Question

我试图从文本框中捕获值输入（是的，我已经验证它有一个niput并且不为null）。我声明了一个类变量，以确保我正在关联正确的表单。但是，我的语法似乎永远不会分配值，它会到达赋值行然后跳到PrintPreviewDlg.ShowDialog();并抛出错误

未处理的类型＆＃39; System.ArgumentNullException＆＃39;发生在System.Windows.Forms.dll中附加信息：值不能为空。

这是我的语法

public static LiveBait MainInstance { get; set; }

private void btnPreview_Click(object sender, EventArgs e)
{
  PrintPreviewDialog PrintPreviewDlg = new PrintPreviewDialog();
  PrintPreviewDlg.ClientSize = new System.Drawing.Size(400, 300);
  PrintPreviewDlg.Location = new System.Drawing.Point(29, 29);
  PrintPreviewDlg.Name = "PrintPreviewDlg";
  PrintPreviewDlg.MinimumSize = new System.Drawing.Size(375, 250);

  PrintPreviewDlg.WindowState = FormWindowState.Maximized;
  PrintPreviewDlg.UseAntiAlias = true;

  dynamic PD = CreatePrintDocument();
  PD.DefaultPageSettings.Landscape = true;
  PrintPreviewDlg.Document = PD;

  PrintPreviewDlg.ShowDialog();
}

PD CreatePrintDocument()
{
  PD document = new PD();
  document.SetParentCtrl(this);
  document.PrintData.MagRead = MainInstance.txtMagazineRead.Text;

  //More captures here

  return document;
}

class PraDa
{
    public string MagRead;
}

为什么我的变量永远不会被设置，我必须做什么才能确保它被设置为使btn click事件不会出错？

修改
COpying到剪贴板产生了这个堆栈跟踪

System.ArgumentNullException未处理的HResult = -2147467261 Message = Value不能为null。参数名称：args PARAMNAME = ARGS 来源= mscorlib程序堆栈跟踪：在System.String.Format（String format，Object [] args）在ProdData.PD.OnPrintPage（PrintPageEventArgs e）中的C：\ Users \ Owner \ Dropbox \ ProdData \ PD.cs：第234行在System.Drawing.Printing.PrintDocument._OnPrintPage（PrintPageEventArgs e）在System.Drawing.Printing.PrintController.PrintLoop（PrintDocument文档）在System.Drawing.Printing.PrintController.Print（PrintDocument文档）在System.Drawing.Printing.PrintDocument.Print（）在System.Windows.Forms.PrintPreviewControl.ComputePreview（）在System.Windows.Forms.PrintPreviewControl.CalculatePageInfo（）在System.Windows.Forms.Control.InvokeMarshaledCallbackDo（ThreadMethodEntry tme）在System.Windows.Forms.Control.InvokeMarshaledCallbackHelper（Object obj） at System.Threading.ExecutionContext.RunInternal（ExecutionContext executionContext，ContextCallback callback，Object state，Boolean preserveSyncCtx） at System.Threading.ExecutionContext.Run（ExecutionContext executionContext，ContextCallback callback，Object state，Boolean preserveSyncCtx）在System.Threading.ExecutionContext.Run（ExecutionContext executionContext，ContextCallback回调，对象状态）在System.Windows.Forms.Control.InvokeMarshaledCallback（ThreadMethodEntry tme）在System.Windows.Forms.Control.InvokeMarshaledCallbacks（）在System.Windows.Forms.Control.WndProc（消息＆amp; m）在System.Windows.Forms.PrintPreviewControl.WndProc（消息＆amp; m）在System.Windows.Forms.Control.ControlNativeWindow.OnMessage（Message＆amp; m）在System.Windows.Forms.Control.ControlNativeWindow.WndProc（Message＆amp; m）在System.Windows.Forms.NativeWindow.DebuggableCallback（IntPtr hWnd，Int32 msg，IntPtr wparam，IntPtr lparam）在System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW（MSG＆amp; msg）在System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop（IntPtr dwComponentID，Int32 reason，Int32 pvLoopData）在System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner（Int32 reason，ApplicationContext context）在System.Windows.Forms.Application.ThreadContext.RunMessageLoop（Int32 reason，ApplicationContext context）在System.Windows.Forms.Application.RunDialog（表单表单）在System.Windows.Forms.Form.ShowDialog（IWin32Window所有者）在System.Windows.Forms.Form.ShowDialog（）在C：\ Users \ Owner \ Dropbox \ Drl.cs中的ProdData.Controls.DrawingStageCtrl.btnPreview_Click（Object sender，EventArgs e）：第889行在System.Windows.Forms.Control.OnClick（EventArgs e）在System.Windows.Forms.Button.OnClick（EventArgs e）在System.Windows.Forms.Button.OnMouseUp（MouseEventArgs mevent）在System.Windows.Forms.Control.WmMouseUp（消息＆amp; m，MouseButtons按钮，Int32点击）在System.Windows.Forms.Control.WndProc（消息＆amp; m）在System.Windows.Forms.ButtonBase.WndProc（消息＆amp; m）在System.Windows.Forms.Button.WndProc（消息＆amp; m）在System.Windows.Forms.Control.ControlNativeWindow.OnMessage（Message＆amp; m）在System.Windows.Forms.Control.ControlNativeWindow.WndProc（Message＆amp; m）在System.Windows.Forms.NativeWindow.DebuggableCallback（IntPtr hWnd，Int32 msg，IntPtr wparam，IntPtr lparam）在System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW（MSG＆amp; msg）在System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop（IntPtr dwComponentID，Int32 reason，Int32 pvLoopData）在System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner（Int32 reason，ApplicationContext context）在System.Windows.Forms.Application.ThreadContext.RunMessageLoop（Int32 reason，ApplicationContext context）在System.Windows.Forms.Application.Run（Form mainForm）在C：\ Users \ Owner \ Dropbox \ Program.cs中的ProdData.Program.Main（）：第18行 InnerException：

编辑2
这是我所要求的方法的语法，希望这将有助于解决问题所在。

    protected override void OnPrintPage(PrintPageEventArgs e)
{
    base.OnPrintPage(e);
    int printHeight = 0;
    int printWidth = 0;
    int rightMargin = 0;

    PaperSize ps = default(PaperSize);
    for (int ix = 0; ix <= PrinterSettings.PaperSizes.Count - 1; ix++)
    {
        if (PrinterSettings.PaperSizes[ix].Kind == PaperKind.A4)
        {
            ps = PrinterSettings.PaperSizes[ix];
            DefaultPageSettings.PaperSize = ps;
            break;
        }
    }

    DefaultPageSettings.Margins.Top = PAGE_TOP_MARGIN;
    DefaultPageSettings.Margins.Left = PAGE_LEFT_MARGIN;
    DefaultPageSettings.Landscape = true;

    var CurrentPageSettings = base.DefaultPageSettings;
    printWidth = CurrentPageSettings.PaperSize.Height - CurrentPageSettings.Margins.Top - CurrentPageSettings.Margins.Bottom;
    printHeight = CurrentPageSettings.PaperSize.Width - CurrentPageSettings.Margins.Left - CurrentPageSettings.Margins.Right;
    m_leftMargin = CurrentPageSettings.Margins.Left; //X
    rightMargin = CurrentPageSettings.Margins.Top; //Y

    //Create a rectangle printing are for our document
    m_PrintArea = new RectangleF(m_leftMargin, rightMargin, printWidth, printHeight);

    // Get Normal Row Height
    int charactersFitted = 0;
    int linesFilled = 0;
    SizeF TextSize = new SizeF();
    StringFormat textFormat = new StringFormat();
    //Tell it to Alignment Text in its rectangle 
    textFormat.Alignment = StringAlignment.Near;
    textFormat.FormatFlags = StringFormatFlags.NoClip;
    TextSize = e.Graphics.MeasureString("NORMALROW", SUB_HEADING_FONT, m_PrintArea.Size, textFormat, out charactersFitted, out linesFilled);
    m_NormalRowHeight = (int)TextSize.Height + 3; //Row is bigger than text 

    //Draw First Heading
    Rectangle MainHeaderRect = new Rectangle();
    DrawMainHeading(e, LogoRect, ref MainHeaderRect);
    m_iCurrentLocationY = MainHeaderRect.Bottom + (m_NormalRowHeight);

    int LeftSubHeadingWidth = 200;
    m_SubHeaderTextFieldSize = new Size(LeftSubHeadingWidth, m_NormalRowHeight);
    m_TextValuePairSize = new Size(m_SubHeaderTextFieldSize.Width / 30, m_SubHeaderTextFieldSize.Height);

    bool IPN = !string.IsNullOrEmpty(MainInstance.txtPN.Text);
    bool IPJN = !string.IsNullOrEmpty(MainInstance.txtPJN.Text);
    bool IDt = !string.IsNullOrEmpty(MainInstance.dateIDt.Text);
    bool IPB = !string.IsNullOrEmpty(MainInstance.txtIPB.Text);
    bool ISS = !string.IsNullOrEmpty(MainInstance.txtISS.Text);

    string ST = !IPN ? String.Format("IPN:: ", MainInstance.txtPN.Text) : String.Format("IPN:: ", null);
    string pnt = !IPJN ? String.Format( MainInstance.txtPJN.Text) : String.Format(null);
    string dt = !IDt ? String.Format(MainInstance.dateIDt.Text) : String.Format(null);
    string pbt = !IPB ? String.Format(MainInstance.txtIPB.Text) : String.Format(null);
    string ST = !ISS ? String.Format(MainInstance.txtISS.Text) : String.Format(null);

    m_iCurrentLocationY += (int)(1.5 * m_NormalRowHeight);
    int iAlphaStart_Y = m_iCurrentLocationY;

    DrawSubHeading(e, ST, new Rectangle(new Point(m_leftMargin, m_iCurrentLocationY), m_SubHeaderTextFieldSize), StringAlignment.Near);
    DrawPairsToPage(e, "IPJN: ", pnt, true, m_leftMargin);
    DrawPairsToPage(e, "Date: ", dt, true, m_leftMargin);
    DrawPairsToPage(e, "IPB: ", pbt, true, m_leftMargin);
    DrawPairsToPage(e, "ISS: ", ST, true, m_leftMargin);

    Panel DrawingArea = m_ParentCtrl.GetDrawingArea();
    Bitmap pota = new Bitmap(DrawingArea.Width, DrawingArea.Height);
    m_ParentCtrl.ShowDetails(true);
    foreach (Control c in DrawingArea.Controls)
    {
        c.BringToFront();
    }
    DrawingArea.DrawToBitmap(pota, new Rectangle(0, 0, DrawingArea.Width, DrawingArea.Height));
    foreach (Control c in DrawingArea.Controls)
    {
        c.BringToFront();
    }
    m_ParentCtrl.ShowDetails(false);
    int iDrawingY = iAlphaStart_Y + (5 * m_NormalRowHeight);
    e.Graphics.DrawImage(pota, new Rectangle(m_leftMargin, iDrawingY, printWidth, printHeight - iDrawingY));
}
}

Answer 1

stacktrace告诉你ArgumentNull异常的确切位置：

C：\ Users \ Owner \ Dropbox \ ProdData \ PD.cs：第234行

它还告诉你它正在调用String.Format

我的orbuculum让我相信它就在这一行（为了便于阅读而格式化）：

string ST = !IPN ? 
            String.Format("IPN:: ", MainInstance.txtPN.Text) :
            String.Format("IPN:: ", null);

documenation也列出了这些例外情况。

这条线有几个问题。

首先String.Format("IPN:: ", null);是问题的原因，也没有多大意义。为什么要格式化null的东西呢？只需返回纯字符串。

其次String.Format("IPN:: ", MainInstance.txtPN.Text)不会抛出，但会始终输出IPN::，因为您忘记在格式字符串中包含{0}以指示参数应该去的位置。

最后String.Format(null)也会抛出异常。

我建议你介绍一个帮助器方法，这样就可以替换整个布尔值juggle

// takes any Control and return an emtpy string if Text is null or empty
string GetTextOrEmpty(Control ctl)
{
   var txt = ctl.Text; // base implementation of Text doesn't return null
   if (String.IsNullOrEmpty(txt)) {
     return String.Empty;
   } else {
     return txt;
   }
}

更像这样的东西：

string ST = String.Format("IPN:: {0}", GetTextOrEmpty(MainInstance.txtPN));
string pnt = GetTextOrEmpty(MainInstance.txtPJN.Text);
string dt = GetTextOrEmpty(MainInstance.dateIDt.Text);
string pbt = GetTextOrEmpty(MainInstance.txtIPB.Text);
string ST = GetTextOrEmpty((MainInstance.txtISS.Text);

这将解决参数args的ArgumentNull异常以及参数格式，但是你仍然需要遇到它。

从Windows窗体

1 个答案: