is by handling the RowUpdated event on our GridView.  This event will get fired every time an update is attempted on our datasource, and we can access the exception error details if the update event fails.  We can add the below code to check if an error occurs, and if so display an appropriate error message to the end-user:

Figure 30

Notice above how we have not added any validation specific logic into our UI.  Instead, I am retrieving the validation error message string we raised in our business logic and am using it to display an appropriate message to the end-user (I am then displaying a more generic error message in the event of other failures). 

Notice how I'm also indicating above that I want the GridView to stay in Edit mode when an error occurs - that way the user can avoid losing their changes, and can modify the values they entered and click "update" again to try and save them.  We can then add a <asp:literal> control with the "ErrorMessage" ID anywhere we want on our page to control where where we want the error message to be displayed:

Figure 31

And now when we try and update the Product with an invalid value combination we'll see an error message indicating how to fix it:

Figure 32

The nice thing about using this approach is that I can now add or change my data model's business rules and not have to modify any of my UI tier's code to have them pick up and honor the changes.  The validation rules, and corresponding error messages, can be written and centralized in one place in my data model and automatically applied everywhere.