C# Marshal.SizeOf

Question

I am using Marshal.SizeOf to know the size of my stucture:

struct loginStruct
{
    public string userName;
    public string password;

    public loginStruct(string userName, string password)
    {
        this.userName = userName;
        this.password = password;
    }
}

Here is the use of this function:

int len = Marshal.SizeOf(typeof(loginStruct));

I got 2 programs. In one program len is equals to 8. In the other it equals to 16. It is the same struct. Why I got that differece?

Answer 1

I'd guess one program is compiled for AnyCPU (which on a 64-bit platform will be 64-bit) and one for 32-bit.

Answer 2

Methods don't affect the size given, so effectively we're talking about:

struct loginStruct
{
    public string userName;
    public string password;
}

That struct has two reference type fields. As such, it has two fields in the memory which refer to objects on the heap, or to null.

All reference type fields are 4 bytes in 32-bit .NET and 8 bytes in 64-bit .NET.

Hence the size would be 4 + 4 = 8 in 32-bit .NET or 8 + 8 = 16 in 64-bit .NET.