C# Definition:

private enum MULTIPLE_TRUSTEE_OPERATION
{
    NO_MULTIPLE_TRUSTEE,
    TRUSTEE_IS_IMPERSONATE
}

private enum TRUSTEE_FORM
{
    TRUSTEE_IS_SID,
    TRUSTEE_IS_NAME,
    TRUSTEE_BAD_FORM,
    TRUSTEE_IS_OBJECTS_AND_SID,
    TRUSTEE_IS_OBJECTS_AND_NAME
}

private enum TRUSTEE_TYPE
{
    TRUSTEE_IS_UNKNOWN,
    TRUSTEE_IS_USER,
    TRUSTEE_IS_GROUP,
    TRUSTEE_IS_DOMAIN,
    TRUSTEE_IS_ALIAS,
    TRUSTEE_IS_WELL_KNOWN_GROUP,
    TRUSTEE_IS_DELETED,
    TRUSTEE_IS_INVALID,
    TRUSTEE_IS_COMPUTER
}

//Platform independent (32 & 64 bit) - use Pack = 0 for both platforms. IntPtr works as well.
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto, Pack = 0)]
private struct TRUSTEE : IDisposable
{
    public IntPtr pMultipleTrustee;
    public MULTIPLE_TRUSTEE_OPERATION MultipleTrusteeOperation;
    public TRUSTEE_FORM TrusteeForm;
    public TRUSTEE_TYPE TrusteeType;
    private IntPtr ptstrName;

    void IDisposable.Dispose()
    {
       if (ptstrName != IntPtr.Zero) Marshal.Release(ptstrName);
    }

    public string Name { get { return Marshal.PtrToStringAuto(ptstrName); } }
}

VB Definition:

    <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Auto, Pack:=4)> _
    Public Structure TRUSTEE
    Dim pMultipleTrustee As IntPtr ' must be null
    Dim MultipleTrusteeOperation As Integer
    Dim TrusteeForm As Integer
    Dim TrusteeType As Integer
    <Some Marshalling attribute here>
    Dim ptstrName As String
    End Structure

User-Defined Field Types:

None.

Notes:

ptstrName is actually a LPTSTR in windows API (whereas string in .NET is unicode), so a marshalling attribute is needed to make it work.