如何在foreach循环中处理任何内容

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，以及如何修复？

Answer 1

我会更改GetElementsByTagName，使其位于For Each语句之外，这样可以更容易地检查Collection是Null还是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