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?

ForEach is a List method, not LINQ. LINQ is a query language. Why use it at all if you want nested loops? — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Jan 22, 2021 at 20:15
ForEach is absolutely wrong here as are Cast, i.GetType() == typeof(SubItem), and ToList — Aluan Haddad
– Aluan Haddad, Commented Jan 22, 2021 at 20:16
.Where(i => i.GetType() == typeof(SubItem)).Cast<SubItem>() is equivalent to OfType<SubIttem>(). What are you trying to do anyway? ForEach doesn't return anything, so i.SomeArray.Add(i.SomeObject.ForEach won't even compile. Whatever you want to do, it's far easier to do with simple loops — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Jan 22, 2021 at 20:18
Maybe you want i.SomeArray.AddRange(i.SomeObject.Select(x => x.SomeString)) though that only works if SomeArray is a List<T> and not an array as the name implies. — juharr
– juharr, Commented Jan 22, 2021 at 20:26
GetType() is slow and will miss any types that derive from SubItem. ToList isn't necessarily wrong but it's confusing here and people often use it without thinking. Instead use foreach (var item in items.OfType<SubItem>()) {...} — Aluan Haddad
– Aluan Haddad, Commented Jan 22, 2021 at 21:08

Callum Watkins · Accepted Answer · 2021-01-22 21:36:44Z

3

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.

edited Jan 22, 2021 at 21:36

answered Jan 22, 2021 at 21:28

Callum Watkins

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Comments

John Wu · Accepted Answer · 2021-01-22 23:22:42Z

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 )
);

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