How to find correct values for HWND window handles? - winapi

Question

WinRestore,% hwnd([1])
i have found in many programming language the use of hwnd. after searching on google it comes out to be handle. I didnt got more information on this. how programmer knows the value to put in, eg.
Const LB_GETTEXTLEN = &H18A
Const LB_GETTEXT = &H189
Const LB_GETCOUNT = &H18B
&h18a how he known, how will he use this?
this is the example program
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hWnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long
Const LB_GETTEXTLEN = &H18A
Const LB_GETTEXT = &H189
Const LB_GETCOUNT = &H18B
Private Function GetListItems(ByVal hList As Long) As Variant
Dim i As Long, nCount As Long, lItemLength As Long
Dim sItem() As String
nCount = SendMessage(hList, LB_GETCOUNT, 0, ByVal 0&)
For i = 0 To nCount - 1
lItemLength = SendMessage(hList, LB_GETTEXTLEN, i, ByVal 0&)
ReDim Preserve sItem(i)
sItem(i) = String(lItemLength, 0)
Call SendMessage(hList, LB_GETTEXT, i, ByVal sItem(i))
Next i
GetListItems = sItem
End Function
there are many such examples in all different languages but concept will be the same. so i want to learn it. what does it mean and how to use it.
another example from ahk
Gui,2:+hwndhwnd
hwnd(2,hwnd)

Answer 1

Those are all window messages that you can find information about on the MSDN Documentation by googling them. See below links:
LB_GETTEXTLEN
LB_GETTEXT
LB_GETCOUNT
You can find them and other related messages by checking the documentation for the native List Box control.
As for the numbers they're hexadecimal numbers which are (usually) mentioned in the documentation. But since these aren't you'll have to google them and check other websites/forums, or find their values on your own by experimenting with them in C or C++ .
In VB hexadecimal numbers are represented by prepending the number with &H, whereas in C, C++, C# or alike they're prepended with 0x.

Answer 2

In a forms editor each window/control has a hwnd property. For windows not created by your forms package you use the API calls FindWindow (easiest but not reliable) or EnumWindows. Also GetForegroundWindow and GetDesktopWindow.
To find out the value of constants you download the C header files as part of the Windows SDK https://developer.microsoft.com/en-us/windows/downloads/windows-10-sdk. It also has the documentation for all these API calls. This is online documentation listing all the windows' functions https://msdn.microsoft.com/en-us/library/windows/desktop/ms633505(v=vs.85).aspx.