C# Feature - Default interface methods

Overview

Default interface methods allow interfaces to provide method bodies, which helps evolve contracts without breaking every implementation.

Introduced In

C# 8.0 (2019)

Before

public interface ILogger
{
  void Log(string message);
}

After

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

Gotchas & Best Practices

• Use this for interface evolution, not as a replacement for good abstraction design.
• Keep default implementations small and unsurprising.