.NET (Core and Framework)

#Region "API"
    Private Declare Function GetDesktopWindow Lib "user32" () As IntPtr
    Private Declare Function GetWindowRect Lib "user32" (ByVal hWnd As IntPtr, ByRef rect As Rectangle) As Boolean
    Private Declare Function PrintWindow Lib "user32" (ByVal hWnd As IntPtr, ByVal hDC As IntPtr, ByVal nFlags As Integer) As Integer
#End Region

Private Shared Function ImageFromWindow(ByVal hWnd As IntPtr) As Image
    If IntPtr.Equals(hWnd, IntPtr.Zero) Then Return Nothing

    Dim myImage As Image
    Try
        Dim rc As Rectangle
        GetWindowRect(hWnd, rc)
        Dim lW As Integer = rc.Width
        Dim lH As Integer = rc.Height
        If lW = 0 OrElse lH = 0 Then Return Nothing

        myImage = New Bitmap(lW, lH)
        Dim grImg As Graphics
        Try
            grImg = Graphics.FromImage(myImage)
            Dim iDC As IntPtr
            Try
                iDC = grImg.GetHdc()
                PrintWindow(hWnd, iDC, 0)
            Finally
                grImg.ReleaseHdc(iDC)
            End Try
        Catch
            myImage.Dispose()
            myImage = Nothing
        Finally
            grImg.Dispose()
            grImg = Nothing
        End Try
    Finally

    End Try
    Return myImage
End Function

Public Shared ReadOnly Property DesktopAsBitmap() As Image
    Get
        Return ImageFromWindow(GetDesktopWindow())
    End Get
End Property
Public ReadOnly Property WindowAsBitmap() As Image
    Get
        Return ImageFromWindow(hWnd)
    End Get
End Property