Pinvoke command length is incorrect

Question

I am trying to get the FILE_ID_BOTH_DIR_INFO with the GetFileInformationByHandle function. All my values are set to zero after calling.
I get the win32 error message:
ERROR_BAD_LENGTH The program issued a command but the command length is incorrect.

IntPtr handle = CreateFile(dir, EFileAccess.GenericRead, EFileShare.Read, IntPtr.Zero, ECreationDisposition.OpenExisting, EFileAttributes.BackupSemantics | EFileAttributes.Normal, IntPtr.Zero);
FILE_ID_BOTH_DIR_INFO fileStruct = new FILE_ID_BOTH_DIR_INFO();
GetFileInformationByHandleEx(handle, FILE_INFO_BY_HANDLE_CLASS.FileIdBothDirectoryInfo, out fileStruct, (uint)Marshal.SizeOf(fileStruct));

FILE_ID_BOTH_DIR_INFO structure

    typedef struct _FILE_ID_BOTH_DIR_INFO {
    DWORD         NextEntryOffset;
    DWORD         FileIndex;
    LARGE_INTEGER CreationTime;
    LARGE_INTEGER LastAccessTime;
    LARGE_INTEGER LastWriteTime;
    LARGE_INTEGER ChangeTime;
    LARGE_INTEGER EndOfFile;
    LARGE_INTEGER AllocationSize;
    DWORD         FileAttributes;
    DWORD         FileNameLength;
    DWORD         EaSize;
    CCHAR         ShortNameLength;
    WCHAR         ShortName[12];
    LARGE_INTEGER FileId;
    WCHAR         FileName[1];
    } FILE_ID_BOTH_DIR_INFO, *PFILE_ID_BOTH_DIR_INFO

In C#

    public struct FILE_ID_BOTH_DIR_INFO
    {
        uint NextEntryOffset;
        uint FileIndex;
        LARGE_INTEGER CreationTime;
        LARGE_INTEGER LastAccessTime;
        LARGE_INTEGER LastWriteTime;
        LARGE_INTEGER ChangeTime;
        LARGE_INTEGER EndOfFile;
        LARGE_INTEGER AllocationSize;
        uint FileAttributes;
        uint FileNameLength;
        uint EaSize;
        char ShortNameLength;
        [MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 12)]
        string ShortName;
        LARGE_INTEGER FileId;
        [MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 1)]
        string FileName;
    }

    [StructLayout(LayoutKind.Explicit, Size = 8)]
    struct LARGE_INTEGER
    {
        [FieldOffset(0)]
        public Int64 QuadPart;
        [FieldOffset(0)]
        public UInt32 LowPart;
        [FieldOffset(4)]
        public Int32 HighPart;
    }

Answer 1

The character set for ByValTStr is determined by the CharSet argument of the StructLayout attribute. Since you've not specified that, the default of 8 bit ANSI is used. If you specify the CharSet as CharSet.Unicode then that should deal with the problem. Add this attribute to your struct:

[StructLayout(LayoutKind.Sequential, 
    CharSet=CharSet.Unicode)]

For the purposes of debugging write a C++ program that outputs the size of the struct. Make sure that your C# program matches that value.