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Close a Window Application from another application.

 This is just a demo application code to show how the WM_CLOSE message can be sent to the target process which has a titled window to close the application.

To achieve this, either we can use SendMessage or PostMessage APIs to send required Windows messages to the target application.

Though both the APIs are dispatching WM_XXXXX message to target application two APIs has some differences, these are as below:

1. SendMessage () call is a blocking call but PostMessage is a non-blocking call(Asynchronous)

2. SendMessage() APIs return type is LRESULT (LONG_PTR) but PostMessage() APIs return type is BOOL(typedef int). In Short, SendMessage () APIs return type depends on what message has been sent to the Windowed target process. For the other one, it's always a non-zero value, which indicates the message has been successfully placed on the target process message queue.

Now let's see how can I close a target windowed application "Solitaire & Casual Games" from my custom-made command prompt application. There are very few lines of code and it's very straightforward. We are getting / Seeking HWND of the target Windowed application by searching the window via FindWindow () API with the window title. Soon after that, we can shoot the message to the message WM_CLOSE to the target applications message queue via SendMessage or PostMessage. The sample code is here:

#include <iostream>
#include <windows.h>

int main() {
    // Find the main window handle of the target process.
    // In this case using FindWindow API to get the handle of the target process.
    // This technique only works for the process which has Window(UI with title)

    // 2nd Param is the title of the Window
    HWND targetWindow = FindWindow(nullptr, L"Solitaire & Casual Games"); 

    if (nullptr == targetWindow) 
        std::cout << "Target window not found." << std::endl;
        return 1;

    // Send the WM_CLOSE message to the target window
    SendMessage(targetWindow, WM_CLOSE, 0, 0);
    return 0;

Note: WM_CLOSE is a request to the target application to close but it may get ignored by the targeted windowed process. We will see soon how can we cleanly shut down a process (irrespective of windowed or not). There is an API named TerminateProcess() but it never terminates the process cleanly, I mean it doesn't detach dlls from the process, clean resources, etc. So will see soon how can we gracefully terminate a process from another custom process. 



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