why CPaintDC(this) can't make effect in OnInitDialog?

dear all,i write a program based on MFC Dialog i put code as below in OnInitdialog:

Code:

CPaintDC dc(this);
dc.MoveTo(..);
dc.LineTo(..);

but it seems that nothing happened when dialog initiate.

and when i put these code into OnPaint,it make effect, i don't know why and i guessed that maybe the dialog refresh its window after its init, Could anyone help me?