Help with TypeLoadException (marshaling unmanaged code)

I thought I'd go back to first principles, Tom,  to see what I'd come up with from the Win32 definitions alone, the answer being as follows (not necessarily correct, of course :))

[StructLayout(LayoutKind.Sequential)]
struct vc1_sBlk
{
  public vc1_eBlkType eBlkType; /** Block type */
  public short Coded; /** Non zero AC coefficients for Intra,
    non zero AC/DC for Inter */
  public IntraInterUnion u;
}

[StructLayout(LayoutKind.Explicit)]
struct IntraInterUnion
{
   [FieldOffset(0)]public vc1_sBlkIntra sIntra; /** Intra block state information */
   [FieldOffset(0)]public vc1_sBlkInter sInter; /** Inter block state information */
}

[StructLayout(LayoutKind.Sequential)]
struct vc1_sBlkIntra
{
   public ushort NZC; /** NUMZERO and NUMCOEF (excludes DC) */
   public short DC; /** Quantized DC for prediction */
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 7)]
   public short[] ACTop; /** Quantized AC top row for prediction */
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 7)]
   public short[] ACLeft; /** Quantized AC left column for prediction */
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 16)]
   public short[] SmoothRows; /** Bottom two rows kept for overlap smoothing */
}

As far as explicit layout is concerned, the rule is that you can have both value types and reference types in the same struct as long as they don't overlap. However, I don't think you have any reference types here - just structs - so that's not a problem in the union definition. 

Also, AFAIK, there is no need to align members of the struct in any particular way when you are using explicit layout. In effect you're creating a struct with a packing size of 1.

However, I don't think there's any need to use explicit layout for the vc1_sBlk struct, even though it contains a union. You can just use sequential layout with the default packing size.

Finally, there is no need to use the 'unsafe' keyword unless you are explicitly using pointers in your struct definitions which doesn't appear to be the case.