How to group a list with Linq

You can either use GroupBy method on IEnumerable to create IGrouping object that contains a key and grouped objects or you can use ToLookupto create exactly what you want in result:

yourList.ToLookup(m => m.id, m => m.value); 

This creates a hashed collection of keys with their values.
For more information please see below post:
https://www.c-sharpcorner.com/UploadFile/d3e4b1/practical-usage-of-using-tolookup-method-in-linq-C-Sharp/

Just a little more detail to emphasize the difference between the ToLookup approach and the GroupBy approach:

// class definition public class Item { public long Id { get; set; } public string Value { get; set; } } // create your list var items = new List<Item> { new Item{Id = 0, Value = "value0a"}, new Item{Id = 0, Value = "value0b"}, new Item{Id = 1, Value = "value1"} }; // this approach results in a List<string> (a collection of the values) var lookup = items.ToLookup(i => i.Id, i => i.Value); var groupOfValues = lookup[0].ToList(); // this approach results in a List<Item> (a collection of the objects) var itemsGroupedById = items.GroupBy(i => i.Id).ToList(); var groupOfItems = itemsGroupedById[0].ToList(); 

So, if you want to work with values only after grouping, then you could take the first approach; if you want to work with objects after grouping, you could take the second approach. And, these are just a couple example implementations, there are plenty of ways to accomplish your goal.

First convert to a Lookup then select into a list, like so:

var groups = list .ToLookup ( item => item.ID, item => item.Value ) .Select ( item => new { ID = item.Key, Values = item.ToList() } ) .ToList(); 

The resulting JSON looks like this:

[{"ID":1,"Values":["a","b","c"]},{"ID":2,"Values":["t"]}] 

Link to working example on DotNetFiddle.