How to access a window?

Question

I'm trying to access a specific window using its handle (that is System.IntPtr value):

        // Getting the process of Visual Studio program
        var process = Process.GetProcessesByName("devenv")[0];

        // Showing the handle we've got, we've no problem
        MessageBox.Show(this, process.MainWindowHandle.ToString());

        // Attempting to get the main window object by its handle
        var wnd = NativeWindow.FromHandle(process.MainWindowHandle);

        // always fails
        if (wnd == null)
            MessageBox.Show("Failed");
        else
            MessageBox.Show(wnd.ToString(), "Yeeeeees!!");

I have tried also to access another demo .net winforms application's main window, that I have made for this purpose, (i.e. I run the demo application, and attempted to access its main window by this application), and failed, too, although both the demo and this application are .NET applications. However, this successes:

        var process2 = Process.GetCurrentProcess();
        MessageBox.Show(this, process2.MainWindowHandle.ToString());

        var wnd2 = NativeWindow.FromHandle(process2.MainWindowHandle);
        if (wnd2 == null)
            MessageBox.Show("Failed");
        else
            MessageBox.Show(wnd2.ToString(), "Yes");

I think this works because it is invoked from the same application. So, how can I access some another program's window object by its handle? I thought it can work using C\C++ by using header file <windows.h> and then using a P\invoke.

If I can't, is there another way to access a window (i.e. rather than using handles)?

=================== EDIT

I want to deal with the entire window object and its own controls

Answer 1

Then, as Raymond suggested, why don't you try with Automation? Add a console project with references to UIAutomationClient and UIAutomationTypes

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Automation;
using System.Diagnostics;
using System.Threading;

namespace ConsoleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            var pInfo = new ProcessStartInfo("notepad");

            var p = Process.Start(pInfo);

            p.WaitForInputIdle();

            AutomationElement installerEditorForm = AutomationElement.FromHandle(p.MainWindowHandle);

            // menus
            AutomationElementCollection menuBars = installerEditorForm.FindAll(TreeScope.Children, new PropertyCondition(
                AutomationElement.ControlTypeProperty, ControlType.MenuBar));

            var mainMenuItem = menuBars[0];

            AutomationElementCollection menus = mainMenuItem.FindAll(TreeScope.Children, new PropertyCondition(
                AutomationElement.ControlTypeProperty, ControlType.MenuItem));

            var fileMenuItem = menus[0];

            ExpandCollapsePattern fileMenuItemOpenPattern = (ExpandCollapsePattern)fileMenuItem.GetCurrentPattern(
                ExpandCollapsePattern.Pattern);

            fileMenuItemOpenPattern.Expand();

            AutomationElement fileMenuItemNew = fileMenuItem.FindFirst(TreeScope.Children,
                new AndCondition(
                    new PropertyCondition(AutomationElement.ControlTypeProperty, ControlType.MenuItem),
                    new PropertyCondition(AutomationElement.NameProperty, "New")));

            Console.Read();
        }
    }
}

reference

Answer 2

The documentation for NativeWindow.FromHandle explains why that function always returns null for you:

The handle must already be owned by another NativeWindow in the current process; otherwise, null is returned.

But the window that you are targeting is in a different process. So you simply cannot use NativeWindow here. You will have to make do with the window handle as an IntPtr.

In your edit you state:

I want to deal with the entire window object and its own controls

That changes nothing. You can't use NativeWindow. You will have to deal with the raw Win32 API.