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The ValidationRules object maintains a list of rule methods associated with an object. It also executes those methods to check the rules, either for a specific property or for all properties. The end result of that process is that descriptions for broken rules are recorded into the BrokenRulesCollection associated with the business object. The BrokenRulesCollection is a list of BrokenRule objects. Each BrokenRule object represents a validation rule that is currently broken by the data in the business object. These BrokenRule objects are added and removed from the collection by ValidationRules as part of its normal processing. The BusinessBase class uses its BrokenRulesCollection to implement an IsValid property. IsValid returns True only if BrokenRulesCollection contains no items. If it does contain items, the object is in an invalid state. The primary point of interest with the BusinessRulesCollection is that it is designed to not only maintain a list of current broken rules but also to provide read-only access to the UI. This is the reason for implementing a specialized collection object that can change its own data but is seen by the UI as being read-only. On top of that, the base class implements support for data binding so that the UI can display a list of broken rule descriptions to the user by simply binding the collection to a list or grid control. Additionally, the implementation of IDataErrorInfo uses BrokenRulesCollection to return error text for the object or for individual properties. Supporting this interface allows WPF data binding and the Windows Forms DataGridView and ErrorProvider controls to automatically display validation error text to the user.

If you consider the validation rules applied to most properties, there s a set of common behaviors that occur time and time again. For example, there s the idea that a string value is required or that a string has a maximum length. Rather than requiring every business application to implement these same behaviors over and over again, you can have them be supplied by the framework. As you ll see in 11, the implementation makes use of reflection so there s a performance cost. If you find in your particular application that performance cost to be too high, you can always do what you would have done anyway that is, write the rule implementation directly into the application. In most cases, however, the benefit of code reuse will outweigh the small performance cost incurred by reflection.

public ref class XYZConnection { [DllImport("XYZLib.dll")] static XYZHandle^ XYZConnect(); [DllImport("XYZLib.dll")] static double XYZGetData(XYZHandle^ xyzHandle); XYZHandle^ xyzHandle; public: XYZConnection() { xyzHandle = XYZConnection::XYZConnect(); } ~XYZConnection() { delete xyzHandle; } double GetData() { if (this->xyzHandle->IsInvalid) throw gcnew ObjectDisposedException("XYZConnection"); return XYZConnection::XYZGetData(this->xyzHandle); } }; There is an important limit of SafeHandle that you must be aware of. The P/Invoke layer marshals SafeHandle-derived types to a native handle of the native pointer size 32-bit on a 32-bit CLR and 64-bit on a 64-bit CLR. If the wrapped native API works with handles that have a different size, SafeHandle must not be used. As an example, the Event Tracing for Windows API uses 64-bit handles, even in the Win32 API. For more information on this API, consult the documentation of the RegisterTraceGuids function. If your wrapper library explicitly allows callers with restricted CAS permissions (which is not covered in this book), I highly recommend using SafeHandle, because it avoids a special exploit: the handle-recycling attack. For more information on handle-recycling attacks, see http://blogs.msdn.com/bclteam/archive/2006/06/23/644343.aspx and www. freepatentsonline.com/20060004805.html.

Supporting object persistence the ability to store and retrieve an object from a database can be quite complex. This was covered earlier in the chapter during the discussion about basic persistence and the concept of ORM. As you ll see in 18, data access logic is encapsulated within the formal data access layer assembly, which is invoked by the business objects. This data access assembly must be deployed to the physical tier that will execute the data access code. At the same time, however, you don t want to be in a position in which a change to your physical architecture requires every business object in the system to be altered. The ability to easily switch between having the data access code run on the client machine and having it run on an application server is the goal, with that change driven by a configuration file setting. On top of this, when using an application server, not every business object in the application should be directly exposed by the server. This would be a maintenance and configuration nightmare because it would require updating configuration information on all client machines any time a business object is added or changed.

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