How to convert nested foreach loops with conditions to LINQ

Question

I'd like to ask if it is possible to convert below nested foreach loops to LINQ expression.

public interface IFoo
{
  bool IsCorrect(IFoo foo);
  void DoSomething(IFoo foo);
}

List<IFoo> listA; //assume that contains items
List<IFoo> listB; //assume that contains items


foreach (var a in listA)
{
  foreach (var b in listB)
  {
    if (b.IsCorrect(a))
    {
      b.DoSomething(a);
    }
  }
}

Possible duplicate of [LINQ equivalent of foreach for IEnumerable](http://stackoverflow.com/questions/200574/linq-equivalent-of-foreach-for-ienumerablet) — EtherDragon, Nov 23 '16 at 16:24

score 4 · Accepted Answer · answered Nov 23 '16 at 16:24

4

This should work:

var result =
    from a in listA
    from b in listB
    where b.IsCorrect(a)
    select new {a, b};

foreach (var item in result)
    item.b.DoSomething(item.a);

answered Nov 23 '16 at 16:24

Matthew Watson

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2

Not only is this readable, but it avoids `List.ForEach`, which is an abomination :-) – Gary McGill Nov 23 '16 at 16:30

score 2 · Answer 2 · answered Nov 23 '16 at 16:34

2

With method syntax you would use this query:

var correctPairs = listA
    .SelectMany(a => listB.Where(b => b.IsCorrect(a)).Select(b => new { a, b }));

foreach (var x in correctPairs)
    x.b.DoSomething(x.a);

answered Nov 23 '16 at 16:34

Tim Schmelter

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686
939

score 1 · Answer 3 · answered Nov 23 '16 at 16:23

1

I'm not sure exactly how far you want to go with this, but this is a Linq statement doing the same thing:

listA.ForEach(a => listB.ForEach(b =>
{
    if (b.IsCorrect(a)) b.DoSomething(a);
}));

answered Nov 23 '16 at 16:23

LordWilmore

2,829
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Actually, this answer has nothing to do with with LINQ. – itsme86 Nov 23 '16 at 16:27
1

@itsme86 Very true. I always thought the ForEach method was a linq extension, but it appears that it is not! – LordWilmore Nov 23 '16 at 16:29
If you want a linq Foreach, this will give it to you: public static partial class FunctionalExtensions { public static void ForEach(this IEnumerable items, Action action) { foreach (T item in items) action(item); } public static IEnumerable ForEach(this IEnumerable items, Func action) { foreach (T item in items) { yield return action(item); } } } – Jim Nov 23 '16 at 17:17
2

@Jim That would involve no more LINQ than this method. Nor is it representative of a LINQ style of programming. In fact, it's very specifically *contrary* to the LINQ style of programming. – Servy Nov 23 '16 at 21:13

score 1 · Answer 4 · answered Nov 23 '16 at 16:24

You can do this but this is not more efficient what you have so far:

var query= listA.SelectMany(a=>listB.Select(b=>new {a,b}))
                .Where(e=>e.b.IsCorrect(e.a))
                .ToList()// Foreach is a method of List<T>
                .Foreach(e=> e.b.DoSomething(e.a));

To avoid to call ToList, you can implement an extension method like this:

public static void ForEach<T>(this System.Collection.Generic.IEnumerable<T> list, System.Action<T> action)
{
    foreach (T item in list)
        action(item);
}

And then your query would be:

var query= listA.SelectMany(a=>listB.Select(b=>new {a,b}))
                .Where(e=>e.b.IsCorrect(e.a))
                .Foreach(e=> e.b.DoSomething(e.a));

How to convert nested foreach loops with conditions to LINQ

4 Answers4

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