Overview
The Proxy pattern is a structural design pattern that provides an object which acts as a substitute or placeholder for another object. This pattern is used to control access to the original object, allowing you to perform something either before or after the request gets through to the original object. In software systems, it can be beneficial for lazy initialization, access control, logging, monitoring, locking, or smart reference management.
Key Concepts
- Proxy vs. Real Subject: The Proxy maintains a reference to the Real Subject and controls access to it. They both implement the same interface, allowing the Proxy to be used anywhere the Real Subject is expected.
- Types of Proxies: There are several types of Proxies, including virtual proxies (for lazy initialization), protection proxies (for access control), and remote proxies (for accessing objects in different address spaces).
- Use Cases: Proxies are useful for operations like authorization, network connection management, logging, or lazy object initialization, which are not directly related to the actual business logic of the object.
Common Interview Questions
Basic Level
- What is the Proxy pattern, and why would you use it?
- Can you write a simple implementation of the Proxy pattern?
Intermediate Level
- How does a virtual proxy differ from a protection proxy?
Advanced Level
- Describe a scenario where using a Proxy pattern could significantly improve the performance of a software system.
Detailed Answers
1. What is the Proxy pattern, and why would you use it?
Answer: The Proxy pattern is a design pattern where a proxy object controls access to another object, which may be remote, expensive to create, or in need of securing. The Proxy pattern is used for several reasons, including to delay the object creation cost until necessary (lazy initialization), to control access to the object (protection proxy), or to add a layer of abstraction for network operations (remote proxy).
Key Points:
- Control Access: The Proxy can protect the real object from undue or harmful access.
- Additional Responsibilities: It can perform additional tasks when accessing an object, such as logging or monitoring access.
- Lazy Initialization: It can improve performance by delaying the creation and initialization of expensive objects until they're needed.
Example:
public interface ISubject
{
void Request();
}
// RealSubject class
public class RealSubject : ISubject
{
public void Request()
{
Console.WriteLine("RealSubject Request");
}
}
// Proxy class
public class Proxy : ISubject
{
private RealSubject _realSubject;
public void Request()
{
if (_realSubject == null)
{
_realSubject = new RealSubject();
}
// Additional actions can be taken here (e.g., access control, logging)
_realSubject.Request();
}
}
2. Can you write a simple implementation of the Proxy pattern?
Answer: Below is a basic implementation of the Proxy pattern, demonstrating a protection proxy that controls access to the RealSubject based on an access level.
Key Points:
- Interface Implementation: Both Proxy and RealSubject implement the ISubject interface.
- Access Control: The Proxy controls access to the RealSubject based on predefined conditions.
- Delegation: The Proxy delegates the call to the RealSubject once the access is granted.
Example:
public class ProtectionProxy : ISubject
{
private RealSubject _realSubject;
public string User { get; set; }
public void Request()
{
if (User != "AuthorizedUser")
{
Console.WriteLine("ProtectionProxy: Access denied.");
return;
}
if (_realSubject == null)
{
_realSubject = new RealSubject();
}
_realSubject.Request();
}
}
3. How does a virtual proxy differ from a protection proxy?
Answer: A virtual proxy is used for lazy initialization of an object, delaying its creation until it is actually needed. In contrast, a protection proxy controls access to an object, making decisions based on access rights or other security considerations.
Key Points:
- Virtual Proxy: Focuses on resource optimization and delaying object creation.
- Protection Proxy: Focuses on security and access control.
- Implementation Intent: Although both implement the same interface as the real object, their primary purposes differ significantly.
Example:
For a virtual proxy:
public class VirtualProxy : ISubject
{
private RealSubject _realSubject;
public void Request()
{
if (_realSubject == null)
{
Console.WriteLine("Initializing RealSubject...");
_realSubject = new RealSubject();
}
_realSubject.Request();
}
}
For a protection proxy, refer to the example provided in question 2.
4. Describe a scenario where using a Proxy pattern could significantly improve the performance of a software system.
Answer: A common scenario is when a software system deals with large, resource-intensive objects, such as high-resolution images. Using a virtual proxy can significantly improve the system's performance by delaying the loading of these images until they are actually needed to be displayed to the user.
Key Points:
- Lazy Loading: The virtual proxy can instantiate the real object only when its data is required.
- Memory Efficiency: This approach saves memory and other resources by avoiding the loading of objects that may never be needed.
- User Experience: It can also enhance the user experience by speeding up the application startup time.
Example:
public interface IImage
{
void Display();
}
public class HighResolutionImage : IImage
{
public HighResolutionImage(string imagePath)
{
// Assume this is a costly operation, such as loading an image from disk
Console.WriteLine($"Loading image from {imagePath}");
}
public void Display()
{
Console.WriteLine("Displaying image");
}
}
public class ImageProxy : IImage
{
private HighResolutionImage _highResImage;
private string _imagePath;
public ImageProxy(string imagePath)
{
_imagePath = imagePath;
}
public void Display()
{
if (_highResImage == null)
{
_highResImage = new HighResolutionImage(_imagePath);
}
_highResImage.Display();
}
}
This proxy pattern example improves performance by loading the image only when it's actually needed for display, rather than at object creation time.
