C# Definition:

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

VB Definition:

    Structure INPUT_TYPE
        <FieldOffset(0)> Dim dwType As Integer
        <FieldOffset(4)> Dim mi As MOUSEINPUT
        <FieldOffset(4)> Dim ki As KEYBDINPUT
        <FieldOffset(4)> Dim hi As HARDWAREINPUT
    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). It might therefore be more reasonable, to use three structs instead of a hard-coded layout.