VB.Net How to set an object reference to an instan

问题:

Using VB.NET, trying to write a page title to a text file. I am stumped here:

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

But I get an error saying "Object reference not set to an instance of an object." Please help!

回答1:

Most likely you are trying to access the Document property when it is still equal to Nothing. Move your code to the DocumentCompleted event of the WebBrowser control, as such:

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

回答2:

My Guess is 'WebBrowser1.Documen't is null. I'm not sure what conditions have to exist to make the Document not null, but you should definitely check that first before trying to grab its title.

I stole this from: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