What approaches should be used when writing code to facilitate its automated testing?

To facilitate automated testing of code in C#, the following approaches are recommended:

• Single Responsibility Principle — each class or method should perform one task, which simplifies writing tests.
• Dependency Injection — injecting dependencies through constructors or properties allows replacing real objects with mocks or stubs in tests.
• Interfaces and abstractions — using interfaces makes it easier to create test doubles.
• Avoid static methods and states — static methods are hard to mock; it's better to use class instances.
• Pure functions and methods without side effects — such methods are easier to test.
• Logging and exception handling — helps identify errors in tests.

Example of dependency injection via constructor:

public interface IDataService
{
    string GetData();
}

public class DataService : IDataService
{
    public string GetData() => "Real data";
}

public class BusinessLogic
{
    private readonly IDataService _dataService;

    public BusinessLogic(IDataService dataService)
    {
        _dataService = dataService;
    }

    public string ProcessData()
    {
        var data = _dataService.GetData();
        return data.ToUpper();
    }
}

In tests, you can replace IDataService with a mock to control behavior and isolate the tested code.