Once a authentication token has been obtained, you can request a paged response of all the applications. The application GUID is particularly important for updating an application via the API.
PagedResponseCollection<ListAllAppsResponse> apps = await client.Apps.ListAllApps();
foreach (var app in apps)
{
Console.WriteLine("Application {0}, guid {1}, state {2}", app.Name, app.EntityMetadata.Guid, app.State);
}