I'm developing IRC client(not mIRC script) and want to add function for sending files into it.There is a explanation on the DCC protocol at this page I'm sending the following private message with my client "DCC SEND whatever 1566565459 1024 34",but the target client receive it like private message,not like request for receiving file over dcc.What's wrong?

Just to play safe, you sent the $chr(127) / ascii 0127 as well?
PRIVMSG ClientB :<here>DCC SEND filename longip port filesize<and here>

if I send this line to myself in mIRC, the dcc get dialogue triggers:
//raw PRIVMSG $me :
DCC SEND test.txt $longip($ip) 10 10

or
//PRIVMSG $me
DCC SEND test.txt $longip($ip) 10 10

Maybe I missed something, but you wrote "$chr(127)" then used character 1 in your examples.

So, to avoid confusion: $chr(1) is correct and is used to distinguish CTCP messages from normal PRIVMSGs/NOTICEs.

Thank you both for answers.

my fault, used $asc(paste) on the wrong char

One more question...when some client wants to send DCC request for sending file to other client,it must send a message according to the DCC protocol,but mIRC sends a message which violates the DCC protocol,isn't?Here is the raw message which mIRC sends PRIVMSG cygnusX :$chr(1)DCC SEND c1a3.HL3 4294967295 0 28 13$chr(1).
4294967295 is the IP address and 13 must be filesize,and 28 must be the port,then what's the 0?