Subroutine GetFormImage gets a Graphics object for the form. It makes a Bitmap big enough to hold the image and gets a Graphics object for it. It then gets the device context handles (hDC) for the two Graphics objects and uses BitBlt to copy the form's image into the Bitmap.

Private Declare Auto Function BitBlt Lib "gdi32.dll" (ByVal _
    hdcDest As IntPtr, ByVal nXDest As Integer, ByVal _
    nYDest As Integer, ByVal nWidth As Integer, ByVal _
    nHeight As Integer, ByVal hdcSrc As IntPtr, ByVal nXSrc _
    As Integer, ByVal nYSrc As Integer, ByVal dwRop As _
    System.Int32) As Boolean
Private Const SRCCOPY As Integer = &HCC0020

' Save the picture.
Private Sub btnSave_Click(ByVal sender As System.Object, _
    ByVal e As System.EventArgs) Handles btnSave.Click
    Dim bm As Bitmap = GetFormImage()
    bm.Save(txtFile.Text, ImageFormat.Bmp)
    Beep()
End Sub

Private Function GetFormImage() As Bitmap
    ' Get this form's Graphics object.
    Dim me_gr As Graphics = Me.CreateGraphics

    ' Make a Bitmap to hold the image.
    Dim bm As New Bitmap(Me.ClientSize.Width, _
        Me.ClientSize.Height, me_gr)
    Dim bm_gr As Graphics = me_gr.FromImage(bm)
    Dim bm_hdc As IntPtr = bm_gr.GetHdc

    ' Get the form's hDC. We must do this after 
    ' creating the new Bitmap, which uses me_gr.
    Dim me_hdc As IntPtr = me_gr.GetHdc

    ' BitBlt the form's image onto the Bitmap.
    BitBlt(bm_hdc, 0, 0, Me.ClientSize.Width, _
        Me.ClientSize.Height, _
        me_hdc, 0, 0, SRCCOPY)
    me_gr.ReleaseHdc(me_hdc)
    bm_gr.ReleaseHdc(bm_hdc)

    ' Return the result.
    Return bm
End Function