The key to writing C# code well is maintainability and testability. Reasonably divide responsibilities, follow the single responsibility principle (SRP), and take data access, business logic and request processing by Repository, Service and Controller respectively to improve structural clarity and testing efficiency. Multi-purpose interface and dependency injection (DI) facilitate replacement implementation, extension of functions and simulation testing. Unit testing should isolate external dependencies and use Mock tools to verify logic to ensure fast and stable execution. Standardize naming and splitting small functions to improve readability and maintenance efficiency. Adhering to the principles of clear structure, clear responsibilities and test-friendly can significantly improve development efficiency and code quality.

Writing C# code well is not just "just run". What really tests the level is whether the code is easy to maintain and test. Especially in team collaboration and long-term projects, these two points directly affect development efficiency and product quality. Let’s talk about how to do it from a few practical perspectives.

Divide responsibilities reasonably and don't let categories be too "tired"

A type does too many things, and it will be like peeling off a cocoon in the later stage, which will move the whole body. Try to follow the Single Responsibility Principle (SRP) and each class does only one thing, or deals with a highly relevant set of logic.

For example, data access and business logic should be separated. If you connect both the database and do calculations in a class, it will be troublesome to change the database or adjust the algorithm in the future. Use the Repository mode to extract data operations and process business logic in conjunction with the Service class, the structure is clearer and easier to simulate and test.

Common practices:

• Put database operations in Repository
• Leave the business logic to the Service class
• Controller is only responsible for receiving requests and returning results

After this layering, you only need to mock the data layer when testing, and don’t really connect to the database.

Use multiple interfaces, less dependency on specific implementations

The interface is not for showing off skills, but for making the code more flexible. With dependency injection (DI) and interface abstraction, you can easily replace different implementations, which is especially useful for unit testing.

For example: you have an email service that sends emails. If other classes directly new it, it will be difficult to replace it with Mock or Fake implementations for testing. But if you define an IEmailService interface and other classes rely only on this interface, then you can pass in a fake implementation when testing without actually sending emails.

The benefits are more than testing:

• Easier to expand new features (such as adding text messages)
• Reduce coupling between modules
• Convenient integration testing and simulation testing

So, think more when writing code: "Can this function change in the future?" If the answer is yes, it is worth abstracting into an interface.

Unit testing should be simple, don't be dragged down by the environment

It is a good habit to write some tests after writing the code. But many people start to connect to the database and adjust external services as they write. As a result, the test runs slowly, and it is easy to fail - it is not that there is a problem with the code, but that the environment is unstable.

The correct way is: isolate external dependencies and focus on logical verification .

For example, you have a method to calculate discounts based on user level, so don’t check the database to get user information. Instead, you should pass the level as a parameter, or inject an IUserProvider interface through the constructor, and pass a fake implementation in during testing.

suggestion:

• Use the mock tool of Moq or xUnit to simulate dependencies
• Avoid accessing real databases or networks in unit tests
• Test naming should make sense, such as CalculateDiscount_WhenLevelIsVip_Returns10Percent

This way your tests can be executed quickly and are stable and reliable.

Naming norms and small functions, read like sentences

Many codes are difficult to read, not because of the complex logic, but because of the messy variable names and too long functions. Don't be afraid of decomposing functions. A small function only does one small thing, which is easier to understand.

For example, this paragraph:

 if (user != null && user.IsActive && user.Role == "Admin")

Can be encapsulated into:

 if (UserHelper.IsEligibleForAccess(user))

You can tell at a glance that it is judging whether the user has permissions. The readability is improved and it is much more convenient to maintain.

In addition, the naming should be consistent with the style, such as:

• PascalCase
• The interface name begins with I
• Private fields begin with underscore

These details look small, but are very critical in multi-person collaboration.

Basically that's it. Writing maintainable and testable C# code does not rely on any advanced skills. The key is to have clear structure, clear responsibilities, and test-friendly. If you stick to these directions, you will find that changing bugs and adding functions will be much easier over time.

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