s COMPLETING THE FRAMEWORK in Visual Basic .NET

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CHAPTER 5 s COMPLETING THE FRAMEWORK
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This approach is useful for lists, but not reports. By my definition, lists require relatively small amounts of data, so it s acceptable to transfer that data to a client and generate the report there. Reports, on the other hand, require processing large amounts of data, and the closer you can do this to the database, the better. In this case, directly using Crystal Enterprise or some other server-based reporting tool to generate the report physically close to or in the database is often the best solution.
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ObjectAdapter
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The Csla.Data.ObjectAdapter class is a utility that generates a DataSet (or more accurately, a DataTable in a DataSet) based on an object (or a collection of objects). This isn t terribly difficult, because reflection can be used to get a list of the properties or fields on the objects, and then loop through the objects properties to populate the DataTable with their values. ObjectAdapter is somewhat similar to a data adapter object such as OleDbDataAdapter, in that it implements a Fill() method that fills a DataSet with data from an object or a collection. To implement a Fill() method that copies data from a source, such as a business object, into a DataSet, ObjectAdapter needs to support a certain amount of basic functionality. In ADO.NET, data is stored in a DataTable, and then that DataTable is held in a DataSet. This means that object data will be copied into a DataTable object. To do this, ObjectAdapter needs to get a list of the properties exposed by the source object. That list will be used to define the list of columns to be created in the target DataTable object. Alternatively, it will also support the concept of a preexisting DataTable that already contains columns. In that case, ObjectAdapter will attempt to find properties in the source object that match the columns that already exist in the target DataTable object. Also, rather obviously, the data values from the original data source must be retrieved. Reflection will be used to do this because it allows dynamic retrieval of the values.
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Operational Scope
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Figure 5-5 illustrates the possible data sources supported by the ObjectAdapter class.
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Figure 5-5. Data sources supported by ObjectAdapter
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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The code could be simplified by only supporting binding to an object but by supporting any valid data source (including ADO.NET objects, or arrays of simple values), it provides a more flexible solution.
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Ultimately, a list of column, property, or field names will be retrieved from the data source, whether that be a DataView, an array or a collection, simple types (such as Integer or String), or complex types (such as a Structure or an object). In the end, all data sources implement the IList interface that s defined in the .NET Framework. However, sometimes some digging is required to find that interface; or it must be added by creating a collection. Some data source objects, such as a DataSet, don t expose IList directly. Instead, they expose IListSource, which can be used to get an IList. In the case of simple types such as a string or a business object, an ArrayList is created and the item is placed inside it, thus providing an IList with which to work.
Fill Method
Like the OleDbDataAdapter, the ObjectAdapter implements a Fill() method (actually, several overloads of Fill() for easy use). In the end, though, they all route to a single Fill() method that fills a DataTable from data in a source object: Public Sub Fill(ByVal dt As DataTable, ByVal source As Object) If source Is Nothing Then Throw New ArgumentException(My.Resources.NothingNotValid) End If Dim columns As List(Of String) = GetColumns(source) If columns.Count < 1 Then Exit Sub ' create columns if needed For Each column As String In columns If Not dt.Columns.Contains(column) Then dt.Columns.Add(column) End If Next ' get an IList and copy the data CopyData(dt, GetIList(source), columns) End Sub The first thing this method does is get a list of column names (typically, the public properties and fields) from the data source. It does this by calling a GetColumns() method (which will be covered later). Next, the target DataTable is checked to ensure that it has a column corresponding to every column name retrieved from GetColumns(). If any columns are missing, they are added to the DataTable: For Each column As String In columns If Not dt.Columns.Contains(column) Then dt.Columns.Add(column) End If This ensures that all properties or fields from the data source have a column in the DataTable so they can be copied. With that done, all that remains is to initiate the copy of data from the source object to the DataTable: CopyData(dt, GetIList(source), columns)
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