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ScanObject Method
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Back in the GetColumns() method, a ScanObject() method is called, passing the type of the child object as a parameter. The ScanObject() uses reflection against that type. If you recall, the GetColumns() method itself might also call ScanObject() if it detects that the source object wasn t a collection but was a single, complex Structure or object: ' the source is a regular object result = ScanObject(innerSource.GetType) The ScanObject() method uses reflection much like you ve seen in other methods within the framework. But in this case, it not only assembles a list of public properties, but also of public fields: Private Function ScanObject(ByVal sourceType As Type) As List(Of String) Dim result As New List(Of String) ' retrieve a list of all public properties Dim props As PropertyInfo() = sourceType.GetProperties() If UBound(props) >= 0 Then For column As Integer = 0 To UBound(props) If props(column).CanRead Then result.Add(props(column).Name) End If Next End If
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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' retrieve a list of all public fields Dim fields As FieldInfo() = sourceType.GetFields() If UBound(fields) >= 0 Then For column As Integer = 0 To UBound(fields) result.Add(fields(column).Name) Next End If Return result End Function Given that this code is similar to other code you ve seen earlier in the book, I won t go through it in detail. In the end, it returns a list of column names by finding the names of all public properties and fields.
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Copying the Data
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The last step in the Fill() method is to call a CopyData() method to copy the data from the source list to the DataTable. The list of column names from GetColumns() is also passed as a parameter, and that list is used to retrieve the data from each item in the source list. Private Sub CopyData( _ ByVal dt As DataTable, _ ByVal ds As IList, ByVal columns As List(Of String)) ' load the data into the DataTable dt.BeginLoadData() For index As Integer = 0 To ds.Count - 1 Dim dr As DataRow = dt.NewRow For Each column As String In columns Try dr(column) = GetField(ds(index), column) Catch ex As Exception dr(column) = ex.Message End Try Next dt.Rows.Add(dr) Next dt.EndLoadData() End Sub Before doing any changes to the DataTable object, its BeginLoadData() method is called. This tells the DataTable that a batch of changes are about to happen, so it suppresses its normal event-handling process. This not only makes the changes more efficient to process, but avoids the possibility of the UI doing a refresh for every little change to the DataTable. Then the method loops through all the items in the source list. For each item, a new DataRow object is created, the values are copied from the source object, and the DataRow is added to the DataTable. The GetField() method, which is key to this process, is discussed in the following section. When all the data has been copied into the DataTable, its EndLoadData() method is called. This tells the object that the batch of changes is complete so it can resume its normal event, index, and constraint processing.
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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GetField Method
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The workhorse of CopyData() is the GetField() method. This method retrieves the specified column property or field value from the source object. Given that the source object could be a simple or complex type, GetField() is relatively long: Private Shared Function GetField( _ ByVal obj As Object, ByVal fieldName As String) As String Dim result As String Dim dataRowView As DataRowView = TryCast(obj, DataRowView) If dataRowView IsNot Nothing Then ' this is a DataRowView from a DataView result = dataRowView.Item(fieldName).ToString ElseIf TypeOf obj Is ValueType AndAlso obj.GetType.IsPrimitive Then ' this is a primitive value type result = obj.ToString Else Dim tmp As String = TryCast(obj, String) If tmp IsNot Nothing Then ' this is a simple string result = obj.ToString Else ' this is an object or Structure Try Dim sourcetype As Type = obj.GetType ' see if the field is a property Dim prop As PropertyInfo = sourcetype.GetProperty(fieldName) If prop Is Nothing OrElse Not prop.CanRead Then ' no readable property of that name exists - check for a field Dim field As FieldInfo = sourcetype.GetField(fieldName) If field Is Nothing Then ' no field exists either, throw an exception Throw New System.Data.DataException( _ My.Resources.NoSuchValueExistsException & " " & fieldName) Else ' got a field, return its value result = field.GetValue(obj).ToString End If Else ' found a property, return its value result = prop.GetValue(obj, Nothing).ToString End If
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