无法使用 VB 应用程序在 ASP.Net MVC 上查明空引用异常

如何解决无法使用 VB 应用程序在 ASP.Net MVC 上查明空引用异常

使用其他人的 VB.Net 代码。 View 正在抛出 'Object reference not set to an instance of an object.'，我的理解是“该对象在使用前尚未初始化。”。即使在使用断点进行调试之后，也无法确定问题的确切位置。我尝试了 here 提出的建议，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

它中断的 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

解决方法

问题出在 Controller 函数上。当视图被调用时，它向视图传递一个空对象，它显然是 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

我找到了对错误 Object reference not set to an instance of an object. 的很好的解释。