C# Definition:

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