SendMessage() under specific situation

[Greg Roberts]

Hi

I have a program running as a service by using the
instserv myprogram srvany.exe
method.

Under this the application is hidden but running.
Within the application threads talk to the main thread via sendmessage.
By breaking in the debugger i find say that the window handle is 0x1000 (an
example)

When i try to talk to the invisible program via another program also using
SendMessage() using the 0x1000 value, i get an invalid handle error.

>> Has ayone had experiance trying to talk to a program within a service ?

Thanks