RoleList in Visual Basic .NET

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RoleList
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The final object used by Project, ProjectResources, ProjectResource, and Assignment is the RoleList collection. This is a name/value list based on the Roles table from 6. The name (key) values are of type Integer, while the values are the String names of each role. The CSLA .NET framework includes the NameValueListBase class to help simplify the creation of name/value list objects. Such objects are so common in business applications that it is worth having a base class to support this one specialized scenario. 7 includes a template for name/value list classes, and RoleList will follow that template. It includes the Business Methods, Factory Methods, and Data Access regions. The class is declared like this:
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CHAPTER 8 s BUSINESS OBJECT IMPLEMENTATION
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<Serializable()> _ Public Class RoleList Inherits NameValueListBase(Of Integer, String) Notice the generic type parameters. The first specifies the data type of the name or key, while the second specifies the data type of the value. These data types are used to define the name and value types of the NameValuePair child objects contained in the collection.
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The only business method in this class is DefaultRole(), which returns the default role for a resource newly assigned to a project. Not all name/value collections will provide a method to specify the default role, but it is often helpful. Recall that this method is used by ProjectResource as a new ProjectResource object is created. Here s the method: Public Shared Function DefaultRole() As Integer Dim list As RoleList = GetList() If list.Count > 0 Then Return list.Items(0).Key Else Throw New NullReferenceException( _ "No roles available; default role can not be returned") End If End Function The implementation in this application is very simplistic, as it just returns the first item in the collection. In a more complex application, the default value might be specified in the database.
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Factory Methods
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As in the template from 7, RoleList implements a form of caching to minimize load on the database. The GetList() factory method stores the collection in a Shared field and returns it if the object has already been loaded. It only goes to the database if the cache field is Nothing: Private Shared mList As RoleList Public Shared Function GetList() As RoleList If mList Is Nothing Then mList = DataPortal.Fetch(Of RoleList) _ (New Criteria(GetType(RoleList))) End If Return mList End Function Remember that NameValueListBase defines a Criteria class, so one doesn t need to be declared in every business class. As long as no filtering is required, that basic Criteria class can be used; and it meets the needs of RoleList just fine.
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If you do need to filter the name/value list results, you ll need to declare your own criteria class in the Data Access region just like you would with any other root object.
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CHAPTER 8 s BUSINESS OBJECT IMPLEMENTATION
In case the cache needs to be flushed at some point, there s also an InvalidateCache() method: Public Shared Sub InvalidateCache() mList = Nothing End Sub By setting the Shared cache value to Nothing, the cache is reset. The next time any code calls the GetList() method, the collection will be reloaded from the database. This InvalidateCache() method will be called by the Roles collection later in the chapter. Of course, there s also a non-Public constructor in the class to enforce the use of the factory method to retrieve the object.
Data Access
Finally, there s the DataPortal_Fetch() method that loads the data from the database into the collection: Private Overloads Sub DataPortal_Fetch(ByVal criteria As Criteria) Me.RaiseListChangedEvents = False Using cn As New SqlConnection(Database.PTrackerConnection) cn.Open() Using cm As SqlCommand = cn.CreateCommand cm.CommandType = CommandType.StoredProcedure cm.CommandText = "getRoles" Using dr As New SafeDataReader(cm.ExecuteReader) IsReadOnly = False With dr While .Read() Me.Add(New NameValuePair( _ .GetInt32("id"), .GetString("name"))) End While End With IsReadOnly = True End Using End Using End Using Me.RaiseListChangedEvents = True End Sub As with the DataPortal_Fetch() method in Project, the code here opens a connection to the database, sets up a SqlCommand object, and executes it to get a SafeDataReader object. The code then loops through that data reader and creates a new NameValuePair object for each row. Since the collection is normally read-only, the IsReadOnly property is set to False before loading the data and then restored to True once the data has been loaded. The result is a fully populated name/value list containing the data from the Roles table in the database. This completes the Project object family, including all collaboration objects. Next, I ll walk briefly through the Resource object family.
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