Why is more memory allocated than expected?

Question

I'm playing around with BenchmarkDotNet and its MemoryDiagnoser feature.

Considering the following benchmark:

[Benchmark]
public void Dummy()
{
   var buffer = new byte[1];
}

I expect it to allocate exactly 1 byte.

But the benchmark result shows that a total of 32 bytes were allocated. How come? I find this quite misleading.

| Method |     Mean |     Error |    StdDev |   Median | Ratio | Rank |  Gen 0 | Gen 1 | Gen 2 | Allocated |
|------- |---------:|----------:|----------:|---------:|------:|-----:|-------:|------:|------:|----------:|
|  Dummy | 4.486 ns | 0.1762 ns | 0.5196 ns | 4.650 ns |  1.00 |    1 | 0.0038 |     - |     - |      32 B |

                                                                                      why not 1 byte? ^^^^

Answer 1

I am the author of MemoryDiagnoser and I've described how to read the results my blog: https://adamsitnik.com/the-new-Memory-Diagnoser/#how-to-read-the-results

CLR does some aligning. If you try to allocate new byte[1] array, it will allocate byte[8] array.

We need extra space for object header, method table pointer and length of the array. The overhead is 3x Pointer Size. 8 + 3x4 = 20 for 32bit and 8 + 3x8 = 32 for 64bit.