This example shows two aspects that cause a field or a property to be backed by a persistent storage:

• RegistryValueAttribute is an aspect that persists a field or property into a Windows registry value. It is useful in Windows applications that use a lot of registry settings.

• AppSettingValuesAttribute retrieves the value of a field or property from the <appSettings> section of app.config or web.config.

Both aspects work in a similar way. Both aspect classes contain a field named isFetched, which is set to true when the value has already been fetched from the storage. This is an optimization that avoids the application from retrieving and converting the value several times. When the field or property value is retrieved, but the isFetched field is set to false, then the aspect retrieves the value from the storage, converts it from a string to the field/property type, and sets the isFetched field to true.

The RegistryValueAttribute is modifies the registry value when the value of the target field or property is set. However, the AppSettingsValueAttribute is not allowed to change the content of the app.config or web.config file. Therefore, changing the value of the target field or property is forbidden after the field or property value has been retrieved. However, changing the value before the first retrieval is allowed: this is how you should set the default value of this setting.

Both aspect classes use the [MulticastAttributeUsage(TargetMemberAttributes = MulticastAttributes.Static)] custom attribute to specify that they can be applied only to static fields and properties.

In this example, we apply the RegistryValueAttribute custom attribute to the TestRegistryValues class, which has the effect to persist all static fields of this class to registry. We're doing the same with the AppSettingsValueAttribute custom attribute and the TestAppSettingsValue class. If you want to persist only specific fields or properties, remove the custom attribute from the class and add it to these specific fields or properties.

Demonstrated techniques

The aspect demonstrates the use of LocationInterceptionAspect to enhance fields and properties, and [MulticastAttributeUsage] to restrict the aspects to static fields or properties.