ID應該不會重複吧，可以用HashSet儲存
HashSet<string> idSet = new HashSet<string>() { "1", "3", "4" }; List<Element> elementList = new List<Element>(); foreach (var item in elementList) { if (idSet.Contains(item.ID)) { //Do Some Thing... } }

有用過LINQ嗎

https://docs.microsoft.com/zh-tw/dotnet/api/system.linq.enumerable.join?f1url=https%3A%2F%2Fmsdn.microsoft.com%2Fquery%2Fdev15.query%3FappId%3DDev15IDEF1%26l%3DZH-TW%26k%3Dk(System.Linq.Enumerable.Join%60%604);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.6.2);k(DevLang-csharp)%26rd%3Dtrue&view=netframework-4.7.2

List<String> ID = new List<string>(); List<XmlElement> Element = new List<XmlElement>(); var newJoin = ID.Join(Element, X => X, Y => Y.Name, (X, Y) => Y);

看看這樣是否合用
https://drive.google.com/open?id=113ZYAAe_wl7TwlKdsZJdZulY9e6CXRtk

void Main()
{
var Ids = new List<string> { "1", "3", "4" };
var Elements = new List<Element> {
new Element{Id="1",Name="A1"},
new Element{Id="2",Name="A2"},
new Element{Id="3",Name="A3"},
new Element{Id="5",Name="A5"}
};
var result = Elements.Where(item =>Ids.Contains(item.Id) ); // 這是你要的程式
}


class Element {
public string Id;
public string Name;
}