Using Linq to group a list of objects into a new grouped list of list of objects

Question

var groupedCustomerList = userList
    .GroupBy(u => u.GroupID)
    .Select(grp => grp.ToList())
    .ToList();

Your group statement will group by group ID. For example, if you then write:

foreach (var group in groupedCustomerList)
{
    Console.WriteLine("Group {0}", group.Key);
    foreach (var user in group)
    {
        Console.WriteLine("  {0}", user.UserName);
    }
}

that should work fine. Each group has a key, but also contains an IGrouping<TKey, TElement> which is a collection that allows you to iterate over the members of the group. As Lee mentions, you can convert each group to a list if you really want to, but if you're just going to iterate over them as per the code above, there's no real benefit in doing so.

For type

public class KeyValue
{
    public string KeyCol { get; set; }
    public string ValueCol { get; set; }
}

collection

var wordList = new Model.DTO.KeyValue[] {
    new Model.DTO.KeyValue {KeyCol="key1", ValueCol="value1" },
    new Model.DTO.KeyValue {KeyCol="key2", ValueCol="value1" },
    new Model.DTO.KeyValue {KeyCol="key3", ValueCol="value2" },
    new Model.DTO.KeyValue {KeyCol="key4", ValueCol="value2" },
    new Model.DTO.KeyValue {KeyCol="key5", ValueCol="value3" },
    new Model.DTO.KeyValue {KeyCol="key6", ValueCol="value4" }
};

our linq query look like below

var query =from m in wordList group m.KeyCol by m.ValueCol into g
select new { Name = g.Key, KeyCols = g.ToList() };

or for array instead of list like below

var query =from m in wordList group m.KeyCol by m.ValueCol into g
select new { Name = g.Key, KeyCols = g.ToList().ToArray<string>() };