C# Definition:

[StructLayout(LayoutKind.Sequential)]
public struct INPUT
{
  internal InputType type;
  internal InputUnion U;
  internal static int Size
  {
   get { return Marshal.SizeOf(typeof(INPUT)); }
  }
}

public enum InputType : uint
{
  INPUT_MOUSE,
  INPUT_KEYBOARD,
  INPUT_HARDWARE
}

[StructLayout(LayoutKind.Explicit)]
internal struct InputUnion
{
  [FieldOffset(0)]
  internal MOUSEINPUT mi;
  [FieldOffset(0)]
  internal KEYBDINPUT ki;
  [FieldOffset(0)]
  internal HARDWAREINPUT hi;
}

VB Definition:

<StructLayout(LayoutKind.Explicit)> _
Structure InputUnion
  <FieldOffset(0)> Public mi As MOUSEINPUT
  <FieldOffset(0)> Public ki As KEYBDINPUT
  <FieldOffset(0)> Public hi As HARDWAREINPUT
End Structure        

Structure INPUT
  Public type As Integer
  Public u As InputUnion
End Structure

User-Defined Field Types:

MOUSEINPUT, KEYBDINPUT, HARDWAREINPUT

Notes:

The last 3 fields are a union, which is why they are all at the same memory offset.

On 64-Bit systems, the offset of the mi, ki and hi fields is 8, because the nested struct uses the alignment of its biggest member, which is 8 (due to the 64-bit pointer in dwExtraInfo). By separating the union into its own structure, rather than placing the mi, ki and hi fields directly in the INPUT structure, we assure that the .Net structure will have the correct alignment on both 32 and 64 bit.

"internal static int Size" does not work. it should be:

"internal int Size" <-- Not quite correct: Size is not part of the structure itself. It is a static helper property to get the size of INPUT in bytes. See SendInput. The definition above works well.