C# .NET How to capture (screen shot) external application's window without using unmanaged code (user32.dll)

I am trying to capture/copy (screen shot) an external application and save it to a PictureBox. App #1, will be using .NET 4.0.

Load list box

C# pseudo Code for listbox:

using System.Web.UI.WebControls;

public Form1()
{
    InitializeComponent();

    foreach (Process process in Process.GetProcesses())
    {
        if (string.IsNullOrEmpty(process.MainWindowTitle))
            continue;

         listBox1.Items.Add(new ListItem(process.MainWindowTitle,
    process.MainWindowHandle.ToString()));
    }

}

Once user makes selection

C# pseudo code for listbox's button or event.

IntPtr intptr = _listBox1.Items[listBox1.SelectedIndex].value;

Graphics g = Graphics.FromHwnd(intptr);

// get width and height of app #2
int width = (int)g.VisibleClipBounds.Width;
int height = (int)g.VisibleClipBounds.Height;

Size s = new Size(width, height);

// create new bitmap
Bitmap wincapture = new Bitmap(width, height, g);

See notes below, needs code to copy App #2, to wincapture

// once App #2 has been captured, show image in picture box.
PictureBox1.Image = wincapture;

Question

How do I copy Application #2's window and save it in Application #1's Picture Box.

I am unable to use unmanaged code ex: user32.dll.

I am looking for a .NET answer.

Notes

I have tried using the following and it does not work or its missing something:

g.CopyFromScreen(0, 0, 0, 0, new Size(width, height));

g.DrawImage(wincapture, 0, 0, width, height);