Em_setcharformat

Hey, I have a question regarding the windows message EM_SETCHARFORMAT. I am trying to use this and I am having a problem. I have a text editor and it allows the user to set the text to different fonts. This works but if, for example, there are 5 lines of text, each line has a different font, when I highlight all 5 lines and try to change the font, it only changes the font of the first selected line, not all 5 lines. Any idea what is causing this, the carriage return perhaps? How do I fix this so it will change the font for all the lines?