Question

I have a list of Persons which has children [same type]. I get the list from xml file.

Scenario :

Person : Id, Name, Gender, Age, Children [Class with fields]

If personList has 1,2,5 Ids,
2 and 5 with children 3,4 and 6,7,8 respectively.
I have to get the max id as 8.

How do i get the max of Id from PersonList using lambda expression?

Answer 1

You could try a combination of Concat and SelectMany (that’s assuming it’s only nested one level):

var maxId = personList.Concat(personList.SelectMany(p => p.Children)).Max(p => p.Id);

UPDATE
If you have multiple nesting levels, you could also write an extension method to make SelectMany recursive:

public static IEnumerable<T> SelectManyRecursive<T>(this IEnumerable<T> source, Func<T, IEnumerable<T>> selector) {
    if (source == null) { yield break; }
    foreach (var item in source) {
        yield return item;
        foreach (var selected in selector(item).SelectManyRecursive(selector)) {
            yield return selected;
        }
    }
}

That doesn’t handle circular references and it behaves differently than SelectMany by also returning the items in the source collection itself (so you might want to change the name), but otherwise i think it does the job. You could use it quite easily:

var maxId = personList.SelectManyRecursive(p => p.Children).Max(p => p.Id);