asp.net - 如何使用 Linq 获取 XML 元素的值(即使为空)

Question

请原谅我的愚蠢，我倾向于发现遍历 XML 过于复杂。

我在 VB 中使用 ASP.NET。

我有一个 XML 文档，其中包含我公司员工的所有详细信息...

<staff>
    <staffName>Test Staff</staffName>
    <staffTitle>Slave</staffTitle>
    <staffDepartmentName>Finance</staffDepartmentName>
    <staffOffice>London</staffOffice>
    <staffEmail>t.staff@company.co.uk</staffEmail>
    <staffPhone>0207 123 456</staffPhone>
    <staffNotes>Working hours Mon to Thurs 9.15 - 5.15</staffNotes>
    <staffBio></staffBio>
</staff>

正如您所看到的，某些节点并不总是包含所有员工的数据；只有董事才有传记。

我访问这样的值...

For Each staff In ( _
    From matches In myXMLFile.Descendants("staff").Descendants("staffName") _
        Where matches.Nodes(0).ToString.ToLower.Contains(LCase(search)) _
        Order By matches.Value _
        Select matches)
    staffName = staff.Descendants("staffName").Nodes(0).ToString)
    staffTitle = staff.Descendants("staffTitle").Nodes(0).ToString)
    staffOffice = staff.Descendants("staffOffice").Nodes(0).ToString)
    staffEmail = staff.Descendants("staffEmail").Nodes(0).ToString)
    staffPhone = staff.Descendants("staffPhone").Nodes(0).ToString)
    staffNotes = staff.Descendants("staffNotes").Nodes(0).ToString)
    staffBio = staff.Descendants("staffBio").Nodes(0).ToString)

    ' Do something with that data...
Next

一旦到达staffBio，我收到一条错误消息“对象引用未设置到对象的实例”。显然是因为该节点不存在。

我的问题是，即使变量为空，如何将值分配给变量，而不必在每次赋值之前进行条件检查？

Answer 1

首先，myXMLFile.Descendants("staff").Descendants("staffName") 是多余的。 Descendants 返回 XDocument 或 XElement 内任何级别的所有元素。因此，myXMLFile.Descendants("staffName") 会给出相同的结果。

其次，您可以像这样使用 Element 属性和 Value 属性:

staffBio = staff.Element("staffBio").Value

staff 将只有一个 staffBio 元素，因此无需使用 Descendants 属性。 Value 是一个字符串，因此您不需要调用Value.ToString。如果元素为空，则 Value 将返回一个空字符串，这就是您要查找的内容!

第三，在 VB.NET 中有一种更好的(而且我认为更直接的)方法可以做到这一点。这是一个控制台应用程序，演示了我将如何执行此操作:

Module Module1

Sub Main()
    Dim myXMLFile = <allStaff>
                        <staff>
                            <staffName>Test Staff</staffName>
                            <staffTitle>Slave</staffTitle>
                            <staffDepartmentName>Finance</staffDepartmentName>
                            <staffOffice>London</staffOffice>
                            <staffEmail>t.staff@battens.co.uk</staffEmail>
                            <staffPhone>0207 123 456</staffPhone>
                            <staffNotes>Working hours Mon to Thurs 9.15 - 5.15</staffNotes>
                            <staffBio></staffBio>
                        </staff>
                        <staff>
                            <staffName>Other Staff</staffName>
                            <staffTitle>Master</staffTitle>
                            <staffDepartmentName>IT</staffDepartmentName>
                            <staffOffice>Oxford</staffOffice>
                            <staffEmail>o.staff@battens.co.uk</staffEmail>
                            <staffPhone>0207 123 789</staffPhone>
                            <staffNotes></staffNotes>
                            <staffBio>Some guy.</staffBio>
                        </staff>
                    </allStaff>

    Dim search = "Test"

    Dim searchQuery = From staff In myXMLFile...<staff> _
                      Where staff.<staffName>.Value.Contains(search) _
                      Select si = New StaffInfo With {.Name = staff.<staffName>.Value, _
                                                      .Title = staff.<staffTitle>.Value, _
                                                      .Department = staff.<staffDepartmentName>.Value, _
                                                      .Office = staff.<staffOffice>.Value, _
                                                      .Email = staff.<staffEmail>.Value, _
                                                      .Phone = staff.<staffPhone>.Value, _
                                                      .Notes = staff.<staffNotes>.Value, _
                                                      .Bio = staff.<staffBio>.Value}

    For Each staff In searchQuery
        Console.WriteLine("Name: {0}", staff.Name)
        Console.WriteLine("Title: {0}", staff.Title)
        Console.WriteLine("Department: {0}", staff.Department)
        Console.WriteLine("Office: {0}", staff.Office)
        Console.WriteLine("Email: {0}", staff.Email)
        Console.WriteLine("Phone: {0}", staff.Phone)
        Console.WriteLine("Notes: {0}", staff.Notes)
        Console.WriteLine("Bio: {0}", staff.Bio)
        Console.WriteLine()
    Next

    Console.ReadLine()

End Sub

Private Class StaffInfo

    Private _name As String
    Public Property Name() As String
        Get
            Return _name
        End Get
        Set(ByVal value As String)
            _name = value
        End Set
    End Property

    Private _title As String
    Public Property Title() As String
        Get
            Return _title
        End Get
        Set(ByVal value As String)
            _title = value
        End Set
    End Property

    Private _department As String
    Public Property Department() As String
        Get
            Return _department
        End Get
        Set(ByVal value As String)
            _department = value
        End Set
    End Property

    Private _office As String
    Public Property Office() As String
        Get
            Return _office
        End Get
        Set(ByVal value As String)
            _office = value
        End Set
    End Property

    Private _email As String
    Public Property Email() As String
        Get
            Return _email
        End Get
        Set(ByVal value As String)
            _email = value
        End Set
    End Property

    Private _phone As String
    Public Property Phone() As String
        Get
            Return _phone
        End Get
        Set(ByVal value As String)
            _phone = value
        End Set
    End Property

    Private _notes As String
    Public Property Notes() As String
        Get
            Return _notes
        End Get
        Set(ByVal value As String)
            _notes = value
        End Set
    End Property

    Private _bio As String
    Public Property Bio() As String
        Get
            Return _bio
        End Get
        Set(ByVal value As String)
            _bio = value
        End Set
    End Property

End Class

End Module

如果您有 XML 的架构(.xsd 文件)，则可以将该 xmlns 的引用导入到 VB 源文件中，这将为您提供编写 LINQ to XML 查询的智能感知。

(编辑:创建架构的快速方法是在 Visual Studio 中打开 XML 文件，然后从 XML 菜单中选择“创建架构”。)

有关更多信息和帮助，请查看 "How Do I" video series on LINQ by Beth Massi 。