Use a foreach loop within a foreach loop in a linq query

Question

I am wondering on how to do the following. I have the Linq query:

Items items.Where(i => i.GetType() == typeof(SubItem))
                .Cast<SubItem>()
                .ToList()
                .ForEach(i => i.SomeList.Add(i.SomeObject.ForEach(i => i.SomeString)));

My question is about i.SomeList.Add() . I want to return a couple of string values to i.SomeList.Add() from i.SomeObject but I do not know how I can do this in this way? Is it even possible like this to have another ForEach Loop within a Linq ForEach usinq Linq query?

Answer 1

I believe this foreach loop will achieve your goal, if I've understood the problem.

It will loop over any element of items that is a (or is derived from) SubItem . It will then select all SomeObject.SomeString strings and add them to the SomeList .

foreach (var subItem in items.OfType<SubItem>()) {
    subItem.SomeList.AddRange(subItem.SomeObject.Select(o => o.SomeString));
}

This is a compilation of suggestions from Panagiotis Kanavos, juharr, and Aluan Haddad.

Answer 2

LINQ isn't really for running Add operations... it's much more powerful when you think of it as returning a resultset.

So instead of

//Add every value of SomeField to targetList
sourceList.ForEach( x => targetList.Add(x.SomeField) )

Think of doing it this way:

//Create a list of all instances of SomeField and assign it to targetList
targetList = sourceList.Select( x => x.SomeField).ToList();

Or if you need to keep the existing items in the target list, do this:

//Create a list of all SomeFields and add it to targetList
targetList.AddRange
(
     sourceList.Select( x => x.SomeField )
);

Similarly, instead of using a nested foreach, consider using SelectMany .

I'm not completely clear on your requirements but you probably want something like this:

//To SomeList, add the SomeString field from all instances of SomeObject
someList.AddRange
(
    items.OfType<SubItem>().SelectMany( x => x.SomeObject ).Select( x => x.SomeString )
);