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

See example on how you can do this with full source code here:

http://www.csharphelp.com/2006/08/get-current-window-handle-and-caption-with-windows-api-in-c/

[DllImport("user32.dll")]
static extern IntPtr GetForegroundWindow();

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

private string GetActiveWindowTitle()
{
    const int nChars = 256;
    StringBuilder Buff = new StringBuilder(nChars);
    IntPtr handle = GetForegroundWindow();

    if (GetWindowText(handle, Buff, nChars) > 0)
    {
        return Buff.ToString();
    }
    return null;
}

Edited with @Doug McClean comments for better correctness.

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.

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;