无法使用 VB 应用在 ASP.Net MVC 上查明空引用异常

Question

使用其他人的 VB.Net 代码。 A View 抛出 'Object reference not set to an instance of an object.'，我的理解是“对象在使用前尚未初始化”。即使在使用断点调试后也无法确定问题的确切位置。我尝试了 here 的建议，但运气不佳。

分享所涉及的功能，如果有人能发现问题：

Public Function EditBefore(id As String) As UXEmailTemplate
    Dim m_EmailTemplate As UXEmailTemplate = GetEmailTemplate(id)
        Try

            Dim m_GetEmailTemplate As New UXEmailTemplate

            With m_GetEmailTemplate
                .Versions = GetVersions(m_EmailTemplate.ParentID).ToList()
            End With

            With m_EmailTemplate
                .Versions = m_GetEmailTemplate.Versions
            End With

            Return m_EmailTemplate

        Catch ex As Exception
            ex.ToString()
            _c.WriteError(System.Reflection.MethodInfo.GetCurrentMethod.ToString, String.Concat("ERROR: ", ex.ToString))
            Return m_EmailTemplate
        End Try
    End Function


        Public Function GetEmailTemplate(id As String) As UXEmailTemplate

        Dim m_EmailTemplates As List(Of UXEmailTemplate)
        GetEmailTemplate = Nothing

        m_EmailTemplates = GetAllEmailTemplates()

        If m_EmailTemplates IsNot Nothing Then
            For Each m_EmailTemplate As UXEmailTemplate In m_EmailTemplates
                If m_EmailTemplate.ID.Equals(id) Then
                    GetEmailTemplate = m_EmailTemplate
                    Exit For
                End If
            Next
        Else
            Return Nothing
        End If

    End Function

中断的 View 代码是：

  <div Class="col-sm-4">
      @If (Model.Versions.Count > 1) Then            <<<<<<< here exception occurs (returns Nothing)
        @<div Class="cardFull" style="padding-top:20px;">
            <div Class="labelUX">Email Template Versions</div>
        </div>
      @<div Class="cardFull Checkboxlisten">
           <div id="CheckBoxlisten" Class="CheckboxlistenContent" style="background-color: lightgrey;">
               @For Each item In Model.Versions
                @<p>Version <a href="\KI\NewsletterEdit\@item.ID">@item.Version</a></p>Next
                                </div>
      </div>End If
    </div>

控制器：

 <HttpPost()>
    <ValidateInput(False)>
    <ValidateAntiForgeryToken()>
    Function NewsletterEdit(<Bind(Include:="ID, SendFrom, Subject,Text, HtmlText,CreatedDate, Version, ParentID")> ByVal item As UXEmailTemplate, url As String) As ActionResult
        If ModelState.IsValid Then
            Dim m_Error As Boolean = False
            If item Is Nothing Then
                ModelState.AddModelError("", "unexpected error")
                m_Error = True
            End If

            Dim m_Message As String = String.Empty

            If Not m_Error Then
                m_Message = dbEmail.EditEmailTemplate(item)
            End If

            If Not String.IsNullOrEmpty(m_Message) Then
                ModelState.AddModelError("", m_Message)
                m_Error = True
            End If

            If m_Error = True Then
                Dim m_EmailTemplate As New UXEmailTemplate
                Return View(m_EmailTemplate)
            End If

            If String.IsNullOrEmpty(url) Then
                Return RedirectToAction("../KI/Newsletter")
            Else
                Return Redirect(url)
            End If
        Else
            Return View(User)
        End If

    End Function

Answer 1

问题出在 Controller 函数上。调用视图时，它会将一个空对象传递给视图，显然是 Nothing for VB.Net 或 Null in c#.

此处创建了对象但未填充数据。

If m_Error = True Then
   Dim m_EmailTemplate As New UXEmailTemplate <<< this will throw NullReferrence exception
       Return View(m_EmailTemplate)
End If

解决方案：为对象填充一些数据。

If m_Error = True Then
   Dim m_EmailTemplate As New UXEmailTemplate 
   m_EmailTemplate = GetETemplate(id)   <<<<call the function which will return the object for the view 
       Return View(m_EmailTemplate)
End If

我找到了一个很好的错误解释 Object reference not set to an instance of an object.。

无法使用 VB 应用在 ASP.Net MVC 上查明空引用异常

Can't pinpoint null reference exception on ASP.Net MVC with VB app

vb.net

nullreferenceexception

object-reference