关于sendmessage 问题

private const int WM_USER = 0x400;
private const int WS_CHILD = 0x40000000;
private const int WS_VISIBLE = 0x10000000;
private const int WM_CAP_START = WM_USER;
private const int WM_CAP_STOP = WM_CAP_START + 68;
private const int WM_CAP_DRIVER_CONNECT = WM_CAP_START + 10;
private const int WM_CAP_DRIVER_DISCONNECT = WM_CAP_START + 11;
private const int WM_CAP_SAVEDIB = WM_CAP_START + 25;
private const int WM_CAP_GRAB_FRAME = WM_CAP_START + 60;
private const int WM_CAP_SEQUENCE = WM_CAP_START + 62;
private const int WM_CAP_FILE_SET_CAPTURE_FILEA = WM_CAP_START + 20;
private const int WM_CAP_SEQUENCE_NOFILE =WM_CAP_START+ 63;
private const int WM_CAP_SET_OVERLAY =WM_CAP_START+ 51;
private const int WM_CAP_SET_PREVIEW =WM_CAP_START+ 50;
private const int WM_CAP_SET_CALLBACK_VIDEOSTREAM = WM_CAP_START +6;
private const int WM_CAP_SET_CALLBACK_ERROR=WM_CAP_START +2;
private const int WM_CAP_SET_CALLBACK_STATUSA= WM_CAP_START +3;
private const int WM_CAP_SET_CALLBACK_FRAME= WM_CAP_START +5;
private const int WM_CAP_SET_SCALE=WM_CAP_START+ 53;
private const int WM_CAP_SET_PREVIEWRATE=WM_CAP_START+ 52;

上面都是sendmessage的第二个参数，为什么 WM_USER 定义成0x400 而不是其他数值？后面的每个值都在WM_CAP_START基础上增加的数值是什么意思，怎么计算的，？

其实WM_USER的意思就是说可以让用户自由使用，为什么要定义成0x400呢？那是因为有很多系统消息存在占用了前面的值，所以自定义的消息不要小于WM_USER，否则有可能和系统的消息发生冲突。

这些参数是固定的