为什么我得到一个空引用异常？

Question

我使用iTextSharp的，并试图用

PdfAction action = PdfAction.GotoLocalPage(1, pdfDest, writer); 

这是一个静态方法，但我总是得到这个错误

Server Error in '/' Application. 
Object reference not set to an instance of an object. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object. 

Source Error: 


Line 178: 
Line 179:   //create a new action to send the document to our new destination. 
Line 180:   PdfAction action = PdfAction.GotoLocalPage(1, pdfDest, writer); 
Line 181: 
Line 182:   //set the open action for our writer object 


Source File: VoucherService.cs Line: 180 

Stack Trace: 


[NullReferenceException: Object reference not set to an instance of an object.] 
    iTextSharp.text.pdf.PdfWriter.GetPageReference(Int32 page) +326 
    iTextSharp.text.pdf.PdfAction.GotoLocalPage(Int32 page, PdfDestination dest, PdfWriter writer) +49 
    OnlineStudentPlanner.Framework.Services.VoucherService.SetupPdfDoc(String invoiceNumber, String logoPath, Document doc, MemoryStream memoryStream, PdfWriter writer, PdfPTable& table) in VoucherService.cs:180 
    OnlineStudentPlanner.Framework.Services.VoucherService.GenerateVouchers(Int32 qty, Int32 voucherSize, String invoiceNumber, String logoPath, Int32 siteWideQty, IEnumerable`1 validDomains, Boolean siteWideVoucher) in VoucherService.cs:55 
    OnlineStudentPlanner.WebUI.Areas.Admin.Controllers.HomeController.GenerateVouchers(GenerateVouchersVm vm) in Admin\Controllers\HomeController.cs:52 
    lambda_method(Closure , ControllerBase , Object[]) +163 
    System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +17 
    System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +208 
    System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +27 
    System.Web.Mvc.<>c__DisplayClass15.<InvokeActionMethodWithFilters>b__12() +55 
    System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +263 
    System.Web.Mvc.<>c__DisplayClass17.<InvokeActionMethodWithFilters>b__14() +19 
    System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodWithFilters(ControllerContext controllerContext, IList`1 filters, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +191 
    System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +343 
    System.Web.Mvc.Controller.ExecuteCore() +116 
    System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +97 
    System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) +10 
    System.Web.Mvc.<>c__DisplayClassb.<BeginProcessRequest>b__5() +37 
    System.Web.Mvc.Async.<>c__DisplayClass1.<MakeVoidDelegate>b__0() +21 
    System.Web.Mvc.Async.<>c__DisplayClass8`1.<BeginSynchronous>b__7(IAsyncResult _) +12 
    System.Web.Mvc.Async.WrappedAsyncResult`1.End() +62 
    System.Web.Mvc.<>c__DisplayClasse.<EndProcessRequest>b__d() +50 
    System.Web.Mvc.SecurityUtil.<GetCallInAppTrustThunk>b__0(Action f) +7 
    System.Web.Mvc.SecurityUtil.ProcessInApplicationTrust(Action action) +22 
    System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +60 
    System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9 
    System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +8970061 
    System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +184 

这里是我的代码，我有

Document doc = new Document(); 
MemoryStream memoryStream = new MemoryStream(); 
PdfPTable table; 

doc.Open(); 

PdfWriter writer = PdfWriter.GetInstance(doc, memoryStream); 
writer.CloseStream = false; 

SetupPdfDoc(invoiceNumber, logoPath, doc, memoryStream, writer, out table); 

private void SetupPdfDoc(string invoiceNumber, string logoPath, Document doc, MemoryStream memoryStream, PdfWriter writer, out PdfPTable table) 
{ 
    PdfDestination pdfDest = new PdfDestination(PdfDestination.XYZ, 0, doc.PageSize.Height, 0.75f); 
    doc.AddTitle(String.Format("Vouchers-{0}", invoiceNumber)); 
} 

Answer 1

这是因为您试图在没有任何页面的情况下操作空白的PDF文档。

更改以下三行：

 doc.Open(); 

     PdfWriter writer = PdfWriter.GetInstance(doc, memoryStream); 
     writer.CloseStream = false; 

到：

 PdfWriter writer = PdfWriter.GetInstance(doc, memoryStream); 
     writer.CloseStream = false; 
     doc.Open(); 
     doc.NewPage(); 

应该解决您的问题。

Answer 2

你检查过pdfDest和writer是否为空？它以线抛出异常：

iTextSharp.text.pdf.PdfWriter.GetPageReference(Int32 page) +326 

我认为作家不为空，所以无论是pdfDest为空或有缺陷的iTextSharp的，并传递错误的页面是引起异常（错误的例外，它应该是OutOfBounds或者其他的东西）。