s BUSINESS OBJECT IMPLEMENTATION in C#.NET

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CHAPTER 8 s BUSINESS OBJECT IMPLEMENTATION
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When this rule is run, it loops through the list of Role objects in the parent Roles collection to see if any other child object has the same Id value. If there s a match, the method returns false, otherwise it returns true. The rule method is associated with the Id property in the AddBusinessRules() method: protected override void AddBusinessRules() { ValidationRules.AddRule( Csla.Validation.CommonRules.StringRequired, "Name"); ValidationRules.AddRule(NoDuplicates, "Id"); } This custom rule ensures that duplicate Id values are caught as they are entered, so that the data doesn t have to be sent to the database to find out about the problem. As you ll see in 9, this is particularly nice in a Windows Forms UI, since the user gets instant and automatic feedback about what is wrong.
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The first object discussed in the chapter was Project, and I covered all the code in that class except for the Exists() command implementation. Many objects can benefit from implementation of an Exists() command, as it allows the UI to quickly and easily determine if a given object s data is in the database without having to fully instantiate the object itself. Ideally, a UI developer could write conditional code like this: if (Project.Exists(productId)) Implementing an Exists() command also provides an opportunity to make use of Csla.CommandBase to create a command object. This makes sense, since all an Exists() command needs to do is run a stored procedure in the database and report on the result.
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The Project class itself has a static method called Exists(), which is public so it can be called from UI code: public static bool Exists(Guid id) { ExistsCommand result; result = DataPortal.Execute<ExistsCommand> (new ExistsCommand(id)); return result.Exists; } While this code is somewhat like other factory methods, it is different in one key way. It creates an instance of an ExistsCommand object, and has the data portal execute the object on the application server: result = DataPortal.Execute<ExistsCommand> (new ExistsCommand(id)); Notice how the id parameter value is used to initialize the ExistsCommand object as it is created. The important thing to understand is that the ExistsCommand object is created and initialized on the client, and then the data portal transfers the object to the server where the object s DataPortal_ Execute() method is run.
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CHAPTER 8 s BUSINESS OBJECT IMPLEMENTATION
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This means that a command object can do work on the client, then do work on the server, and then do more work on the client. In this particular case, it is initialized on the client, executes a stored procedure on the server, and then exposes the result as a property back on the client. That result value is returned as the result of the Exists() method.
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The real work occurs in the command object itself: ExistsCommand. The ExistsCommand class inherits from Csla.CommandBase and is declared as a private nested class within Project: [Serializable()] private class ExistsCommand : CommandBase Not all command objects are nested within other business classes, but in this case, it makes sense. There s no need for the UI developer to be aware of the ExistsCommand class or its implementation details; they only need to know about the Project.Exists() method. In other cases, you may have public command objects that are directly used by the UI. A good example would be a ShipOrder object that is responsible for shipping a sales order. It is quite realistic to expect that the UI would want to directly ship a sales order, and so there s value in being able to call a ShipOrder.Ship(orderId) method. Command objects, whether public or private, tend to be very simplistic in terms of their structure. ExistsCommand declares some instance fields, one property, and a constructor: private Guid _id; private bool _exists; public bool Exists { get { return _exists; } } public ExistsCommand(Guid id) { _id = id; } The constructor initializes the _id field, so that value is available when the command is executed on the server. The _exists field is set as a result of the command running on the server and is exposed through the Exists property. The code that runs on the server is entirely contained within the DataPortal_Execute() method: protected override void DataPortal_Execute() { using (SqlConnection cn = new SqlConnection(Database.PTrackerConnection)) { cn.Open(); using (SqlCommand cm = cn.CreateCommand()) { cm.CommandType = CommandType.StoredProcedure; cm.CommandText = "existsProject"; cm.Parameters.AddWithValue("@id", _id); int count = (int)cm.ExecuteScalar(); _exists = (count > 0); } } }
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