Using Windows APIs from C#, again!

• Download demo project - 7.01 Kb

Introduction

This article shows how to control other windows and trigger events for their controls using Windows APIs.

In this sample, I simply get a handle for the Calculator window using the FindWindow API, get a handle for the Calculator buttons using FindWindowEx, and trigger the Button Click event for any required buttons, using the SendMessage API.

Background

The main idea I want to demonstrate here is that any form/dialog in Windows must have a window handle; with this handle and using the windows related APIs, you can control the form/dialog and trigger events for its controls.

Here's the code. I think it is well commented and needs no more explanation:

int hwnd=0;
IntPtr hwndChild=IntPtr.Zero;

//Get a handle for the Calculator Application main window
hwnd=FindWindow(null,"Calculator");
if(hwnd == 0)
{
    if(MessageBox.Show("Couldn't find the calculator" + 
                       " application. Do you want to start it?", 
                       "TestWinAPI", 
                       MessageBoxButtons.YesNo)== DialogResult.Yes)
    {
        System.Diagnostics.Process.Start("Calc");
    }
}
else
{
        
    //Get a handle for the "1" button
    hwndChild = FindWindowEx((IntPtr)hwnd,IntPtr.Zero,"Button","1");
    
    //send BN_CLICKED message
    SendMessage((int)hwndChild,BN_CLICKED,0,IntPtr.Zero);

    //Get a handle for the "+" button
    hwndChild = FindWindowEx((IntPtr)hwnd,IntPtr.Zero,"Button","+");
    
    //send BN_CLICKED message
    SendMessage((int)hwndChild,BN_CLICKED,0,IntPtr.Zero);

    //Get a handle for the "2" button
    hwndChild = FindWindowEx((IntPtr)hwnd,IntPtr.Zero,"Button","2");
    
    //send BN_CLICKED message
    SendMessage((int)hwndChild,BN_CLICKED,0,IntPtr.Zero);

    //Get a handle for the "=" button
    hwndChild = FindWindowEx((IntPtr)hwnd,IntPtr.Zero,"Button","=");
    
    //send BN_CLICKED message
    SendMessage((int)hwndChild,BN_CLICKED,0,IntPtr.Zero);

}

Points of interest

I think controlling windows in other processes can be very helpful in many situations like:

1. Ensuring that a main application is running before starting a dependant application.
2. Automating some tasks.