vb.net中的对象引用错误[重复]

Question

这是一个 Windows 窗体应用程序的一部分，它会提示用户“请输入销售订单”，然后会打开一个 CAD 绘图。但是，我在“dsDoc = dsApp.GetActiveDocument()”处遇到错误。谁能帮我解决这个问题？谢谢！

 Private Sub startButton_Click(sender As Object, e As EventArgs) Handles startButton.Click
    Dim salesOrder As Integer
    If Int32.TryParse(txtboxSalesOrder.Text, salesOrder) Then
        If salesOrder > 99999 AndAlso salesOrder <= 999999 Then
            Using process As Process = New Process
                Dim ProcessProperties As New ProcessStartInfo
                ProcessProperties.FileName = "C:\Program Files\Dassault Systemes\DraftSight\bin\DraftSight.exe"
                ProcessProperties.Arguments = ("C:\Users\MyUserName\Desktop\JE5022AA.dwg")
                ProcessProperties.WindowStyle = ProcessWindowStyle.Hidden
                process.Start(ProcessProperties)
            End Using
            Dim dsApp As DraftSight.Application
            Dim application As Object
            dsApp = Nothing
            application = Nothing

            Dim dsDoc As DraftSight.Document
            dsDoc = dsApp.GetActiveDocument()
           'An unhandled exception of type 'System.NullReferenceException' occurred in WindowsApplication2.exe
           'Additional information: Object reference not set to an instance of an object.

            If dsDoc Is Nothing Then
                Return
            End If

Answer 1

问题来了：

dsApp = Nothing
' ...
dsDoc = dsApp.GetActiveDocument()

在这种情况下，对象引用是变量dsApp。当它说它没有设置为对象的实例时，这意味着该变量为空（即Nothing）。显然，由于您将变量显式设置为Nothing，因此它将始终为空，因此可以保证抛出NullReferenceException。

要修复它，您需要将dsApp 设置为指向适当的DraftSight.Application 对象，而不是将其设置为Nothing。