How to convert a nested for each loop into Linq when the two lists are of different types

Question

I want to convert a nested foreach loop in C# to Linq. I know how to do it if the two lists are of same type. But in my case, they are of different type, except that they both have two common properties, DocumentId and IsValid. I have to loop through the outer list and the inner list and whenever the DocumentId of both are the same, then assign the IsValid value from the outer loop object to the inner loop object. I have the following foreach loop is working fine.

foreach (var docs in outerDocuments)
{
     foreach (var presentedDocuments in innerdocuments)
     {
           if (docs.DocumentId.Equals(presentedDocuments.DocumentId))
           {
                 presentedDocuments.IsValid = docs.IsValid;
           }
     }
}

I need to convert it to Linq. Usually ReSharper does a good job of refactoring such code. But in this case it converted only the nested foreach to Linq, and even then it threw a warning , "Access to foreach variable in closure".

Thanks

As you aren't putting out a new list, you're really just going to end up with the built in ForEach anyway. That's probably part of why ReSharper didn't clean it up — Justin Pihony
– Justin Pihony, Commented Oct 4, 2015 at 1:47

sstan · Accepted Answer · 2015-10-04 02:12:18Z

1

You can try using Enumerable.Join:

foreach (var item in outerDocuments.Join(innerdocuments,
                                         outer => outer.DocumentId,
                                         inner => inner.DocumentId,
                                         (outer, inner) => new { OuterDoc = outer, InnerDoc = inner }))
{
    item.InnerDoc.IsValid = item.OuterDoc.IsValid;
}

answered Oct 4, 2015 at 2:12

sstan

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Backs · Accepted Answer · 2015-10-04 01:56:50Z

0

Try this:

foreach (var presentedDocuments in innerdocuments)
{
    var doc = outerDocuments.FirstOrDefault(a => a.DocumentId.Equals(presentedDocuments.DocumentId));
    if (doc != null)
    {
        presentedDocuments.IsValid = doc.IsValid;
    }
}

answered Oct 4, 2015 at 1:56

Backs

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Comments

Vova · Accepted Answer · 2015-10-04 02:58:59Z

0

You can use .Join() extension method and do it in one line like this:

outerDocuments.Join(innerDocuments, o => o.DocumentId, i => i.DocumentId, (o, i) =>  i.IsValid = o.IsValid).ToList();

As noted by @sstan .ToList() call is required to force enumeration.

edited Oct 4, 2015 at 2:58

answered Oct 4, 2015 at 2:17

Vova

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3 Comments

sstan Over a year ago

Interesting idea. Not sure it's the best idea to have state changing code in the resultSelector parameter, it kind of goes against the spirit of how the join method is supposed to work. And in this case, it won't actually work unless you actually loop the result of your join call. If you add a call to ToList() at the end it will work.

Vova Over a year ago

@Enigmativity I used LINQPad to test it and its' .Dump() extension method did ToList's work, that's why my test succeded. I updated code example as per @sstan suggestion.

Enigmativity Over a year ago

@Vova - Until you forced execution it wouldn't work. The .ToList() (or .Dump()) would be needed.

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