Method Table, TypeHandle & Object Header Memory Layout

1Concept

Every reference type instance on the managed heap carries an 8-byte SyncBlockIndex and an 8-byte MethodTable pointer (TypeHandle) on 64-bit architectures, establishing an object header overhead of 16 bytes before any user fields.

2Architecture Diagram

64-Bit Heap Object Layout (Minimum 24 Bytes):
┌───────────────────────────┬───────────────────────────┬───────────────────────────┐
│ SyncBlockIndex (8 Bytes)  │  MethodTable Ptr (8 Bytes)│  User Field Data (8+ Bytes)│
└───────────────────────────┴───────────────────────────┴───────────────────────────┘
   ▲ Lock state, hashcode      ▲ Virtual method vtable     ▲ Instance fields & padding

3Code Example

using System;
using System.Runtime.InteropServices;

public class HeaderDemo
{
    public class SamplePayload
    {
        public int Id;     // 4 bytes
        public int Age;    // 4 bytes
    }

    public static void Main()
    {
        var obj = new SamplePayload { Id = 101, Age = 28 };
        Console.WriteLine($"Type Name: {obj.GetType().FullName}");
        Console.WriteLine($"TypeHandle: {obj.GetType().TypeHandle.Value:X}");
        Console.WriteLine("Object has 16-byte header (SyncBlock + MethodTable) + 8-byte fields.");
    }
}

4Expected Output

Type Name: HeaderDemo+SamplePayload
TypeHandle: 7FF8A4B29010
Object has 16-byte header (SyncBlock + MethodTable) + 8-byte fields.

5Key Takeaways

• ✓Small reference types incur a 16-byte overhead on 64-bit systems.
• ✓SyncBlock holds monitor lock data and lazily computed hash codes.
• ✓Value types (`struct`) have NO object header when placed on stack or embedded in arrays.