problem with SendMessage

First I Get notepad's handle

HANDLE hNote = FindWindow(....)

Now Question is

PostMessage(hNote,WM_KEYDOWN,0x31,0x470001);

is success

but

When i use

SendMessage(hNote,WM_KEYDOWN,0x31,0x470001);

it's does not work

i want to know why

thank you

Because you are posting your message to a window in a different process... SendMessage wait's for the function to return, it can't know where to return to across isolated processes thus the call fails. PostMessage does not wait for the function to return so it can succeed across the process boundary.

FWIW... if you are sending WM_KEYDOWN... don't forget to also send WM_KEYUP as any subsequent WM_KEYDOWN messages will either be ignored (creating a dead key) or cause errors in the target process.