Advanced Topics in VS .NET

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and running an EXE in another AppDomain because loading DLLs with LoadFrom doesn t cause a permission demand.
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Isolated Storage
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Isolated storage is a .NET Framework feature that enables assemblies to access a welldefined area of the file system, even if they don t have full permissions on the entire file system. Assemblies manage isolated storage by means of stores. Each store contains one or more files or subdirectories and is distinct from the store that another user (or even another assembly running in the same user account) uses. This isolation ensures that an untrusted assembly can t read or modify the store used by another assembly. In addition, the system administrator can set a higher limit to the amount of data that can be written to a store (the so-called quota) so that an assembly can t compromise the cli ent system by writing too much data to its hard drive. Isolated storage was primarily designed for partially trusted assemblies that are loaded from the Internet either as Windows Forms applications launched via an HTTP URL or Windows Forms controls running inside Internet Explorer but it can have other uses as well. An ASP.NET application can use isolated storage on the server to maintain a user s data between sessions; the only prerequisite in this latter case is that the ASP.NET application must impersonate the remote user so that each remote user has a distinct store. Isolated data is perfect for storing user preferences but not for configuration settings such as an ADO.NET connection string because the latter type of settings should be under the control of the administrator, not the end user. Likewise, isolated storage should not be used to store secret data because any unmanaged or fully trusted appli cation can access the area of the file system where the store is held.
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Creating a Store
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All the classes you need to use isolated storage in your applications can be found in the System.IO.IsolatedStorage namespace. The most important of such classes are Isolat edStorageFile (which represents a store) and IsolatedStorageFileStream (which repre sents an open file in the store). The following example shows how you get a reference to a store, write a text file into it, and then read the file back:
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Create a store for current user/assembly.
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Dim store1 As IsolatedStorageFile = IsolatedStorageFile.GetStore( _
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IsolatedStorageScope.User Or IsolatedStorageScope.Assembly Or _ IsolatedStorageScope.Domain, Nothing, Nothing) Create a file in the store. Dim ifs1 As New IsolatedStorageFileStream( file1.txt", FileMode.Create, store1) Write a string to it. Dim sw As New StreamWriter(ifs1)
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Programming for the Internet
sw.WriteLine( This is a test string )
sw.Close()
This statement ensures that the store is written to disk.
store1.Close()
Create another store for current user/assembly.
Dim store2 As IsolatedStorageFile = IsolatedStorageFile.GetStore( _ IsolatedStorageScope.User Or IsolatedStorageScope.Assembly Or _ IsolatedStorageScope.Domain, Nothing, Nothing) Read a file in the store.
Dim ifs2 As New IsolatedStorageFileStream( file1.txt", FileMode.Open, store2)
Display its contents.
Dim sr As New StreamReader(ifs2)
MsgBox(sr.ReadToEnd, MsgBoxStyle.Information)
sr.Close()
store2.Close()
In most cases you don t need to know where the actual files in a store are saved, but this information can be useful during the debug phase. The path of the directory that holds isolated storage files depends on the Windows version, the isolation scope being used (see next section), the user identity, whether the store is enabled for roaming users, and in some cases whether the operating system was installed on a clean machine or was an update from a previous version. For example, on Windows XP and Windows Server 2003, isolated storage files are held under the C:\Document and Settings\username\Local Settings\Application Data direc tory for nonroaming stores and C:\Document and Settings\username\Application Data for roaming-enabled stores. (See Figure 34-5.)
Figure 34-5
The actual directory used for a store in Windows Server 2003
Part VII:
Advanced Topics
Understanding Isolation Scope
Different users always access different stores and there is no way to change this default behavior, even though an application with sufficient permissions can impersonate a different user and therefore access a different store. Stores are further isolated at the assembly level, which means that assembly A and assembly B always access a different store, even if they run under the same user identity and in the same AppDomain. Again, you can t change the way stores are isolated at the user or assembly level. However, you can decide whether two instances of the same assembly running under the same user identity but in different AppDomains share the same store or access different stores. You can make this decision by selecting one of the following two isolation lev els for the store you create:
Isolation by User and Assembly If this isolation type is selected, two instances of the same assembly running under the same user identity share the same store, even if the two instances are running in different AppDomains. You should use this isola tion type for data that is to be shared among applications, such as user name and general preferences (colors, fonts, and so on). By default, code loaded from the local intranet can select this type of isolation, but code loaded from the Internet can t. Isolation by User, Domain, and Assembly If this isolation type is selected, two instances of the same assembly running under the same user identity always access different stores and can t share data via isolated storage. This setting is more secure and should be adopted unless applications must share data.
You select the type of isolation you want to enforce in the first argument to the GetStore shared method or by using either the GetUserStoreForAssembly or the GetUserStoreForDomain shared methods:
These two statements create a store that is isolated by user and assembly. Dim store1 As IsolatedStorageFile = IsolatedStorageFile.GetStore( _ IsolatedStorageScope.User Or IsolatedStorageScope.Assembly, Nothing, Nothing) Dim store2 As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForAssembly() These two statements create a store that is isolated by user, domain, and assembly. Dim store3 As IsolatedStorageFile = IsolatedStorageFile.GetStore( _ IsolatedStorageScope.User Or IsolatedStorageScope.Assembly Or _ IsolatedStorageScope.Domain, Nothing, Nothing) Dim store4 As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForDomain()
Stores that support roaming users that is, users who can log on to any machine on the network and still access their isolated storage data require that you specify the Isolat edStorageScope.Roaming bit in the first argument of the GetStore shared method:
Create a roaming-enabled store that is isolated by user, domain, and assembly. Dim store5 As IsolatedStorageFile = IsolatedStorageFile.GetStore( _ IsolatedStorageScope.User Or IsolatedStorageScope.Assembly Or _ IsolatedStorageScope.Domain Or IsolatedStorageScope.Roaming, Nothing, Nothing)
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