Lesson 1: Creating Application Domains in VB.NET

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Lesson 1: Creating Application Domains
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// C# AppDomain d = AppDomain.CreateDomain("New Domain"); d.ExecuteAssemblyByName("ShowWinIni");
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4. Build the project and resolve any errors. Verify that the Console application successfully calls the ShowWinIni.exe assembly and that it displays the text file successfully.
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Lesson Summary
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An application domain is a logical container that allows multiple assemblies to run within a single process but prevents them from directly accessing memory belonging to other assemblies. Create an application domain anytime you want to start an assembly. The AppDomain class contains methods for defining privileges, folders, and other properties for a new application domain, starting an assembly, and unloading an application domain. To create an instance of the AppDomain class, call the static AppDomain. CreateDomain method. AppDomain does not have any traditional constructors. To load an assembly in an application domain, create an instance of the AppDomain class and then call the App.Domain.ExecuteAssembly method. To unload an application domain, call the AppDomain.Unload static method.
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You can use the following questions to test your knowledge of the information in Lesson 1, Creating Application Domains. The questions are also available on the companion CD if you prefer to review them in electronic form.
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Answers to these questions and explanations of why each answer choice is right or wrong are located in the Answers section at the end of the book.
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1. Which of the following are valid reasons to create an application domain (Choose all that apply.) A. It is the only way to start a separate process. B. You can remove the application domain to free up resources. C. Application domains improve performance. D. Application domains provide a layer of separation and security.
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Application Domains and Services
2. Which of the following are valid ways to run an assembly within an application domain (Choose all that apply.) A. AppDomain.CreateDomain B. AppDomain.ExecuteAssembly C. AppDomain.ExecuteAssemblyByName D. AppDomain.ApplicationIdentity 3. Which command would you use to close the application domain in the following code sample
' VB Dim d As AppDomain = AppDomain.CreateDomain("New Domain") d.ExecuteAssemblyByName("MyAssembly") // C# AppDomain d = AppDomain.CreateDomain("New Domain"); d.ExecuteAssemblyByName("MyAssembly");
A. d.DomainUnload() B. d = null C. d.Unload() D. AppDomain.Unload(d)
Lesson 2: Configuring Application Domains
Lesson 2: Configuring Application Domains
You can configure application domains to create customized environments for assemblies. The most important application of modifying the default settings for an application domain is restricting permissions to reduce the risks associated with security vulnerabilities. When configured ideally, an application domain not only provides a unit of isolation, but it limits the damage that attackers can do if they successfully exploit an assembly.
After this lesson, you will be able to: Start assemblies in an application domain with limited privileges. Configure application domain properties to control folder locations and other settings. Estimated lesson time: 25 minutes
How to Use an Application Domain to Start Assemblies with Limited Privileges
Restricting the permissions of an application domain can greatly reduce the risk that an assembly you call will perform some malicious action. Consider the following scenario: You purchase an assembly from a third party and use the assembly to communicate with a database. An attacker discovers a security vulnerability in the third-party assembly and uses it to configure a spyware application to start automatically. To the user, the security vulnerability is your fault, because your application trusted the third-party assembly and ran it with privileges sufficient to install software. Now consider the same scenario using an application domain with limited privileges: An attacker discovers a security vulnerability in the third-party assembly. However, when the attacker attempts to exploit the vulnerability to write files to the local hard disk, the file input/output (I/O) request is rejected because of insufficient privileges. Although the security vulnerability still exists, the limited privileges assigned to the application domain prevent it from being exploited. In this example, starting assemblies with limited privileges is an example of defensein-depth. Defense-in-depth is the security principle of providing multiple levels of protection so that you are still protected in the event of a vulnerability. Defense-in-depth is particularly important when calling external code because external code might have vulnerabilities that you are not aware of, cannot prevent, and cannot fix.
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