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

Question

I don t know if this is possible in Linq but here goes     I have an object   public class User     public int UserID   get  set      public string UserName   get  set      public int GroupID   get  set        I return a list that may look like the following   List lt User gt  userList   new List lt User gt     userList Add  new User   UserID   1  UserName    UserOne   GroupID   1      userList Add  new User   UserID   2  UserName    UserTwo   GroupID   1      userList Add  new User   UserID   3  UserName    UserThree   GroupID   2      userList Add  new User   UserID   4  UserName    UserFour   GroupID   1      userList Add  new User   UserID   5  UserName    UserFive   GroupID   3      userList Add  new User   UserID   6  UserName    UserSix   GroupID   3        I want to be able to run a Linq query on the above list that groups all the users by GroupID  So the output will be a list of user lists that contains user  if that makes sense    Something like   GroupedUserList     UserList         UserID   1  UserName    UserOne   GroupID   1         UserID   2  UserName    UserTwo   GroupID   1         UserID   4  UserName    UserFour   GroupID   1     UserList         UserID   3  UserName    UserThree   GroupID   2     UserList         UserID   5  UserName    UserFive   GroupID   3         UserID   6  UserName    UserSix   GroupID   3   I ve tried using the groupby linq clause but this seems to return a list of keys and its not grouped by correctly   var groupedCustomerList   userList GroupBy  u   gt  u GroupID   ToList

User · Accepted Answer

var groupedCustomerList   userList      GroupBy u   gt  u GroupID       Select grp   gt  grp ToList         ToList

User · Answer

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 lt TKey  TElement gt  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

User · Answer

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 lt string gt

User · Answer

Still an old one  but answer from Lee did not give me the group Key as result  Therefore  I am using the following statement to group a list and return a grouped list   public IOrderedEnumerable lt IGrouping lt string  User gt  gt  groupedCustomerList   groupedCustomerList           from User in userList         group User by User GroupID into newGroup         orderby newGroup Key         select newGroup    Each group now has a key  but also contains an IGrouping which is a collection that allows you to iterate over the members of the group

[c#] Using Linq to group a list of objects into a new grouped list of list of objects

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