Overhead of a .NET array?

Question

I was trying to determine the overhead of the header on a .NET array (in a 32-bit process) using this code:

long bytes1 = GC.GetTotalMemory(false);
object[] array = new object[10000];
    for (int i = 0; i < 10000; i++)
        array[i] = new int[1];
long bytes2 = GC.GetTotalMemory(false);
array[0] = null; // ensure no garbage collection before this point

Console.WriteLine(bytes2 - bytes1);
// Calculate array overhead in bytes by subtracting the size of 
// the array elements (40000 for object[10000] and 4 for each 
// array), and dividing by the number of arrays (10001)
Console.WriteLine("Array overhead: {0:0.000}", 
                  ((double)(bytes2 - bytes1) - 40000) / 10001 - 4);
Console.Write("Press any key to continue...");
Console.ReadKey();

The result was

    204800
    Array overhead: 12.478

In a 32-bit process, object[1] should be the same size as int[1], but in fact the overhead jumps by 3.28 bytes to

    237568
    Array overhead: 15.755

Anyone know why?

(By the way, if anyone's curious, the overhead for non-array objects, e.g. (object)i in the loop above, is about 8 bytes (8.384). I heard it's 16 bytes in 64-bit processes.)

Overhead of a .NET array?

5 Answers