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 List<empl> lstSource = new List<empl>();

        lstSource.Add(new empl { departmentId = 2, Id = 101, Name = "S1" }); 
        lstSource.Add(new empl { departmentId = 2, Id = 109, Name = "S9" });
        lstSource.Add(new empl { departmentId = 2, Id = 102, Name = "S2" });
        

        lstSource.Add(new empl { departmentId = 4, Id = 101, Name = "S1" });
        lstSource.Add(new empl { departmentId = 4, Id = 102, Name = "S2" });
        lstSource.Add(new empl { departmentId = 4, Id = 108, Name = "S8" });

        lstSource.Add(new empl { departmentId = 3, Id = 105, Name = "S5" });
        lstSource.Add(new empl { departmentId = 3, Id = 103, Name = "S3" });
        lstSource.Add(new empl { departmentId = 3, Id = 102, Name = "S2" });

should result {Id = 102, Name = "S2"} if I add

lstSource.Add(new empl { departmentId = 3, Id = 101, Name = "S1" });

should result {Id = 102, Name = "S2"} {Id = 101, Name = "S1"}

Hint : we can group with departmentId and find common Id in 3 group.

Anish Sinha
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  • What if there are multiple common elements? How are you determining a group? What makes an item a "common" one with another? – DavidG Jan 18 '22 at 16:11
  • Where exactly are you stuck trying this? Showing your own effort would probably make the question more clear. – Gert Arnold Jan 18 '22 at 16:18
  • Where exactly are you stuck trying this? - I want to fetch list which is common in all group or in can say intersection of grouped (departmentId) items. – Anish Sinha Jan 18 '22 at 16:32
  • or refer close question - https://stackoverflow.com/questions/15183982/is-there-a-way-to-find-common-elements-in-multiple-lists – Anish Sinha Jan 18 '22 at 16:34
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    What keeps yo from showing your own effort? The other question is completely different. – Gert Arnold Jan 18 '22 at 19:02
  • as i said it is very close to my question, difference is only i have complex type class (emp class) shared link having array of primitive data type. output is same like find common items in all sets/group – Anish Sinha Jan 19 '22 at 03:50

2 Answers2

1

Based on your comments and example above, I take it that the Name associated with any given Id is always the same. In that case, you could split the Ids registered on each department into separate lists, then intersect those lists to find the common Ids, and then find the associated Name for each common Id.

You have done something similar in your own example. By rewriting the code (e.g. by replacing the foreach loop with an Aggregate() function) you could achieve a more straight forward approach:

var idsPerDepartment = lstSource
    .GroupBy(item => item.departmentId)
    .Select(gr => gr.Select(item => item.Id));

var commonIds = idsPerDepartment
    .Aggregate((common, next) => common.Intersect(next));

var commonItems = commonIds
    .Select(id => lstSource.First(item => item.Id == id))
    .Select(commonItem => new { commonItem.Id, commonItem.Name })
    .OrderByDescending(commonItem => commonItem.Name);

commonItems is empty (not null) if there are no common items.

It could all be written as one single expression, but I've spilt it into several variables to clarify what is happening along the way.

Astrid E.
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var groups = lstSource.GroupBy(t1=> t1.departmentId)
                     .ToList();
        var validIds = groups.First().Select(t1 => t1.Id).ToList();
        foreach (var g in groups.Skip(0))
        {
            var otherGroupItemIds = g.Select(t1 => t1.Id).ToList();
            validIds = validIds.Intersect(otherGroupItemIds).ToList();
           
        }
if (validSRIds.Count > 0)
                return lstSource.FindAll(t1 => validSRIds.Contains(t1.Id)).GroupBy(t2 => new { t2.Id, t2.Name }).Select(g => g.First()).OrderByDescending(t => t.Name).ToList();

you will get all common id,name which belongs to all group

Anish Sinha
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