Article source code: isolatedstorage.zip

Introduction

Among the vast number of classes and functions provided by the Microsoft .NET Framework, exists a new and powerful feature for storing information relating to users and applications called Isolated Storage. The System.IO.IsolatedStorage namespace allows data to be written to the hard disk where sometimes this is not allowed. This provides the capability to store things such as application state and user preference information.

Both ASP.NET and Windows Form Applications can use the full capabilities of isolated storage. ASP.NET applications may use it to store personalized information which is useful for customizing a site for each user. Windows Applications traditionally place application data in Windows .INI files and Windows Registry settings but now developers can also use isolated storage.

This article provides two Windows Application examples on how information can be accessed and used with isolated storage by utilizing ADO.NET and the XML namespaces. Data can be easily read and written using IO streams with Isolated Storage.

How does Isolated Storage Work?

Isolated storage helps to manage the reading and writing of data to a particular storage location. Think of it as a sandbox type area on a hard drive that various applications can write to, including those that normally don't have authorization to do so. Applications can include anything from a website to a windows application. This encompasses all trust levels of code written under the .NET framework, from no trust to full trust.

There are two types of isolation that can be used:

1. Isolation by user and assembly
2. Isolation by user, domain and assembly

Both types of isolation require that the storage area be associated with a user and assembly. Only the user and assembly that created the storage have access to it.

Isolation by user and assembly: Information stored with this type of isolation can be accessed by multiple applications with appropriate levels of trust. For example an intranet site can have access to this information but an internet site cannot, by default.

Isolation by user, domain and assembly: This type of isolation has the same requirements as the first type for user and assembly, and adds application domain on top if it. With application domain (or just domain, for short), isolation becomes even more granular. Now, only the code that created the storage can have access to it, and that must be an assembly run from just one application. No other application has access to the storage. This provides an extra layer of security, which is mandatory when running code from non-trusted sources such as the internet.

Which Isolation Type to Use?

Use the user and assembly storage type when running applications from trusted sources that require sharing of information among multiple applications for that assembly. Use the user, assembly and domain type when running applications from non-trusted sources such as components downloaded from the internet. You may also employ this type of storage when the requirement is that each application has its own separate storage location.

Example 1: Creating XML files with Isolated Storage

A common usage of isolated storage is a Windows Application that stores user settings. To demonstrate how to do this I have created an XML file that stores a window's background color and size, and a textbox control's text property. Try it out for yourself by creating Visual Basic.NET Windows Application in Visual Studio.NET. The Form will contain three buttons, a TextBox control and a ColorDialog control. Add controls to the Form with the following settings:

Click the View Code button on the Solution Explorer and add in the following statements on the first line of code:

Imports System
Imports System.IO
Imports System.IO.IsolatedStorage
Imports System.Text
Imports System.Xml

This imports the necessary namespaces that are required for this program.

Add the following lines of code in the btnBackColor's Click event:

If cdBackground.ShowDialog() = DialogResult.OK Then
    Me.BackColor = cdBackground.Color
End If

This is standard code for handling results from the ColorDialog control.

I'll first describe how to save a file with isolated storage. Later in this example I'll show how to retrieve a file that has already been stored with isolated storage. The first task, with regards to isolated storage, is to create the necessary object to work with a storage file. An IsolatedStorageFile object is created. I've chosen to use the user, domain and assembly type of isolation. I don't expect to share this storage area with other applications, so this type of isolation is sufficient for my needs. The second line of code sets the isolation type with the GetUserStoreForDomain option. If I want to use user and assembly isolation instead, I'd just replace that with GetUserStoreForAssembly.

The following code segments are placed in the btnSave Click event:

Dim isoStorage As IsolatedStorageFile
isoStorage = IsolatedStorageFile.GetUserStoreForDomain

The next line of code creates an IsolatedStorageFileStream object. The first parameter is the filename/path of the XML file to be written. The second parameter contains the FileMode with which the file is opened. In this instance, a new file will be created and overwritten if the same file already exists. Finally, the last parameter is the IsolatedStorageFile object in this case isoStorage.

Dim stmWriter As New IsolatedStorageFileStream ("UserSettingsXML.xml", _
    FileMode.Create, isoStorage) 

Since an XML file is being written, an XmlTextWriter object is created and associated with the stream object.

Dim writer As New XmlTextWriter(stmWriter, Encoding.UTF8)

The following lines of code simply write out the XML file one line at a time:

writer.Formatting = Formatting.Indented
writer.WriteStartDocument()
writer.WriteStartElement("UserSettings")
writer.WriteStartElement("BackColor")
writer.WriteString(Me.BackColor.ToArgb.ToString)
writer.WriteEndElement()
writer.WriteStartElement("Width")
writer.WriteString(Me.Width.ToString)
writer.WriteEndElement()
writer.WriteStartElement("Height")
writer.WriteString(Me.Height)
writer.WriteEndElement()
writer.WriteStartElement("SampleData")
writer.WriteString(txtSampleData.Text)
writer.WriteEndElement()
writer.WriteEndElement()

Flushing and closing the writer object will complete writing of the XML file. It's a good practice to close the file storage and stream objects. This allows for later processing of the isolated storage file without encountering any errors.

writer.Flush()
writer.Close()

stmWriter.Close()
isoStorage.Close()

That's all there is to creating an XML file using isolated storage. But what about the next time the application is started? After the settings have been saved there needs to be a way of reading them back in. The best place for this is in the Form's Load event.

Again, as in the btnSave Click event we declare and initialize a IsolatedStorageFile object.

Dim isoStorage As IsolatedStorageFile
isoStorage = IsolatedStorageFile.GetUserStoreForDomain

The next few lines determine if an XML settings file currently exists:

Dim StoreFileNames As String()
Dim StoreFile As String
StoreFileNames = isoStorage.GetFileNames("UserSettingsXML.xml")

For Each StoreFile In StoreFileNames