vb.net - 如何在 foreach 循环中不处理任何内容

Question

我有以下功能:

 Public Sub performautowebrowserOperations()
    Try
        For Each link As HtmlElement In WebBrowser2.Document.GetElementsByTagName("input") 'sometimes throws a null reference exception
            If link.GetAttribute("value") IsNot Nothing Then
                If link.GetAttribute("value") = "Compare prices" Then
                    link.InvokeMember("click")
                End If
            End If
        Next
    Catch ex As Exception
        MsgBox(ex.Message)
    End Try
End Sub

有时注释行会抛出 NullReferenceException。为什么会这样，如何解决？

Answer 1

我会更改 GetElementsByTagName ，使其位于 For Each 语句之外，这样可以更轻松地检查 Collection 是否为 Null 或 Empty。

Public Sub performautowebrowserOperations()
    Try
        Dim elements As HtmlElementCollection = WebBrowser2.Document.GetElementsByTagName("input")
        If Not IsNothing(elements) And elements.Count > 0 Then
            For Each link As HtmlElement In elements
                If link.GetAttribute("value") IsNot Nothing Then
                    If link.GetAttribute("value") = "Compare prices" Then
                        link.InvokeMember("click")
                    End If
                End If
            Next
        End If
    Catch ex As Exception
        MsgBox(ex.Message)
    End Try
End Sub