Bits of .NET

🔌 Default Interface Methods = API Evolution

Interfaces break existing code. Default interface methods add methods without breaking. Evolve APIs safely.

📝 Default Interface Basics

// Interface with default method
public interface ILogger
{
    void Log(string message);
    
    // Default implementation
    void LogError(string message)
    {
        Log($"ERROR: {message}");
    }
    
    // Default implementation
    void LogWarning(string message)
    {
        Log($"WARNING: {message}");
    }
}

// Implementation
public class ConsoleLogger : ILogger
{
    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}

// Usage - doesn't need to implement LogError
var logger = new ConsoleLogger();
logger.Log("Info");
logger.LogError("Something went wrong"); // Works!

// Override default
public class DatabaseLogger : ILogger
{
    public void Log(string message)
    {
        // Write to database
    }
    
    public void LogError(string message)
    {
        // Custom error logging
        Log($"CUSTOM ERROR: {message}");
    }
}

🎯 Advanced Usage

// Static methods in interfaces
public interface IMath
{
    static int Add(int a, int b) => a + b;
}

// Private methods in interfaces
public interface ICalculator
{
    int Calculate(int a, int b);
    
    private int Multiply(int a, int b) => a * b;
    
    int Power(int a, int b)
    {
        int result = 1;
        for (int i = 0; i < b; i++)
        {
            result = Multiply(result, a);
        }
        return result;
    }
}

// Multiple inheritance
public interface ILoggable
{
    void Log(string message);
    
    void LogInfo(string message) => Log($"INFO: {message}");
}

public interface IAuditable
{
    void Audit(string action);
    
    void AuditCreate(string entity) => Audit($"CREATE: {entity}");
}

public class UserService : ILoggable, IAuditable
{
    public void Log(string message) => Console.WriteLine(message);
    public void Audit(string action) => Console.WriteLine(action);
    
    // Both interfaces work
}

// Diamond problem - explicit implementation
public interface IA
{
    void Method() => Console.WriteLine("A");
}

public interface IB
{
    void Method() => Console.WriteLine("B");
}

public class C : IA, IB
{
    public void Method()
    {
        // Must be implemented
        Console.WriteLine("C");
    }
}

💡 Best Practices

• Use for API evolution
• Keep defaults simple
• Document default behavior
• Use sparingly (avoid complexity)
• Consider interface segregation