不满足条件时继续评估条件

Question

在 Visual Studio 表单中的 WebBrowser 控件中，这段代码工作正常：

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

    WebBrowser1.DocumentText =
    "<html><body>Search in Google:<br/>" &
    "<form method='get' action='http://www.google.com/search'>" &
    "<input type='text' name='as_q'/><br/>" &
    "<input type='submit' value='Search'/>" &
    "</form></body></html>"

End Sub

当我将以下代码添加到通过 WebBrowser 文档验证 HTML 表单时属性收到此错误消息：对象引用未设置为对象的实例。

 Private Sub WebBrowser1_Navigating(sender As Object, e As WebBrowserNavigatingEventArgs) Handles WebBrowser1.Navigating

    Dim document = WebBrowser1.Document
    If document IsNot Nothing And
    document.All("as_q") IsNot Nothing And
    String.IsNullOrEmpty(
    document.All("as_q").GetAttribute("value")) Then
        e.Cancel = True
        MsgBox("Please enter a search term.")
    End If

End Sub

Answer 1

将所有 And 更改为 AndAlso... 截至目前，您仍在尝试评估逻辑。

当你使用AndAlso时它会短路并在不满足条件时跳出

不满足条件时继续评估条件

Evaluating condition continues when condition is not met

vb.net

webbrowser-control

visual-studio