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I was trying to get the text of a control from an application. So I first tried using GetWindowText and got a string back but to my surprise it wasn't the text in the actual control. So I used WM_GETTEXT instead and now I got the text of the control. Why is it that I get two different results using these two approaches? Doesn't GetWindowText just send the WM_GETTEXT message?
The GetWindowText function copies the text of the specified window's title bar (if it has one) into a buffer. If the specified window is a control, the text of the control is copied. However, GetWindowText cannot retrieve the text of a control in another application.
Yes I know what the function is supposed to do. I'm also aware that the MSDN library claimes that it can't be used to retrieve the text of a textbox from another process, but I've used it many times retrieving for example the text of a button and the like. What I wondered was what the difference between the WM_GETTEXT message and the GetWindowText function really is? I also wondered why I got two different results with a specific control in another process? If GetWindowText doesn't send the WM_GETTEXT message I also wonder why not?
There is two different ways a window can handle its text. Either let system handle it or handle it themself. If the system handles it it will store the text passed to CreateWindow/CreateWindowEx in a dedicated memory area. This is returned by GetWindowText or by sending WM_GETTEXT. However the window/control can handle it themselfs, which is pretty common for a control to do. In that case the control handles the WM_GETTEXT and WM_GETTEXTLENGTH messages and returns whatever text they want/use.
So when you send a WM_GETTEXT message, if the window/control doesn't handle that message itself, Windows will return the string stored in the dedicated memory area when the window was created, and it will replace it if a WM_SETTEXT message is sent. This is the default behaviour.
Now, if you use the GetWindowText function it will send a WM_GETTEXT message if the window you're requesting the text for have been created by the same process that calls GetWindowText. But if you use GetWindowText on a control in another process it will return the text stored in this special memory area and not send a WM_GETTEXT message. The reason for this is simply because if GetWindowText would send the message to a window that is hung it would never return, hence hanging the process that call the function. That would cause a huge problem for such things as the task switcher that would hang if one window is hung. A hung application/window shouldn't cause problems for other applications/windows.
This is also the reason you can get different values using GetWindowText and sending WM_GETTEXT to a control. The control was created with a call to CreateWindow or CreateWindowEx with one text (that will be returned by GetWindowText) and then handles the WM_GETTEXT message itself and returns something else. So if you want to be sure to get the correct text you should use WM_GETTEXT for other processes, however you should also be aware that if the window/control in question is hung SendMessage will never return. So the best approach is to use SendMessageTimeout instead of SendMessage.
I'm sorry for the long delay, but thank you very much Joacim. That makes it very clear.