C# Definition:

     [StructLayout(LayoutKind.Sequential)]
    internal struct MOUSEINPUT
    {
        internal LONG dx;
        internal LONG dy;
        internal MouseEventDataXButtons mouseData;
        internal MOUSEEVENTF dwFlags;
        internal DWORD time;
        internal ULONG_PTR dwExtraInfo;
    }

VB Definition:

Structure MOUSEINPUT
  Public dx As Integer
  Public dy As Integer
  Public mouseData As Integer
  Public dwFlags As Integer
  Public time As Integer
  Public dwExtraInfo As IntPtr
End Structure

User-Defined Field Types:

    [Flags]
    internal enum MouseEventDataXButtons : uint
    {
        Nothing = 0x00000000,
        XBUTTON1 = 0x00000001,
        XBUTTON2 = 0x00000002
    }

    [Flags]
    internal enum MOUSEEVENTF : uint
    {
        ABSOLUTE = 0x8000,
        HWHEEL = 0x01000,
        MOVE = 0x0001,
        MOVE_NOCOALESCE = 0x2000,
        LEFTDOWN = 0x0002,
        LEFTUP = 0x0004,
        RIGHTDOWN = 0x0008,
        RIGHTUP = 0x0010,
        MIDDLEDOWN = 0x0020,
        MIDDLEUP = 0x0040,
        VIRTUALDESK = 0x4000,
        WHEEL = 0x0800,
        XDOWN = 0x0080,
        XUP = 0x0100
    }

Notes:

None.