How can I get the active screen dimensions?

Screen.FromControl, Screen.FromPoint and Screen.FromRectangle should help you with this. For example in WinForms it would be:

class MyForm : Form
{
  public Rectangle GetScreen()
  {
    return Screen.FromControl(this).Bounds;
  }
}

I don't know of an equivalent call for WPF. Therefore, you need to do something like this extension method.

static class ExtensionsForWPF
{
  public static System.Windows.Forms.Screen GetScreen(this Window window)
  {
    return System.Windows.Forms.Screen.FromHandle(new WindowInteropHelper(window).Handle);
  }
}

You can use this to get desktop workspace bounds of the primary screen:

System.Windows.SystemParameters.WorkArea

This is also useful for getting just the size of the primary screen:

System.Windows.SystemParameters.PrimaryScreenWidth System.Windows.SystemParameters.PrimaryScreenHeight

Also you may need:

• SystemParameters.VirtualScreenWidth
• SystemParameters.VirtualScreenHeight

to get the combined size of all monitors and not one in particular.

Adding a solution that doesn't use WinForms but NativeMethods instead. First you need to define the native methods needed.

public static class NativeMethods
{
    public const Int32 MONITOR_DEFAULTTOPRIMERTY = 0x00000001;
    public const Int32 MONITOR_DEFAULTTONEAREST = 0x00000002;


    [DllImport( "user32.dll" )]
    public static extern IntPtr MonitorFromWindow( IntPtr handle, Int32 flags );


    [DllImport( "user32.dll" )]
    public static extern Boolean GetMonitorInfo( IntPtr hMonitor, NativeMonitorInfo lpmi );


    [Serializable, StructLayout( LayoutKind.Sequential )]
    public struct NativeRectangle
    {
        public Int32 Left;
        public Int32 Top;
        public Int32 Right;
        public Int32 Bottom;


        public NativeRectangle( Int32 left, Int32 top, Int32 right, Int32 bottom )
        {
            this.Left = left;
            this.Top = top;
            this.Right = right;
            this.Bottom = bottom;
        }
    }


    [StructLayout( LayoutKind.Sequential, CharSet = CharSet.Auto )]
    public sealed class NativeMonitorInfo
    {
        public Int32 Size = Marshal.SizeOf( typeof( NativeMonitorInfo ) );
        public NativeRectangle Monitor;
        public NativeRectangle Work;
        public Int32 Flags;
    }
}

And then get the monitor handle and the monitor info like this.

        var hwnd = new WindowInteropHelper( this ).EnsureHandle();
        var monitor = NativeMethods.MonitorFromWindow( hwnd, NativeMethods.MONITOR_DEFAULTTONEAREST );

        if ( monitor != IntPtr.Zero )
        {
            var monitorInfo = new NativeMonitorInfo();
            NativeMethods.GetMonitorInfo( monitor, monitorInfo );

            var left = monitorInfo.Monitor.Left;
            var top = monitorInfo.Monitor.Top;
            var width = ( monitorInfo.Monitor.Right - monitorInfo.Monitor.Left );
            var height = ( monitorInfo.Monitor.Bottom - monitorInfo.Monitor.Top );
        }