Take screenshot of external application

[Edgemeal]

a. if there are other windows on top of that application, it takes the screenshot of that top most window.

The PrintWindow API can capture windows even if they are behind other windows or off to the sides of the desktop area.
One minor drawback is PrintWindow doesn't support opacity so captured windows are always solid.

Public Class Form1
' Tested on: WinXP-32 and Win7-64.
    Public Structure RECT
        Public left As Int32
        Public top As Int32
        Public right As Int32
        Public bottom As Int32
    End Structure
    Private Declare Function GetWindowRect Lib "user32.dll" (ByVal hwnd As IntPtr, ByRef lpRect As RECT) As Int32
    Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As IntPtr
    Private Declare Function PrintWindow Lib "User32.dll" (ByVal hWnd As IntPtr, ByVal hdcBlt As IntPtr, ByVal nFlags As Int32) As Int32

    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        ' get handle of app to capture by the titlebar text
        Dim hWnd As IntPtr = FindWindow(Nothing, "Calculator") '<<< WINDOWS CALCULATOR

        If hWnd = IntPtr.Zero Then
            MessageBox.Show("Window not found!", "No Window Match", MessageBoxButtons.OK, MessageBoxIcon.Information)
        Else
            ' Capture app to Bitmap
            Dim bmp As Bitmap = Capture_hWndToBitmap(hWnd)
            If bmp IsNot Nothing Then
                Try
                    ' save bitmap to a public folder
                    bmp.Save("C:\Users\Ed\Pictures\TEST_CAPTURE.png", Imaging.ImageFormat.Png)                    
                Catch ex As Exception
                    MessageBox.Show(ex.Message, "ERROR", MessageBoxButtons.OK, MessageBoxIcon.Error)
                End Try
                bmp.Dispose()
            Else
                MessageBox.Show("Capture failed!", "Capture Fail", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)
            End If
        End If
    End Sub

    ' Capture window by handle to bitmap
    Private Function Capture_hWndToBitmap(ByVal WindowHandle As IntPtr) As Bitmap
        Try
            Dim WinRect As RECT
            GetWindowRect(WindowHandle, WinRect)
            Dim bmp As Bitmap = New Bitmap(WinRect.right - WinRect.left, WinRect.bottom - WinRect.top)
            Dim g As Graphics = Graphics.FromImage(bmp)
            Dim hDC As IntPtr = g.GetHdc()
            Dim pwRet As Integer = PrintWindow(WindowHandle, hDC, 0)
            g.ReleaseHdc(hDC)
            If pwRet = 0 Then ' printwindow failed!
                Return Nothing
            Else
                Return bmp
            End If
        Catch
            Return Nothing ' something went wrong!
        End Try
    End Function

End Class

I had posted something similar yesterday but removed it because I was getting an error when run under Win7, turns out I didn't have permission to write the file to the drive/folder I selected!