s COMPLETING THE FRAMEWORK in VB.NET

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CHAPTER 5 s COMPLETING THE FRAMEWORK
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Catch ex As Exception Throw New System.Data.DataException( _ My.Resources.ErrorReadingValueException & " " & fieldName, ex) End Try End If End If Return result End Function One of the supported data sources is an ADO.NET DataView. A DataView contains a list of DataRowView objects. Because of this, GetField() handles DataRowView objects as follows: Dim dataRowView As DataRowView = TryCast(obj, DataRowView) If dataRowView IsNot Nothing Then ' this is a DataRowView from a DataView result = dataRowView.Item(fieldName).ToString The source list might also be an array of simple values such as Integer. In that case, a simple value is returned: ElseIf TypeOf obj Is ValueType AndAlso obj.GetType.IsPrimitive Then ' this is a primitive value type result = obj.ToString Similarly, the data source might be an array of String data, as shown here: Dim tmp As String = TryCast(obj, String) If tmp IsNot Nothing Then ' this is a simple string result = obj.ToString If the data source was none of these, then it s a more complex type a Structure or an object. In this case, there s more work to do, since reflection must be used to find the property or field and retrieve its value. The first thing to do is get a Type object in order to provide access to type information about the source object, as follows: ' this is an object or Structure Try Dim sourcetype As Type = obj.GetType The code then checks to see if there s a Property with the name of the specified column, as shown here: ' see if the field is a property Dim prop As PropertyInfo = sourcetype.GetProperty(fieldName) If prop Is Nothing OrElse Not prop.CanRead Then If there s no such property (or if the property isn t readable), then the assumption is that there s a matching field instead. However, if there is a readable property, its value is returned: Else ' found a property, return its value result = prop.GetValue(obj, Nothing).ToString On the other hand, if no readable property is found, then a similar process is used to look for a field: ' no readable property of that name exists - check for a field Dim field As FieldInfo = sourcetype.GetField(fieldName) If field Is Nothing Then
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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If there s no field by this name, then an exception is thrown to indicate that the GetField() method was unsuccessful: Throw New System.Data.DataException( _ My.Resources.NoSuchValueExistsException & " " & fieldName) However, if there is a matching field, then its value is returned, as follows: ' got a field, return its value result = field.GetValue(obj).ToString If any other exception occurs during the process, it is caught and included as an inner exception. The reason for doing this is so the exception message can include the field name that failed to make debugging easier: Catch ex As Exception Throw New System.Data.DataException( _ My.Resources.ErrorReadingValueException & " " & fieldName, ex) End Try The end result is that the GetField() method will return a property or field value from a row in a DataView, from an array of simple values, or from a Structure or an object. At this point, the ObjectAdapter is complete. Client code can use the Fill() methods to copy data from virtually any object or collection of objects into a DataTable. Once the data is in a DataTable, commercial reporting engines such as Crystal Reports or Active Reports can be used to generate reports against the data.
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Much of the focus in 3 was on ensuring that business objects support Windows Forms data binding. That support from the objects is useful, but can be made even more useful by adding some functionality to each form. This can be done using a type of Windows Forms control called an extender control. Extender controls are added to a form, and they in turn add properties and behaviors to other controls on the form, thus extending those other controls. A good example of this is the ErrorProvider control, which extends other controls by adding the ability to display an error icon with a tooltip describing the error.
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