Question

使用VB.NET，尝试将页面标题写入文本文件。我在这里难过：

Private Sub
    Dim pagetitle As String
    pagetitle = WebBrowser1.Document.Title
    My.Computer.FileSystem.WriteAllText("page title.txt", pagetitle, False)

但是我收到一条错误，说“对象引用没有设置为对象的实例”。请帮忙！

Answer 1

当Document属性仍然等于Nothing时，您很可能会尝试访问该属性。将代码移至DocumentCompleted控件的WebBrowser事件，如下所示：

Private Sub WebBrowser1_DocumentCompleted(sender As System.Object, e As System.Windows.Forms.WebBrowserDocumentCompletedEventArgs) Handles WebBrowser1.DocumentCompleted
        If WebBrowser1.Document IsNot Nothing Then
            Dim pagetitle As String
            pagetitle = WebBrowser1.Document.Title
            My.Computer.FileSystem.WriteAllText("page title.txt", pagetitle, False)
        End If
End Sub

Answer 2

我的猜测是'WebBrowser1.Documen't为空。我不确定什么条件才能使Document不为null，但在尝试获取它的标题之前，你应该先检查它。

我偷了这个：http://msdn.microsoft.com/en-us/library/system.windows.forms.webbrowser.document.aspx

Private Sub webBrowser1_Navigating( _
    ByVal sender As Object, ByVal e As WebBrowserNavigatingEventArgs) _
    Handles webBrowser1.Navigating

    Dim document As System.Windows.Forms.HtmlDocument = _
    webBrowser1.Document
    If document IsNot Nothing And _
        document.All("userName") IsNot Nothing And _
        String.IsNullOrEmpty( _
        document.All("userName").GetAttribute("value")) Then

        e.Cancel = True
        MsgBox("You must enter your name before you can navigate to " & _
            e.Url.ToString())
    End If 

End Sub

VB.Net如何设置对象实例的对象引用？

2 个答案: