Default interface methods C# 8 and later

Context: C# 8 introduced default implementations for interface methods. This allows adding new members to interfaces without breaking existing implementations.

Example

public interface ILogger
{
    void Log(string message);
    // Default implementation
    void LogError(string error) => Log($"ERROR: {error}");
}

public class ConsoleLogger : ILogger
{
    public void Log(string message) => Console.WriteLine(message);
    // LogError is optional; uses default if not overridden
}

Overriding Default Method

public class FileLogger : ILogger
{
    public void Log(string message) => File.WriteAllText("log.txt", message);
    public void LogError(string error) => Log($"Custom error: {error}");
}

Access via Interface

Default methods are only accessible through the interface type.

ILogger logger = new ConsoleLogger();
logger.LogError("Oops"); // calls default or overridden

Use Cases

• Adding methods to widely used interfaces (e.g., IEnumerable<T>).
• Providing backward compatibility.