Code sample: Validation (WPF)

This sample shows DevForce validation at work in a WPF application.

• Platform: WPF
• Language: C#, VB
• Download: Validation (WPF)
  
  

Problem

You'd like to better understand DevForce validation and see it in action.

Solution

The sample shows a simple application displaying employees and their orders.  As you modify various fields you can see property-level validation in action.  For example, the First Name and Last Name fields can't contain more than 30 characters.

These are examples of simple declarative validation through standard attributes.  By default when you generated the code for your model DevForce added validation attributes for string length restrictions and required fields:

These validation attributes help ensure that your simple database-defined rules are enforced.

Although not shown in the sample, you can also add custom attributes, and define rules to enforce your business logic using both predefined and fully custom verifiers.

Exceptions vs. notifications

The DevForce verification engine by default is set to an ErrorNotificationMode of Notify.  This means that when a property validation fails DevForce will use the INotifyDataErrorInfo and IDataErrorInfo interfaces to alert the binding of the problem.

Unfortunately, WPF bindings don't recognize INotifyDataErrorInfo at all, and don't by default listen for IDataErrorInfo. 

This leaves us with two choices:  we can modify the DevForce default to throw validation exceptions, or we can modify the binding to listen on the IDataErrorInfo interface.

Let's take exceptions first.  We must do both of the following: 

1. Change the ErrorNotificationMode on the VerifierEngine to ThrowException.
2. Set the bindings to use ValidatesOnExceptions = "true".

If you dragged properties from a data source using the designer it created bindings that look something like this: