using System;

using System.Linq;

using System.Xml.Linq;

​

public class Program

{

    public static void Main()

    {

        const string xml = @"<FieldsDefinitions>

  <FieldsDefinition>

    <Field>DEMOFIELD</Field>

    <Length>15</Length>

    <Label>IIS: </Label>

    <Type>DropDown</Type>

  </FieldsDefinition>

  <FieldsDefinition>

    <Field>IIS</Field>

    <Length>15</Length>

    <Label>IIS: </Label>

    <Type>DropDown</Type>

  </FieldsDefinition>

  <FieldsDefinition>

    <Field>DEMOFIELD</Field>

    <Length>20</Length>

    <Label>Demo Field</Label>

    <Type>Text</Type>

  </FieldsDefinition>

</FieldsDefinitions>";

​

        XDocument doc = XDocument.Parse(xml);

        var fieldDefs = doc

            .Descendants("FieldsDefinition")

            .Where(x => (string) x.Element("Field") == "DEMOFIELD" &&

                        (string) x.Element("Label") == "IIS: ");

​

        foreach (var def in fieldDefs)

        {

            Console.WriteLine(def);

        }

    }

}