Linq query between two list objects

Question

I have two objects:

ObjectA
{
   string code;
   string country;
}

ObjectB
{
   string code;
   string otherstuff;
}

And I have List<objectA> and List<ObjectB> and I need to find all objects in List<ObjectB> which contains objectA.Code. But cannot manage to implement it on LINQ query.

1. Do you have single objectA or you need it for all objects from List<objectA>. 2. What is the layout of result you expect? 3. What have you tried so far? — Krizz
– Krizz, Commented Jan 26, 2012 at 16:41
Have also a look at IEnumerable.Intersect: msdn.microsoft.com/en-us/library/bb460136.aspx You need to override Equals and GetHashCode in both and allow to compare with each other via Code. — Tim Schmelter
– Tim Schmelter, Commented Jan 26, 2012 at 16:43

JaredPar · Accepted Answer · 2012-01-26 16:40:14Z

54

It sounds like you are trying to find all instances of ObjectB which have a code value present in any of the List<ObjectA> values. If so try the following

List<ObjectA> listA = ...;
List<ObjectB> listB = ...;
var all = listB.Where(b => listA.Any(a => a.code == b.code));

answered Jan 26, 2012 at 16:40

JaredPar

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Comments

jeroenh · Accepted Answer · 2012-01-26 16:42:06Z

17

It sounds like you want to join the list of ObjectA with the list of ObjectB on the code property. This is one way:

List<ObjectA> listOfA = ...;
List<ObjectB> listOfB = ...;
var all = from a in listOfA
          join b in listOfB on a.code equals b.code
          select new {a,b};

The result is a list of anonymous objects, containing 2 properties: a of type ObjectA, b of type ObjectB, with the same code

answered Jan 26, 2012 at 16:42

jeroenh

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Comments

BrokenGlass · Accepted Answer · 2012-01-26 16:41:21Z

7

To do this effectively you can first put the codes into a HashSet<string> and then use a Contains() query to check if the B in question has a code that is contained in the hashset:

var codes = new HashSet<string>(listOfAs.Select(x => x.code));
var selectedBs = listOfBs.Where( x=> codes.Contains(x.code));

answered Jan 26, 2012 at 16:41

BrokenGlass

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Olivier Jacot-Descombes · Accepted Answer · 2017-06-01 14:28:50Z

6

I would put the codes of the ObjectA list into a HashSet, otherwise your query would become an O(n²) operation. Like this it is an O(n) operation:

var aList = new List<ObjectA>();
var bList = new List<ObjectB>();

var aCodes = new HashSet<string>(aList.Select(a => a.code));
var result = bList.Where(b => aCodes.Contains(b.code));

edited Jun 1, 2017 at 14:28

answered Jan 26, 2012 at 16:46

Olivier Jacot-Descombes

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2 Comments

Reno Over a year ago

Thanks Olivier. Do you know any nice tutorials in linq or books with examples in speed and etc?

Olivier Jacot-Descombes Over a year ago

For LINQ Google LINQ tutorial C#. As for speed, what is relevant here, is to have a basic knowledge of the different collection types along with their pros and cons. James Michael Hare gives a good overview here: Choosing the Right Collection Class

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