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How to control the type of a projection (LINQ to XML)

  • บทความ

Projection is the process of filtering a set of data, changing its shape and perhaps its type. Most query expressions do projections. Most of the query expressions in this section evaluate to IEnumerable<T> of XElement, but you can create collections of other types. This article shows how to do this.

Example: Define a new type and create a query that creates an IEnumerable of that type

The following example defines a new type, Customer, and the query expression instantiates new Customer objects in the Select clause. This causes the type of the query expression to be IEnumerable<T> of Customer. The example uses XML document Sample XML file: Customers and orders.

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

This example produces the following output:

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

See also