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CHAPTER 5 s COMPLETING THE FRAMEWORK
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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 { Type sourcetype = 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 PropertyInfo prop = sourcetype.GetProperty(fieldName); if(prop == null || !prop.CanRead) 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 return prop.GetValue(obj, null).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 FieldInfo field = sourceType.GetField(fieldName); if (field == null) If there s no field by this name, then an exception is thrown to indicate that the GetField() method was unsuccessful: throw new DataException( 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(Exception ex) { throw new DataException( Resources.ErrorReadingValueException + " " + fieldName, ex); } 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 struct or 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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CHAPTER 5 s COMPLETING THE FRAMEWORK
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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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3 added authorization code to business objects, making them aware of whether each property can be read or changed. The CanReadProperty() and CanWriteProperty() methods were made public so that code outside the object could easily determine whether the current user is allowed to get or set each property on the object. One primary user of this functionality is the UI, which can decide to alter its appearance to give users clues as to whether they are able to view or alter each piece of data. While this could be done by hand for each control on every form, the ReadWriteAuthorization control helps automate the process of building a UI that enables or disables controls based on whether properties can be read or changed. If a control is bound to a property, and the user does not have read access to that property due to authorization rules, the ReadWriteAuthorization control will disable that control. It also adds a handler for the control s Format event to intercept the value coming from the data source, substituting an empty value instead. The result is that data binding is prevented from displaying the data to the user. Similarly, if the user doesn t have write access to a property, ReadWriteAuthorization will attempt to mark any controls bound to that property as being read-only (or failing that, disabled); ensuring that the user can t attempt to alter the property value. Like all Windows Forms components, extender controls inherit from System.ComponentModel. Component. Additionally, to act as an extender control, the ReadWriteAuthorization control must implement the IExtenderProvider interface: [DesignerCategory("")] [ProvideProperty("ApplyAuthorization", typeof(Control))] public class ReadWriteAuthorization : Component, IExtenderProvider { public ReadWriteAuthorization(IContainer container) { container.Add(this); } } The [ProvideProperty()] attribute is quite important. It specifies that ReadWriteAuthorization extends components of type Control by adding an ApplyAuthorization property to them. In other words, when a ReadWriteAuthorization control is on a form, all other controls on the form get a dynamically added ApplyAuthorization property. Figure 5-6 shows a text box control s Properties window with the dynamically added ApplyAuthorization property. The UI developer can set this property to true or false to indicate whether the ReadWriteAuthorization control should apply authorization rules to that particular control. You ll see how this works as the control is implemented. The [DesignerCategory()] attribute is just used to help Visual Studio decide what kind of visual designer to use when editing the control. The value used here specifies that the default designer should be used.
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