Get the Active Window?

Try this console app:

using System;
using System.Runtime.InteropServices;
using System.Threading;
using System.Text;

[StructLayout(LayoutKind.Sequential)]
public struct RECT
{
   public int Left;   
   public int Top;   
   public int Right;   
   public int Bottom;

   public int Width
   {
     get{return Right - Left;}
   }

   public int Height
   {
     get{return Bottom - Top;}
   }
}

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

    [DllImport("user32.dll")]
    static extern int GetWindowTextLength(IntPtr hWnd);

    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

    [DllImport("user32.dll")]
    static extern int GetWindowRect(IntPtr hWnd, out RECT rect);

    static void Main()
    {
        Console.WriteLine("You have 5 seconds to make another window active");
        Console.WriteLine("otherwise dimensions will be for this console window"); 
        Thread.Sleep(5000);
        IntPtr hWnd = GetForegroundWindow();
        int length = GetWindowTextLength(hWnd);
        StringBuilder sb = new StringBuilder(length +1);
        GetWindowText(hWnd, sb, sb.Capacity);       
        RECT rect;
        GetWindowRect(hWnd, out rect);
        Console.WriteLine("\nActive window title is '{0}'", sb.ToString());
        Console.WriteLine("Width is {0} pixels",rect.Width);
        Console.WriteLine("Height is {0} pixels", rect.Height);
    }
}