Correct way (in .NET) to switch the focus to another application

Get the window handle (hwnd), and then use this user32.dll function:

VB.net declaration:

Declare Function SetForegroundWindow Lib "user32.dll" (ByVal hwnd As Integer) As Integer 

C# declaration:

[DllImport("user32.dll")] public static extern int SetForegroundWindow(int hwnd) 

One consideration is that this will not work if the window is minimized, so I've written the following method which also handles this case. Here is the C# code, it should be fairly straight forward to migrate this to VB.

[System.Runtime.InteropServices.DllImport("user32.dll")]
[return: System.Runtime.InteropServices.MarshalAs(System.Runtime.InteropServices.UnmanagedType.Bool)]
private static extern bool ShowWindow(IntPtr hWnd, ShowWindowEnum flags);

[System.Runtime.InteropServices.DllImport("user32.dll")]
private static extern int SetForegroundWindow(IntPtr hwnd);

private enum ShowWindowEnum
{
    Hide = 0,
    ShowNormal = 1, ShowMinimized = 2, ShowMaximized = 3,
    Maximize = 3, ShowNormalNoActivate = 4, Show = 5,
    Minimize = 6, ShowMinNoActivate = 7, ShowNoActivate = 8,
    Restore = 9, ShowDefault = 10, ForceMinimized = 11
};

public void BringMainWindowToFront(string processName)
{
    // get the process
    Process bProcess = Process.GetProcessesByName(processName).FirstOrDefault();

    // check if the process is running
    if (bProcess != null)
    {
        // check if the window is hidden / minimized
        if (bProcess.MainWindowHandle == IntPtr.Zero)
        {
            // the window is hidden so try to restore it before setting focus.
            ShowWindow(bProcess.Handle, ShowWindowEnum.Restore);
        }

        // set user the focus to the window
        SetForegroundWindow(bProcess.MainWindowHandle);
    }
    else
    {
        // the process is not running, so start it
        Process.Start(processName);
    }
}

Using that code, it would be as simple as setting the appropriate process variables and calling BringMainWindowToFront("processName");

There is another way, which uses the not well-known UI Automation API:

AutomationElement element = AutomationElement.FromHandle(process.MainWindowHandle);
if (element != null)
{
    element.SetFocus();
}

Most of the time, this will work if it's possible to switch to that window. There are a lot of limitations in Windows (security, UAC, specific configuration, etc...) that can prevent you to change the end-user focus.

In VB.Net, you can use the AppActivate function.

Dim App As Process() = Process.GetProcessesByName("program.exe")
If App.Length > 0 Then
   AppActivate(App(0).Id)
End If

Create a New Class in your project and copy-paste the below code in it.

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Threading;

namespace MyProject
{
    public class ProcessHelper
    {
        public static void SetFocusToExternalApp(string strProcessName)
        {
            Process[] arrProcesses = Process.GetProcessesByName(strProcessName);
            if (arrProcesses.Length > 0)
            {

                IntPtr ipHwnd = arrProcesses[0].MainWindowHandle;
                Thread.Sleep(100);
                SetForegroundWindow(ipHwnd);

            }
        }

    //API-declaration
    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    public static extern bool SetForegroundWindow(IntPtr hWnd);

    }
}

Now copy-paste the below code in your required area.

string procName = Process.GetCurrentProcess().ProcessName;
ProcessHelper.SetFocusToExternalApp(procName);

Here you are calling the function to bring focus to the other application's window.