Compare two Lists using LINQ

Question

I have three lists, BeforeList, AfterList and FinalList

BeforeList has the following values

ID Description Operation
1  Descr1      Null
2  Descr2      Null
3  Descr3      Null

AfterList has the following values

ID Description Operation
1  Descr1      Null
2  DescrC      Null
4  Descr4      Null

My FinalList would be

ID Description Operation
1  Descr1      Null
2  DescrC      Update
3  Descr3      Delete
4  Descr4      Add

How do I get the FinalList using LINQ

what is the relationship between the BeforeList and AfterList. Is there a key on ID? — Matt Evans, Nov 02 '11 at 14:52
I'm interested in how you get Op=Update or Delete? and... what happened to Descr2 and why? — kenny, Nov 02 '11 at 14:54
Descr2 updated to DescrC by the user and the new Descr4 added by the user, Descr3 deleted by the user — Gansun, Nov 02 '11 at 14:55
It would be a little cumbersome to do with Linq. I would just do a helper method and use normal for/foreach loops to make things cleaner and easier. — Bala R, Nov 02 '11 at 14:55

Ani · Accepted Answer · 2011-11-02T15:24:13.240

How about something like:

var beforeListById = beforeList.ToDictionary(item => item.Id); 
var afterListById = afterList.ToDictionary(item => item.Id); 

var finalResult = from id in beforeListById.Keys.Union(afterListById.Keys)

                  let before = beforeListById.ContainsKey(id) ? beforeListById[id] : null
                  let after = afterListById.ContainsKey(id) ? afterListById[id] : null

                  select new Record
                  {
                      Id = id,
                      Description = (after ?? before).Description,
                      Operation = (before != null && after != null) 
                                   ? (after.Description == before.Description ? "Null" : "Update")
                                   : (after != null) ? "Add" : "Delete"
                  };

EDIT: Made query simpler (assuming items are non-null).

James Johnson · Answer 2 · 2011-11-02T15:06:46.080

1

This is a simple implementation, but you could try something like this:

var differences = listA.Where(a => !listB.Any(b => b.id == a.id)).Union(listB.Where(b => !listA.Any(a => a.id == b.id)));

EDIT

According to MSDN, you can just use Union to merge the two lists, and duplicates will be filtered out by default. If not, you can always call Distinct():

List<Something> = beforeList.Union(afterList).ToList();

edited Nov 02 '11 at 15:06

answered Nov 02 '11 at 15:00

James Johnson

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score 0 · Answer 3 · edited May 23 '17 at 12:27

0

Looks like you need distinct union on your lists. Here there was a discussion about unions which could give an example of a syntax: Linq to entities : Unions + Distinct

edited May 23 '17 at 12:27

Community

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answered Nov 02 '11 at 15:03

ElDog

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Compare two Lists using LINQ

3 Answers3