Unable to Simulate Alt-Tab Virtual Key Press, works fine with other keys

Question

I have a simple code

KEYBDINPUT k1 = { VK_MENU, 0, 0, 0, 0 };
KEYBDINPUT k2 = { VK_TAB, 0, 0, 0, 0 };
KEYBDINPUT k3 = { VK_TAB, 0, KEYEVENTF_KEYUP, 0, 0 };
KEYBDINPUT k4 = { VK_MENU, 0, KEYEVENTF_KEYUP, 0, 0 };
int cnt = 0;

while (cnt < 10)
{
    Sleep(1000);
    INPUT in1 = { INPUT_KEYBOARD };
    in1.ki = k1;
    SendInput(1, &in1, sizeof(INPUT));

    INPUT in2 = { INPUT_KEYBOARD };
    in2.ki = k2;
    INPUT in3 = { INPUT_KEYBOARD };
    in3.ki = k3;
    SendInput(1, &in2, sizeof(INPUT));
    SendInput(1, &in3, sizeof(INPUT));

    cnt++;
}

INPUT in4 = { INPUT_KEYBOARD };
in4.ki = k4;
SendInput(1, &in4, sizeof(INPUT));

return 0;

This does not work. If I change from VK_TAB to VK_ESCAPE it works.

I would want emulate Alt-Tab as Alt-Esc does not have a visual representation. Is there something special about VK_TAB?

Answer 1

When VKs fail, using SendInput with keyboard scan codes instead often works.

The example below demonstrates how to simulate alt-tab with SendInput and keyboard scan codes.

The alt_tab function takes a count. It will simulate holding down the left alt key, hitting the tab key "count" times, then letting go of the left alt key.

The keypress delay is pretty big, mostly to give adequate time to visually demonstrate the effect. Shorter delays will be more efficient for automation, but you probably cannot remove them altogether.

I pass 1 as the input count to SendInput because I only want to send one at a time. Manipulating KEYBDINPUT timestamps does not work the way one might expect.

To build this example, start with a Win32 Console project.

#include <Windows.h>

void sendkey(WORD scan_code, bool tf_down_up, bool extended=false) {
    INPUT input = {0};
    input.type = INPUT_KEYBOARD;
    input.ki.wScan = scan_code;
    input.ki.dwFlags                  = KEYEVENTF_SCANCODE;
    if(!tf_down_up) input.ki.dwFlags |= KEYEVENTF_KEYUP;
    if(extended)    input.ki.dwFlags |= KEYEVENTF_EXTENDEDKEY;
    SendInput(1, &input, sizeof(input));
}

void keydown(WORD scan_code, bool extended=false) {
    sendkey(scan_code, true, extended);
}

void keyup(WORD scan_code, bool extended=false) {
    sendkey(scan_code, false, extended);
}

static const WORD TAB_SCANCODE      = 0x0f;
static const WORD LEFT_ALT_SCANCODE = 0x38;

static const DWORD keypress_delay_ms = 500;

void alt_tab(int tab_count=1) {
    keydown(LEFT_ALT_SCANCODE);
    Sleep(keypress_delay_ms);
    for(int i=0; i<tab_count; ++i) {
        keydown(TAB_SCANCODE);
        Sleep(keypress_delay_ms);
        keyup(TAB_SCANCODE);
        Sleep(keypress_delay_ms);
    }
    keyup(LEFT_ALT_SCANCODE);
}

int main() {
    Sleep(1000);
    alt_tab(3);
}