Isolated storage gives you access to a small segment of hard-disk space, with certain limitations. For example we don't know exactly where our files are being stored. We also can't read the files left by another Silverlight application or recorded for another user. In essence, isolated storage provides carefully restricted, tamperproof file access for applications that need to store permanent information in the local PC, so that information can be retrieved the next time the user runs the application.

How it works:

Isolated storage provides a virtual file system that lets you write data to a small, user-specific and application-specific slot of space. There's no way to know beforehand exactly where the data will be written, and the default space limit is a mere 1 MB (although you can request that the user grant you more).

Is it similar to browser Cookie?

1. Essentially, isolated storage is the Silverlight equivalent of persistent cookies in an ordinary web page. It allows small bits of information to be stored in a dedicated location that has specific controls in place to prevent malicious attacks.
    
2. Isolated storage is persistent–unlike the browser cache, it never expires, and it's not removed if the user chooses to explicitly delete temporary Internet files.
    
3. Isolated storage isn't a good storage place for important documents, because they're never backed up, are easily deleted, and can be even more easily lost.
    
4. Isolated storage is intended to be a limited-size storage location for data, not a handcrafted replacement for HTTP Caching.

Scope of Isolated Storage:

With isolated storage, a unique storage location is created for every combination of user and application. In other words, the same computer can have multiple isolated storage locations for the same application, assuming each one is for a different user. Similarly, the same user can have multiple isolated storage locations, one for each Silverlight application. Isolated storage isn't affected by browser, so a Windows user switching from Internet Explorer to Firefox will get the same isolated storage location in both browsers.

What all can we store in an Isolated Storage: Good choices include user-specific details, user preferences, and information about recent user actions. Isolated storage is also great temporary storage.

So now we are ready to create a application which will demonstrate how it works.

Step 1: Create a Silverlight Application project and name it IsolatedStorage.

Step 2: Now we will try to understand how to write data and read data from these storages Medias. We will also see if we want to increase the space on the handdisk; how can we do that as well.

So we created a simple xmal like this:

<Grid x:Name="LayoutRoot"  Margin="10"  Background="WhiteSmoke" Width="340" Height="200" HorizontalAlignment="Center" VerticalAlignment="Center">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"></RowDefinition>
            <RowDefinition Height="Auto"></RowDefinition>         

        </Grid.RowDefinitions>

            <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" ></ColumnDefinition>
            <ColumnDefinition Width="Auto" ></ColumnDefinition>           

        </Grid.ColumnDefinitions>

            <TextBlock Text="Enter Your Name" Margin="3" Padding="5" HorizontalAlignment="Left" 
                       VerticalAlignment="Center" Grid.Row="0" Grid.Column="0">               
            </TextBlock>

            <TextBox x:Name="txtname" Margin="3" Padding="5"
                        HorizontalAlignment="Left" VerticalAlignment="Center"
                        Grid.Row="0" Grid.Column="1" Height="30" Width="200" ></TextBox>          

            <Grid Grid.Column="1" Grid.Row="1"  Margin="5" >
                <Grid.RowDefinitions>
                    <RowDefinition Height="50" ></RowDefinition>
                    <RowDefinition ></RowDefinition>
                    <RowDefinition ></RowDefinition>
            </Grid.RowDefinitions>

                <Grid.ColumnDefinitions>
                    <ColumnDefinition ></ColumnDefinition>
                    <ColumnDefinition ></ColumnDefinition>
                </Grid.ColumnDefinitions>

            <Button x:Name="btnWrite"  Margin="5" Padding="5" Width="90" Height="30"
                    Content="Click to Write" HorizontalAlignment="Center" VerticalAlignment="Top"
                    Click="btnWrite_Click">
            </Button>

            <Button x:Name="btnRead"  Margin="5" Padding="5" Width="90" Height="30"
                    Content="Click to Read"
                    HorizontalAlignment="Center" VerticalAlignment="Top" Click="btnRead_Click"
                    Grid.Row="0" Grid.Column="1">
            </Button>

            <Button x:Name="btnSize"  Padding="5" Width="180" Height="30"
                    Content="Click to Increase Disk Storage Space"
                     HorizontalAlignment="Center" VerticalAlignment="Top"
                    Click="btnSize_Click" Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="2">
            </Button>

            <TextBlock x:Name="lblData" Margin="5"  HorizontalAlignment="Center" VerticalAlignment="Top"                       
                    Grid.Row="2" Grid.Column="0" Grid.ColumnSpan="2"></TextBlock>

        </Grid>

Step 3: Now come to Code Behind and write handler for btnWrite_Click.

Also add this Isolated storage namespace.

using System.IO.IsolatedStorage;

private void btnWrite_Click(object sender, RoutedEventArgs e)
        {
            // Write to isolated storage.
            try
            {               
                //this gives you exactly what you want: an application-specific, user-specific
                //location where you can store data.

               IsolatedStorageFile store = IsolatedStorageFile.GetUserStoreForApplication();

                 //If we would have used
                //IsolatedStorageFile.GetUserStoreForSite ,
                //then a storage site that's accessible to all the Silverlight applications on the same website domain.
                using (IsolatedStorageFileStream fs = store.CreateFile("data.txt"))
                {
                    StreamWriter sw = new StreamWriter(fs);                  
                    sw.Write(txtname.Text);                  
                    sw.Close();
                }
                txtname.Text = string.Empty;

            }
            catch
            {

            }
        }

Let us concentrate on this line.

IsolatedStorageFile store = IsolatedStorageFile.GetUserStoreForApplication();

Silverlight creates isolated stores automatically. To interact with an isolated store, you use the IsolatedStorageFile class. You get the IsolatedStorageFile object for the current user and application by calling the static IsolatedStorageFile.GetUserStoreForApplication() method, as shown here:

Step 4: Press F5 and test write and read buttons.

Step 5: Now let's concentrate on how disk storage works. Initially, each Silverlight application gets 1 MB of space in it's isolated store. You can examine the IsolatedStorageFile.AvailableFreeSpace property to find out how much free space remains. If your application needs more space, you can use an option: the IsolatedStorageFile IncreaseQuotaTo() method.

We must request a value that's higher than the current quota. Otherwise, you'll receive an exception. That means you can't use the IncreaseQuotaTo() method to ensure that there's a certain level of free space.

If everything goes well, you will be presented with something such as shown in the figure below.

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Observable Collections are great to preserve the latest data in data controls without having to rebind data to the data controls. In this article we will explore how we can work with Observable Collections in Silverlight 5.

Silverlight – A Quick Look

Silverlight is a browser plug-in that promotes a collaborative development environment of rich online media content that enables developers and designers alike to integrate multimedia and graphics into web pages within the context of the managed environment. Over the past few years, Silverlight, formerly known as Windows Presentation Foundation Everywhere (WPF/E), has become popular worldwide for developing the next generation of cross-browser, cross-platform Rich Internet Applications (RIAs).

Observable Collections

The generic List<T> represents a strongly typed list of elements or a CLR or non-CLR type. When we bind data to a DataControl in ASP.NET using such a list, the data in the data bound control is up to date with the data in the list. However, when the data in the list changes, the data control has to be re-bound to reflect the changes. In other words, we have to rebind the data in the list (now the list contains updated data) to reflect the change. This problem is solved in using ObservableCollection<T>. This collection is capable of providing notifications of items added/deleted/edited through the INotifyCollectionChanged interface. The INotifyCollectionChanged interface has an event called PropertyChanged, which is fired when a property value is changed.

ObservableCollection is a generic dynamic data structure that has the ability to send notifications when items are added, removed or when the collection is refreshed. The MSDN states: "Represents a dynamic data collection that provides notifications when items get added, removed, or when the whole list is refreshed." Reference: http://msdn.microsoft.com/en-us/library/ms668604.aspx

The ObservableCollection class is shown below:

[SerializableAttribute]
public class ObservableCollection<T> : Collection<T>,
            INotifyCollectionChanged, INotifyPropertyChanged

You can instantiate an Observable collection as shown in the code snippet below:

using System.Collections.ObjectModel;
ObservableCollection<Product> products = new ObservableCollection<Product>();

Consider the following class:

public class Contact
    {
        private string firstName;
        private string lastName;
        private string address; 

        public Contact(string firstName, string lastName, string address)
        {
            this.firstName = firstName;
            this.lastName = lastName;
            this.address = address;
        } 

        public string FirstName
        {
            get { return firstName; }
            set { firstName = value; }
        } 

        public string LastName
        {
            get { return lastName; }
            set { lastName = value; }
        } 

        public string Address
        {
            get { return address; }
            set { address = value; }
        }
    }

The following class creates an Observable Collection out of the Contact class:

public class EmployeeList : ObservableCollection<Contact>
    {
        public EmployeeList()
            : base()
        {
            Add(new Contact("Joydip", "Kanjilal", "Hyderabad"));
            Add(new Contact("Debanjan", "Banerjee", "Kolkata"));
            Add(new Contact("Shaik", "Tajuddin","Hyderabad"));
            Add(new Contact("Firdous", "Khan", "Hyderabad"));
        }
    }

You can now use the Observable Collection you have created to bind data to a Silverlight Data Grid. Here is how the mark-up code would look:

<Grid x:Name="LayoutRoot" Background="White">
        <sdk:DataGrid x:Name="dgData" Grid.Row="2" AutoGenerateColumns="True"  ItemsSource="{Binding Source}" >

        </sdk:DataGrid>
    </Grid>

And, here's the code you would write to bind data to the DataGrid control named dgData:

public partial class MainPage : UserControl
    {
        EmployeeList empList = new EmployeeList();

        public MainPage()
        {
            InitializeComponent();          

            dgData.ItemsSource = empList;

        }
    }

To ensure that the DataGrid is refreshed if any item in the collection is changed, we should take advantage of the PropertyChangedEventHandler event handler. The NotifyPropertyChange event handler would be automatically called whenever the list is changed.

public partial class MainPage : UserControl
    {
        EmployeeList empList = new EmployeeList();

        public MainPage()
        {
            InitializeComponent();          
            dgData.ItemsSource = empList;

        }

        public EmployeeList Data
        {

            get { return empList; }

            set
            {

                empList = value;

                NotifyPropertyChange("empList");

            }

        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected void NotifyPropertyChange(string propertyName)
        {

            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));

        }
    }

Summary

In this article we discussed how we can work with Observable Collections in Silverlight 5. We explored Observable Collections, why they are helpful and how we can bind data to data controls in Silverlight using Observable Collections. Happy reading!