Question

我试图在VB.Net中使用Webbrowser组件构建我的第一个HTML UI。我在Microsoft网站上找到了这个代码示例

https://msdn.microsoft.com/en-us/library/system.windows.forms.webbrowser.document(v=vs.110).aspx：

 Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs) _
    Handles Me.Load

        WebBrowser1.DocumentText =
        "<html><body>Please enter your name:<br/>" &
        "<input type='text' name='userName'/><br/>" &
        "<a href='http://www.microsoft.com'>continue</a>" &
        "</body></html>"

    End Sub

    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

当我把它放到测试中时，大部分时间抛出异常＆＃39; System.NullReferenceException＆＃39;在这部分代码中：

If document IsNot Nothing And
        document.All("userName") IsNot Nothing And
        String.IsNullOrEmpty(
        document.All("userName").GetAttribute("value")) Then

有时它有效，但大多数情况下根本不起作用。知道如何解决这个问题吗？我对.Net平台很新，如果有拼写错误，请对不起。任何帮助表示赞赏。

Answer 1

如果document为空，那么If语句的其他子句将生成例外，因为您在document为Nothing时尝试访问属性。你需要像这样重写代码：

Dim document As System.Windows.Forms.HtmlDocument = WebBrowser1.Document

If document IsNot Nothing Then
    If document.All("userName") IsNot Nothing Then
        If 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 If
End If

Web浏览器控件在导航时抛出NullReferenceException

1 个答案: