Understanding the Command Design Pattern in C# Quiz

Test your basic knowledge of the Command Design Pattern in C#, including core concepts, structure, and practical scenarios. This quiz covers the essentials of command objects, handlers, and dependency injection as used in modern .NET applications.

1. Concept of Command Pattern

   Which of the following best describes the main purpose of the Command design pattern in software development?

   1. It ensures that only one instance of a class exists in an application.
   2. It encapsulates a request as an object, allowing for parameterization and queuing of requests.
   3. It provides a way to access parts of an object’s data directly.
   4. It decorates objects with additional functionality at runtime.

   Explanation: The Command pattern turns a request into a standalone object, making it easy to pass, queue, or delay execution. The other options describe different design patterns: singleton (only one instance), decorator (adds functionality), and accessor patterns (direct access). None of the distractors focus on encapsulating a request.

2. Components of the Pattern

   In a C# implementation of the Command pattern, what is the typical role of the 'command handler' class?

   1. It processes the command object and executes the desired business logic.
   2. It manages database connections for the application.
   3. It stores and replays requests to increase application speed.
   4. It holds global constants needed by all classes.

   Explanation: The command handler's main job is to handle the command by executing the related business logic. Replaying requests and managing database connections are not specific responsibilities here. Holding global constants is a different structural concern.

3. Scenarios of Use

   When using the Command pattern in a controller method in C#, where is the command model most commonly populated from?

   1. The HTTP request body payload using the [FromBody] attribute.
   2. A random file on the server filesystem.
   3. A hardcoded string inside the controller.
   4. A static method inside another class.

   Explanation: In .NET, command models are usually bound from the HTTP request payload via [FromBody]. Using files, hardcoded strings, or static methods is not standard practice for creating command objects in this context.

4. Generic Interface

   What is the primary benefit of defining a generic command handler interface such as ICommandHandleru003CTCommandu003E in C#?

   1. It prevents the use of dependency injection.
   2. It allows handling of various command types using type safety.
   3. It forces all handlers to use the same command object.
   4. It only supports commands with integer properties.

   Explanation: Generics ensure that handlers can process different types of commands in a type-safe way. Forcing sameness or restricting to integers is incorrect. Generics do not prevent dependency injection; they often enhance flexibility.

5. Handler Execution

   Given a command handler class that implements ICommandHandleru003CCreateOrderCommandu003E, what does the ExecuteAsync method typically process?

   1. It removes unused code from the application automatically.
   2. It processes business logic for creating an order based on the command’s properties.
   3. It generates random orders for testing.
   4. It updates configuration files according to the latest command.

   Explanation: The handler’s ExecuteAsync method runs business logic using the information in the command. Removing code, generating random data, or changing configuration files are separate from the command handling duties.

6. Dependency Injection

   How are command handler classes typically provided to the controllers in modern C# applications?

   1. Via dependency injection, allowing automatic instantiation and management.
   2. By manually instantiating handlers inside each action method.
   3. By using global variables accessible to all controllers.
   4. By calling static helper methods in every request.

   Explanation: Dependency injection is standard for providing handlers, resulting in better modularity and testability. Using globals, statics, or manual instantiation leads to tighter coupling and is less maintainable.

7. Command Handler Factory

   What is the main function of the CommandHandlerFactory in a C# command pattern implementation?

   1. To store all HTTP responses for future reference.
   2. To encrypt commands before processing.
   3. To locate and execute the correct command handler for each command.
   4. To generate random command objects for seeding databases.

   Explanation: The factory pattern is used here to find and use the right handler for a given command, often leveraging dependency injection. Storing responses, seeding data, and encrypting commands are unrelated to the factory’s main task.

8. Advantages

   Why does the Command pattern help improve maintainability in C# applications?

   1. It requires manual memory management.
   2. It combines all logic in a single class.
   3. It hardcodes all business rules into the controller.
   4. It separates request creation from request processing.

   Explanation: Separating command creation from handling makes code more organized and easier to update. Combining logic, hardcoding rules, or managing memory manually go against maintainability principles.

9. Pattern Category

   To which general category of design patterns does the Command pattern belong?

   1. Structural
   2. Behavioral
   3. Transactional
   4. Creational

   Explanation: The Command pattern is a behavioral design pattern, focusing on the flow of control and the way objects interact. Structural patterns deal with object composition, and creational patterns focus on object creation, while transactional is not a standard category in design patterns.

10. Practical Example

    If you have a CreateOrderCommand with CustomerName set to 'Alex' and Items as a list of products, what part of the pattern defines what happens when this command is handled?

    1. The list property inside the command model.
    2. The controller method that receives the HTTP request.
    3. The configuration file for the application.
    4. The command handler class specifically written for CreateOrderCommand.

    Explanation: The command handler class defines how the command is processed, including business operations. The controller method only receives the request, the list is just data storage, and configuration files do not contain business logic for handling commands.