Application Settings

Saving objects in persistent state is such a common task that the isolated storage API includes an IDictionary<string, object> class named IsolateStorageSettings that is automatically persisted to isolated storage when your app is closed or tombstoned. IsolateStorageSettings is a convenient way to store global application state (see Figure 28.2).

FIGURE 28.2 An app’s IsolatedStorageSettings instance is retrieved using the IsolatedStorageSettings.ApplicationSettings property.

IsolateStorageSettings is a singleton, accessible via the IsolatedStorageSettings.ApplicationSettings property. The ApplicationSettings property can be used the same as you would any other IDictionary.

To place an object in persistent state, add a key/value pair to the collection, like so:

IsolatedStorageSettings.ApplicationSettings["Foo"] = "Bah";

To retrieve the item from persistent state, the following can be used:

string value = (string)IsolatedStorageSettings.ApplicationSettings["Foo"];

If it is uncertain whether an item exists in ApplicationSettings, use the TryGetValue method as shown:

Click here to view code image

string value;
if (!IsolatedStorageSettings.ApplicationSettings.TryGetValue("Foo",
                                                             out value))
{
    value = "Unknown";
}

The Contains method can also be used to test whether an item exists in the dictionary.

When the user exits your app, Windows Phone automatically calls Save to preserve the IsolatedStorageSettings object so that it can be automatically reinstated when your application starts again.