I need some help understanding the SendMessage API

If this is not the correct place to ask this question my apologies, could someone please point me to the proper place ?

I am in need of some ELI5 help to understand the SendMessage function of User32.DLL.

I have a very simple application working so far in which I send a simple arrow down key to another application. It works great, but I need to understand more about it, and what I have found from MS leaves me scratching my head. http://ift.tt/1L4zGhV

Here is the code I am using to send the key down and key up messages.

Win32.SendMessage(iHandle, Win32.WM_KEYDOWN, 0X28, 0X1500001); Win32.SendMessage(iHandle, Win32.WM_KEYUP, 0X28, 0X1500001);

iHandle of course is the Int result of the FindWindow API.

What I do not understand is the 4th parameter that is being passed. How specific is this to the arrow down key I am sending, and how would it vary for a up arrow key ? Where can I find more information about the requirements of these parameters as they would relate to the type of message I am sending ?

by KingLuxor via /r/csharp

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