C# Linq to Group to a list if a Condition meets else do not

Question

I have a list My Requirement I need a LINQ lambda query to Group to a list if a Condition meets else do not group. i.e On a condition I want it to be grouped else it should not be grouped I have searched net - I get details on grouping on condition and I couldn't get and understand on how the remaining item should be included without grouping. Some info found on net was for grouping Conditionally - but with that those items not meeting conditions do not include in the resultant list. For Example List = [{ 1   a }{ 40    ){ 9   a }{ 52   b ){ 2   b }{ 99    ){ 88   b }] The expected resultant list is to be grouped by ab but  should not be grouped ResultantList = Group[0] ==> [{1 a } {9 a }] Group[1] ==>[ {52 b ) {2 b } {88 b }]  // all other items which is  should be included without groups Group[3] [ {40  }] Group[4][ {99  } ] Please share your solution for this. What I have tried: var resultantList = sigList .GroupBy(s => s.SignalGroup) .Select(grp => grp.ToList()) //.Where(g => !g.Any(grp => grp.SignalGroup == )) .ToList(); With the above as expected Uncommenting Where clause groups only a and b==> All those items with empty value (  ) does not get included Commenting Where clause groups ab and  string to make a list with 3 groups(ab and ).

Tapan Patel · Answer

I like this question. (Kind of tricky). Basically you just need to concat two groupings (one with null/empty values where you want to group by id becuase you want to gropu individual items and another with the strings with values). Please try below and let me know if you face difficulties. List lstx = new List { new Demo { id=44name= A } new Demo { id = 42 name = A } new Demo { id = 43 name = A } new Demo { id = 54 name = B } new Demo { id = 22 name= B } new Demo { id = 73 name=  } new Demo { id = 66 name=  } }; var grp1 = lstx.Where(l => l.name != string .Empty).GroupBy(t => t.name); var grp2 = lstx.Where(l => l.name == string .Empty).GroupBy(t => t.id.ToString()); var allgrp = grp1.Concat(grp2); allgrp.ToList().ForEach(obj => { Console.WriteLine($ Grouping Key : {obj.Key} ); foreach (var item in obj) { Console.WriteLine($ 	Item ID: {item.id} Item Name: {item.name} ); } Console.WriteLine(); }); Also Demo is just a class with two properties. public class Demo { public int id { get ; set ; } public string name { get ; set ; } } Output:

J Antoni · Answer

I got the below and it works as expected var resultantList = sigList .GroupBy(s => s.SignalGroup ==  ? s.SignalID.ToString() : s.SignalGroup ) .Select(grp => grp.ToList()) .ToList();