Default Interface Methods (DIM) & Non-Breaking API Evolution

1Concept

Default Interface Methods (DIM) allow interface authors to add new methods with default implementations without breaking existing third-party implementations.

2Architecture Diagram

[ ILogger Interface ]
├── void Log(string msg) ── [Abstract]
└── void LogError(string err) => Log("[ERROR] " + err) ── [Default Implementation]
       │
       ▼
[ LegacyLogger ] ── Implements Log(); inherits LogError() automatically!

3Code Example

using System;

public interface IMetricsCollector
{
    void Record(string metricName, double value);

    // Default Interface Method (DIM)
    void RecordLatency(string operation, double milliseconds)
    {
        Record($"latency.{operation}", milliseconds);
    }
}

public class SimpleCollector : IMetricsCollector
{
    public void Record(string metricName, double value)
    {
        Console.WriteLine($"Metric: {metricName} = {value}");
    }
}

public class DimDemo
{
    public static void Main()
    {
        IMetricsCollector collector = new SimpleCollector();
        collector.RecordLatency("CheckoutApi", 12.45);
    }
}

4Expected Output

Metric: latency.CheckoutApi = 12.45

5Key Takeaways

• ✓DIM provides trait-like capabilities to C# interfaces.
• ✓Default methods are only accessible via the interface reference variable (`IMetricsCollector c = ...`).
• ✓Does not create multiple inheritance diamond state issues because interfaces have no instance fields.