C# Definition:

[StructLayout(LayoutKind.Explicit)]
struct INPUT {
  [FieldOffset(0)] public int type;
  [FieldOffset(4)] public MOUSEINPUT mi;
  [FieldOffset(4)] public KEYBDINPUT ki;
  [FieldOffset(4)] public HARDWAREINPUT hi;
}

VB Definition:

Structure INPUT
   Public TODO
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.