Using User32.dll SendMessage To Send Keys With ALT Modifier

EDN Admin · Dec 2, 2012

I am writing an application that sends keystrokes to another application using the SendMessage function defined in *user32.dll*. I have figured out how to send a single keystroke but I am stumped trying to send the keystroke along with the ALT key. 
 
For the purposes of my question I will focus on sending F1, and ALT + F1. 
 
As stated above, I am able to send the F1 key no problem. Here is a snippet of my code that sends the F1 key: 
 
// DLL Imports 
 
//Set the active window 
[DllImport("user32.dll")] 
public static extern IntPtr SetActiveWindow(IntPtr hWnd); 
 
//sends a windows message to the specified window 
[DllImport("user32.dll")] 
public static extern int SendMessage(IntPtr hWnd, int Msg, uint wParam, uint lParam); 
 
// ... 
 
// Some constants 
#define WM_SYSKEYDOWN 260 
#define WM_SYSKEYUP 261 
#define WM_CHAR 258 
#define WM_KEYDOWN 256 
#define WM_KEYUP 257 
 
// ... 
 
// activate the window and send F1 
SetActiveWindow(hWnd); 
ushort action = (ushort)WM_SYSKEYDOWN; 
ushort key = (ushort)System.Windows.Forms.Keys.F1; 
SendMessage(hWnd, action, key, 0); 
 
One interesting side note is that even though the above code works in sending the F1 key to the target application it is not the same as what I see using Spy++. Here is the output of the Spy++ log whenever I hit the F1 key while monitoring the target application: 
 
<00001> 00050412 P WM_KEYDOWN nVirtKey:VK_F1 cRepeat:1 ScanCode:3B fExtended:0 fAltDown:0 fRepeat:0 fUp:0 
<00002> 00050412 P WM_KEYUP nVirtKey:VK_F1 cRepeat:1 ScanCode:3B fExtended:0 fAltDown:0 fRepeat:1 fUp:1 
 
Note that there are two messages sent, WM_KEYDOWN and WM_KEYUP. 
 
*My first question would be, why am I succesful sending F1 using WM_SYSKEYDOWN when Spy++ tells me that WM_KEYDOWN/WM_KEYUP is the proper message sequence?* 
 
Moving on to my next challenge of trying to send ALT + F1. 
 
I have used Spy++ to monitor the messages passed when pressing ALT + F1 on my keyboard and this is what I see: 
 
<00001> 00050412 P WM_SYSKEYDOWN nVirtKey:VK_MENU cRepeat:1 ScanCode:38 fExtended:1 fAltDown:1 fRepeat:0 fUp:0 
<00002> 00050412 P WM_SYSKEYDOWN nVirtKey:VK_F1 cRepeat:1 ScanCode:3B fExtended:0 fAltDown:1 fRepeat:0 fUp:0 
<00003> 00050412 P WM_SYSKEYUP nVirtKey:VK_F1 cRepeat:1 ScanCode:3B fExtended:0 fAltDown:1 fRepeat:1 fUp:1 
<00004> 00050412 P WM_KEYUP nVirtKey:VK_MENU cRepeat:1 ScanCode:38 fExtended:1 fAltDown:0 fRepeat:1 fUp:1 
 
Given the above Spy++ message capture I tried to send the exact message sequence using the following code (simplified): 
 
SetActiveWindow(hWnd); 
SendMessage(hWnd, (ushort)WM_SYSKEYDOWN, (ushort)System.Windows.Forms.Keys.Menu, 0); 
SendMessage(hWnd, (ushort)WM_SYSKEYDOWN, (ushort)System.Windows.Forms.Keys.F1, 0); 
SendMessage(hWnd, (ushort)WM_SYSKEYUP, (ushort)System.Windows.Forms.Keys.F1, 0); 
SendMessage(hWnd, (ushort)WM_KEYUP, (ushort)System.Windows.Forms.Keys.Menu, 0); 
 
This did not work. 
 
*And so this leads to my next question. Is there anything else I can try or is there something that I am doing wrong here?* 
 
Whenever I captured the output of my program using Spy++ here is what was logged: 
 
<00001> 00050412 S WM_SYSKEYDOWN nVirtKey:VK_MENU cRepeat:0 ScanCode:00 fExtended:0 fAltDown:0 fRepeat:0 fUp:0 
<00002> 00050412 R WM_SYSKEYDOWN 
<00003> 00050412 S WM_SYSKEYDOWN nVirtKey:VK_F1 cRepeat:0 ScanCode:00 fExtended:0 fAltDown:0 fRepeat:0 fUp:0 
<00004> 00050412 R WM_SYSKEYDOWN 
<00005> 00050412 S WM_SYSKEYUP nVirtKey:VK_F1 cRepeat:0 ScanCode:00 fExtended:0 fAltDown:0 fRepeat:0 fUp:0 
<00006> 00050412 R WM_SYSKEYUP 
<00007> 00050412 S WM_KEYUP nVirtKey:VK_MENU cRepeat:0 ScanCode:00 fExtended:0 fAltDown:0 fRepeat:0 fUp:0 
<00008> 00050412 R WM_KEYUP 
 
Notice that there are *extra* messages being sent at lines 2, 4, 6, and 8. *Could this be the reason why things are not working?* 
 
I have one final question regarding the difference between the messages captured from actual keyboard input and those captured using my application. Notice the *cRepeat*, *ScanCode*, *fExtended*, etc. *arguments*. They are non-zero in the messages that were 
captured using my keyboard as the input and they are all zero in the messages sent by my application. *Could this be the reason why my code is not working?* *If so, how do I modify these values?* (I am assuming they come from the 4th argument to the SendMessage 
function, which I have set to zero in all cases.) 
 
Thank you

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Using User32.dll SendMessage To Send Keys With ALT Modifier

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