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I have some XML that contains Payment information including details on the Invoices the payments are being applied to. I would like to use LINQ to XML to gather only those payments that are applied to certain invoices (111,222,333) for example. If it were SQL I could use the IN (111,222,333) keyword but I am not sure how to do the same in LINQ.

<Payment>
 <PaymentId>1</PaymentId>
 <Total>50</Total>
 <Invoice>
 <Id>111</Id>
 <Amount>20</Amount>
 </Invoice>
 <Invoice>
 <Id>555</Id>
 <Amount>30</Amount>
 </Invoice>
</Payment>
<Payment>
 <PaymentId>2</PaymentId>
 <Total>20</Total>
 <Invoice>
 <Id>222</Id>
 <Amount>20</Amount>
 </Invoice>
</Payment>
<Payment>
 <PaymentId>3</PaymentId>
 <Total>80</Total>
 <Invoice>
 <Id>888</Id>
 <Amount>80</Amount>
 </Invoice>
</Payment>

LINQ

var result = xml.Select(x => x.Element("Payment"))
 Where(x => x.Element("Id").Value.Contains("111","222","333"))

In this example, I would like to select "PaymentId 1" and "PaymentId 2" since they apply to invoices that have Ids that match 111, 222, or 333.

You can do a match/intersect against a list of these wellknown Id's.

List<string> ids = new List<string> "111", "222", "333" ;

var result = xml
 .Elements("Payment")
 .Where(p => 
 var invoiceIds = p.Elements("Invoice").Elements("Id").Select(o => o.Value);
 return ids.Intersect(invoiceIds).Any();
 
);

Full code and NetFiddle.

const string XML = @"
 <root>
 <Payment>
 <PaymentId>1</PaymentId>
 <Total>50</Total> 
 <Invoice>
 <Id>555</Id>
 <Amount>30</Amount>
 </Invoice>
 <Invoice>
 <Id>111</Id>
 <Amount>30</Amount>
 </Invoice>
 </Payment>
 <Payment>
 <PaymentId>2</PaymentId>
 <Total>20</Total>
 <Invoice>
 <Id>222</Id>
 <Amount>20</Amount>
 </Invoice>
 </Payment>
 <Payment>
 <PaymentId>3</PaymentId>
 <Total>80</Total>
 <Invoice>
 <Id>888</Id>
 <Amount>80</Amount>
 </Invoice>
 </Payment>
 </root>
 ";

XElement xml = XElement.Parse(XML);
List<string> ids = new List<string> "111", "222", "333" ;

var result = xml
 .Elements("Payment")
 .Where(p => 
 var invoiceIds = p.Elements("Invoice").Elements("Id").Select(o => o.Value);
 return ids.Intersect(invoiceIds).Any();
 
 );

foreach (var item in result)

 Console.WriteLine("PaymentId: 0", (string)item.Element("PaymentId"));