[c#] How do I get the title of the current active window using c#?

I'd like to know how to grab the Window title of the current active window (i.e. the one that has focus) using C#.

Use the Windows API. Call GetForegroundWindow().

GetForegroundWindow() will give you a handle (named hWnd) to the active window.

Documentation: GetForegroundWindow function | Microsoft Docs

If it happens that you need the Current Active Form from your MDI application: (MDI- Multi Document Interface).

Form activForm;
activForm = Form.ActiveForm.ActiveMdiChild;

Based on GetForegroundWindow function | Microsoft Docs:

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern IntPtr GetForegroundWindow();

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern int GetWindowText(IntPtr hWnd, StringBuilder text, int count);

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern int GetWindowTextLength(IntPtr hWnd);

private string GetCaptionOfActiveWindow()
{
    var strTitle = string.Empty;
    var handle = GetForegroundWindow();
    // Obtain the length of the text   
    var intLength = GetWindowTextLength(handle) + 1;
    var stringBuilder = new StringBuilder(intLength);
    if (GetWindowText(handle, stringBuilder, intLength) > 0)
    {
        strTitle = stringBuilder.ToString();
    }
    return strTitle;
}

It supports UTF8 characters.

you can use process class it's very easy. use this namespace

using System.Diagnostics;

if you want to make a button to get active window.

private void button1_Click(object sender, EventArgs e)
    {            
       Process currentp = Process.GetCurrentProcess();
       TextBox1.Text = currentp.MainWindowTitle;  //this textbox will be filled with active window.
    }

If you were talking about WPF then use:

 Application.Current.Windows.OfType<Window>().SingleOrDefault(w => w.IsActive);

Loop over Application.Current.Windows[] and find the one with IsActive == true.