debugger disconnect on embed mono

I embed mono by options:

const char* options[] = {
"--soft-breakpoints",
"--debugger-agent=transport=dt_socket,server=y,address=127.0.0.1:55555,timeout=9999999"
};
mono_jit_parse_options(sizeof(options) / sizeof(char*), (char**)options);

mono_debug_init(MONO_DEBUG_FORMAT_MONO);

mono debug can accept this socket ,but debug thread will get a disconnect event soon, then thread will exit:
res = transport_recv (header, HEADER_LENGTH);

	/* This will break if the socket is closed during shutdown too */
	if (res != HEADER_LENGTH) {
		PRINT_DEBUG_MSG (1, "[dbg] transport_recv () returned %d, expected %d.\n", res, HEADER_LENGTH);

this is the launch.json:
{
// Use IntelliSense to learn about possible attributes.
// Hover to view descriptions of existing attributes.
// For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
"version": "0.2.0",
"configurations": [
{
"name": "Launch",
"type": "mono",
"request": "launch",
"program": "F:/TProject/Module/CSharpTestMono/bin/Debug/CSharpTestMono.exe",//"${workspaceRoot}/program.exe",
"cwd": "F:/TProject/Module/EmbedMono/x64/Debug"//"${workspaceRoot}"
},
{
"name": "Attach",
"type": "mono",
"request": "attach",
"address": "localhost",
"port": 55555,
},
]
}

I can debug by Launch,But Attach is not ok now.

Start by reproducing the Attach configuration in launch.json against the embedded Mono setup using port 55555. Inspect the debug thread around transport_recv and compare its behavior with the working Launch configuration; done means Attach stays connected and debugging can proceed.