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如何控制投影的类型 (LINQ to XML)

  • 项目

投影是过滤一组数据、改变其形状和类型的过程。 大多数查询表达式都会进行投影。 本节中的大多数查询表达式的计算结果为 XElement 的 IEnumerable<T>，但你可以创建其他类型的集合。 本文介绍如何执行此操作。

示例：定义一个新类型并创建一个创建该类型的 IEnumerable 的查询

下面的示例定义了一个新类型 Customer，并且查询表达式在 Select 子句中实例化了新的 Customer 对象。 这样，查询表达式的类型就是 IEnumerable<T> 的 Customer。 此示例使用 XML 文档 示例 xml 文件：客户和订单。

public class Customer
{
    private string customerID;
    public string CustomerID{ get{return customerID;} set{customerID = value;}}

    private string companyName;
    public string CompanyName{ get{return companyName;} set{companyName = value;}}

    private string contactName;
    public string ContactName { get{return contactName;} set{contactName = value;}}

    public Customer(string customerID, string companyName, string contactName)
    {
        CustomerID = customerID;
        CompanyName = companyName;
        ContactName = contactName;
    }

    public override string ToString()
    {
        return $"{this.customerID}:{this.companyName}:{this.contactName}";
    }
}

class Program
{
    static void Main(string[] args)
    {
        XElement custOrd = XElement.Load("CustomersOrders.xml");
        IEnumerable<Customer> custList =
            from el in custOrd.Element("Customers").Elements("Customer")
            select new Customer(
                (string)el.Attribute("CustomerID"),
                (string)el.Element("CompanyName"),
                (string)el.Element("ContactName")
            );
        foreach (Customer cust in custList)
            Console.WriteLine(cust);
    }


}

Public Class Customer
    Private customerIDValue As String
    Public Property CustomerID() As String
        Get
            Return customerIDValue
        End Get
        Set(ByVal value As String)
            customerIDValue = value
        End Set
    End Property

    Private companyNameValue As String
    Public Property CompanyName() As String
        Get
            Return companyNameValue
        End Get
        Set(ByVal value As String)
            companyNameValue = value
        End Set
    End Property

    Private contactNameValue As String
    Public Property ContactName() As String
        Get
            Return contactNameValue
        End Get
        Set(ByVal value As String)
            contactNameValue = value
        End Set
    End Property

    Public Sub New(ByVal customerID As String, _
                   ByVal companyName As String, _
                   ByVal contactName As String)
        CustomerIDValue = customerID
        CompanyNameValue = companyName
        ContactNameValue = contactName
    End Sub

    Public Overrides Function ToString() As String
        Return String.Format("{0}:{1}:{2}", Me.CustomerID, Me.CompanyName, Me.ContactName)
    End Function
End Class

Sub Main()
    Dim custOrd As XElement = XElement.Load("CustomersOrders.xml")
    Dim custList As IEnumerable(Of Customer) = _
        From el In custOrd.<Customers>.<Customer> _
        Select New Customer( _
            el.@<CustomerID>, _
            el.<CompanyName>.Value, _
            el.<ContactName>.Value _
        )
    For Each cust In custList
        Console.WriteLine(cust)
    Next
End Sub

该示例产生下面的输出：

GREAL:Great Lakes Food Market:Howard Snyder
HUNGC:Hungry Coyote Import Store:Yoshi Latimer
LAZYK:Lazy K Kountry Store:John Steel
LETSS:Let's Stop N Shop:Jaime Yorres

请参阅