C# Get Monitor Size As Inch

GetMonitorSize.cs

[DllImport("gdi32.dll")]
static extern int GetDeviceCaps(IntPtr hdc, int nIndex);

private void Button_Click(object sender, RoutedEventArgs e)
{
    Graphics graphics = Graphics.FromHwnd(IntPtr.Zero); // using System.Drawing; (add from referances)
    IntPtr desktop = graphics.GetHdc();
    int monitorHeight = GetDeviceCaps(desktop, 6);
    int monitorWidth = GetDeviceCaps(desktop, 4);
    Content = $"Height: {monitorHeight} mm., Width: {monitorWidth} mm. This Monitor Size: {Math.Sqrt(Math.Pow(monitorHeight, 2) + Math.Pow(monitorWidth, 2)) / 25,4:#,##0.00} inch.";
}

Source

Edit: required System.Drawing reference