C# Evolved

Default Interface Members

C# 8.0 NETCore 3.0

Published Updated Author Jeffrey T. Fritz Reading time —

Evolve interfaces by adding members with default implementations so existing implementers do not break immediately.

C# 8.0 introduced default interface members, allowing interfaces to include method bodies. This enables API evolution without forcing every existing implementation to update at once.

Why it matters

  • Helps library authors add functionality without immediate breaking changes.
  • Supports gradual migrations where implementations can override when ready.
  • Keeps shared fallback behavior close to the interface contract.

Cautions

Use default members for compatibility, not as a replacement for solid abstraction design. Overusing defaults can hide too much behavior in interfaces and make implementations harder to reason about.

Add default behavior to an interface

Default members let existing implementations keep working when an interface evolves.

Valid since C# 8.0 

using System;

public interface ITemperatureFormatter
{
    string FormatCelsius(double celsius) => celsius.ToString("0.0") + " C";
}

public class BasicFormatter : ITemperatureFormatter
{
}

public static class Program
{
    public static void Main()
    {
        Console.WriteLine(((ITemperatureFormatter)new BasicFormatter()).FormatCelsius(21.34));
    }
}

Override interface default behavior

Implementations can still provide custom behavior when needed.

Valid since C# 8.0 

using System;

public interface ITemperatureFormatter
{
    string FormatCelsius(double celsius) => celsius.ToString("0.0") + " C";
}

public class FancyFormatter : ITemperatureFormatter
{
    public string FormatCelsius(double celsius) => celsius.ToString("0.00") + " °C";
}

public static class Program
{
    public static void Main()
    {
        Console.WriteLine(((ITemperatureFormatter)new FancyFormatter()).FormatCelsius(21.34));
    }
}

Local functions

C# 7.0 NETFx 4.7

Start with Local functions when you want an earlier building block for Default Interface Members.

Local functions are methods defined inside other methods or properties, letting you encapsulate helper logic at the point of use. They reduce namespace clutter, improve readability by keeping related logic together, and naturally capture variables from their enclosing scope.

Static Abstract Members

C# 11.0 .NET 7.0

Continue to Static Abstract Members for the next guide in this theme.

Define compile-time contracts for static members so generic algorithms can call them safely.

LINQ

C# 3.0 NETFx 3.5

Explore LINQ for another earlier stepping stone in the same theme.

LINQ gives C# a consistent way to filter, transform, group, and sort data with either query syntax or fluent method syntax.

Learn more

Default interface methods versioning tutorial (Microsoft Learn)