Interface Dispatch: VTable vs Interface Map (ITable)

1Concept

While class virtual methods are resolved via direct VTable indexing, interface method calls (`callvirt`) require looking up the class's Interface Map (ITable), adding a level of indirection.

2Architecture Diagram

Class Virtual Call:
Object ──> MethodTable ──> VTable[Index] ──> Direct Jump

Interface Call:
Object ──> MethodTable ──> Interface Map ──> Interface Slot ──> VTable[Index] ──> Jump

3Code Example

using System;

public interface IDataStore
{
    void Save(string key, string data);
}

public sealed class MemoryDataStore : IDataStore
{
    public void Save(string key, string data)
    {
        Console.WriteLine($"[Store] Key: '{key}' saved with data payload: '{data}'");
    }
}

public class InterfaceDispatchDemo
{
    public static void Main()
    {
        IDataStore store = new MemoryDataStore();
        store.Save("session_99", "{ active: true }");
    }
}

4Expected Output

[Store] Key: 'session_99' saved with data payload: '{ active: true }'

5Key Takeaways

• ✓Sealing the implementation class allows RyuJIT to devirtualize interface calls.
• ✓Use interface abstractions at module boundaries; use concrete types in hot inner loops.
• ✓Interface method dispatch is fast but carries 1 extra pointer hop compared to sealed calls.