Question

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Marshal.SizeOf structure returns excessive number

As MSDN states that sizeof(bool) is 1 byte. But when I put bool in struct , sizeof struct become 4 byte. Can someone explain this behavior?

[StructLayout(LayoutKind.Sequential)]
public struct Sample1
{
    public bool a1;
}

int size1 = Marshal.SizeOf(typeof (Sample1));  // 4
int size2 = sizeof (bool);                     // 1

Answer 1

You can’t directly compare sizeof and Marshal.SizeOf. For example, if we measure it the same way, we get the same result:

static unsafe void Main() { // unsafe is needed to use sizeof here
    int size1 = sizeof(Sample1); // 1
}

Presumably, Marshal is assuming per-field alignment of 4 bytes.