Marshal.SizeOf structure returns excessive number

Question

I have the following structure

[StructLayout(LayoutKind.Sequential)]
public struct SFHeader
{

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 5)]
    pu         


        

Answer 1

You could use a fixed size buffer instead of a string.

[StructLayout(LayoutKind.Sequential)]
public unsafe struct SFHeader
{
    public fixed char FileName[5];
    public int Offset;
    public short Size;
    public byte Flags;
    public byte Source;
    public long LastWriteTime;

    public byte[] GetBytes()
    {
        //omitted
    }

    public static SFHeader FromBytes(byte[] buffer)
    {
        //omitted
    }
}

Answer 2

You're running into a byte-alignment issue. In an attempt to keep fields on word boundaries for speed of access, the compiler is padding your string with 3 extra bytes. To fix this, use the Pack field of the StructLayoutAttribute.

[StructLayout(LayoutKind.Sequential, Pack=1)]  // notice the packing here
public struct SFHeader
{
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 5)]
    public string FileName;

    public int Offset;

    public short Size;

    public byte Flags;

    public byte Source;

    public long LastWriteTime;
}