Can I change a user's keyboard input?

Question

I found this keyboard hook code, which I'm trying to slightly modify for my purposes: http://blogs.msdn.com/toub/archive/2006/05/03/589423.aspx

As an overview, I want to have the user press a key, say 'E', and have the keyboard return a different character, 'Z', to whatever app is in focus.

The relevant method I changed now looks like:

    private static IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam)
    {
        if (nCode >= 0 && wParam == (IntPtr)WM_KEYDOWN)
        {
            //The truely typed character:
            int vkCode = Marshal.ReadInt32(lParam);
            Console.WriteLine((Keys)vkCode);

            KBDLLHOOKSTRUCT replacementKey = (KBDLLHOOKSTRUCT)Marshal.PtrToStructure(lParam, typeof(KBDLLHOOKSTRUCT));
            replacementKey.vkCode = 90; // char 'Z'
            Marshal.StructureToPtr(replacementKey, lParam, false);

            //Now changed to my set character
            vkCode = Marshal.ReadInt32(lParam);
            Console.WriteLine((Keys)vkCode);
        }
        return CallNextHookEx(_hookID, nCode, wParam, lParam);
    }

The console correctly outputs this as:

E
Z
T
Z
G
Z
etc.

HOWEVER, the in focus app still types 'E' instead of 'Z'. Why? I changed the hooked keyboard input to contain 'Z' instead of 'E', and the console lines show that it was changed correctly!

As I understand it, calling the return CallNextHookEx(_hookID, nCode, wParam, lParam); is what sends the "print this now" command to the open app. Is that not how it works? Is there something that's preventing me from typing the character I want? I know apps like AutoHotkey take an input key, check it, and return a different character. How do I do the same here?

Thanks!

Answer 1

You have, most likely, installed the hook "thread wide" and not "system wide", which means that the key translation will occur only for the thread installing the hook.

In order to install it "system wide" you will need two pieces: one dll having the "hook provider" and an exe managing it. Here is a good tutorial http://www.codeproject.com/KB/system/hooksys.aspx and here an example: http://www.codeguru.com/cpp/com-tech/shell/article.php/c4509/

But: 1. Installing system wide hooks can seriously screw up you system (make sure that you forward the keys that you don't translate). 2. Please... don't create another keylogger

Answer 2

I've done this before but a little different.
Instead of trying to change the parameters sent to CallNextHookEx, I 'swallowed' the key press (you can do this by returning a nonzero value from the hook procedure to prevent subsequent procedures from being called).

Then I used SendInput to send the new key that I wanted to 'inject'.

So basically it works like this:

• Hook procedure identifies a target key is pressed
• Call to SendInput, with the new key
• Return 1 from the hook procedure to ignore the original key

Be careful of cyclic redirects, i.e. 'a' redirected to 'b' redirected to 'a', it can easily blow up ;)