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net 2010，我有以下代码将面板及其内容带到打印机，但每次我尝试将面板转换为 PDF 表单时，按钮单击中的 pd.Print() 总是返回 nullreferencedunhandled 任何想法解决这个问题？？谢谢

公开课形式1

Dim img As Bitmap
Dim WithEvents pd As PrintDocument

'Returns the Form as a bitmap
Function CaptureForm1() As Bitmap

    Dim g1 As Graphics = Me.CreateGraphics()
    Dim MyImage = New Bitmap(Me.ClientRectangle.Width, (Me.ClientRectangle.Height), g1)
    Dim g2 As Graphics = Graphics.FromImage(MyImage)
    Dim dc1 As IntPtr = g1.GetHdc()
    Dim dc2 As IntPtr = g2.GetHdc()
    BitBlt(dc2, 0, 0, Me.ClientRectangle.Width, (Me.ClientRectangle.Height), dc1, 0, 0, 13369376)
    g1.ReleaseHdc(dc1)
    g2.ReleaseHdc(dc2)
    'saves image to c drive just, u can comment it also
    'MyImage.Save("c:\abc.bmp")
    Return MyImage
End Function

<DllImport("gdi32.DLL", EntryPoint:="BitBlt", _
 SetLastError:=True, CharSet:=CharSet.Unicode, _
 ExactSpelling:=True, _
 CallingConvention:=CallingConvention.StdCall)> _
Private Shared Function BitBlt(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

    ' Leave function empty - DLLImport attribute forwards calls to MoveFile to
    ' MoveFileW in KERNEL32.DLL.
End Function

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    'img = CaptureForm1()
    'pd = New PrintDocument
    'pd.Print()

    Dim PrintDialog1 As New PrintDialog()
    PrintDialog1.Document = pd
    Dim result As DialogResult = PrintDialog1.ShowDialog()

    If (result = DialogResult.OK) Then
        pd.Print()
    End If


End Sub
'this method will be called each time when pd.printpage event occurs
Sub pd_PrintPage(ByVal sender As Object, ByVal e As PrintPageEventArgs) Handles pd.PrintPage

    Dim x As Integer = e.MarginBounds.X
    Dim y As Integer = e.MarginBounds.Y
    e.Graphics.DrawImage(img, x, y)
    e.HasMorePages = False

End Sub

结束类

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