Use LINQ to concatenate multiple rows into single row (CSV property)

I'm looking for the LINQ equivalent to the Sybase's LIST() or MySQL's group_concat()

It'll convert:

User  Hobby
--------------
Bob   Football 
Bob   Golf 
Bob   Tennis 
Sue   Sleeping 
Sue   Drinking

To:

User  Hobby
--------------
Bob   Football, Golf, Tennis 
Sue   Sleeping, Drinking

From stackoverflow

• That's the GroupBy operator. Are you using LINQ to Objects?

  Here's an example:

  using System;
  using System.Collections.Generic;
  using System.Linq;
  
  public class Test
  {
      static void Main()
      {
          var users = new[]
          {
              new { User="Bob", Hobby="Football" },
              new { User="Bob", Hobby="Golf" },
              new { User="Bob", Hobby="Tennis" },
              new { User="Sue", Hobby="Sleeping" },
              new { User="Sue", Hobby="Drinking" },
          };
  
          var groupedUsers = users.GroupBy(user => user.User);
  
          foreach (var group in groupedUsers)
          {
              Console.WriteLine("{0}: ", group.Key);
              foreach (var entry in group)
              {
                  Console.WriteLine("  {0}", entry.Hobby);
              }
          }
      }
  }
  

  That does the grouping - can you manage the rest yourself?

  Hosam Aly : It's also possible to replace the inner loop with `String.Join(", ", group.ToArray())`, as long as the number of elements is not very large.

  John Paul Jones : Thanks! I'm going to learn LINQ myself instead of constantly bugging you ;-)

  Carter : Am I the only one that noticed the man is into sports while the woman's hobbies are sleeping and drinking? You're terrible Jon Skeet!

  Jon Skeet : @Carter: They're not *my* examples...

  Carter : Ha, you're right, my bad. Still funny.

• See if this solution helps you:

  List<User> users = new List<User>() 
  { 
      new User {Name = "Bob", Hobby = "Football" },
      new User {Name = "Bob", Hobby = "Golf"},
      new User {Name = "Bob", Hobby = "Tennis"},
      new User {Name = "Sue", Hobby = "Sleeping"},
      new User {Name = "Sue", Hobby = "Drinking"}
  };
  
  var groupedUsers = from u in users
           group u by u.Name into g
           select new
           {
               Name = g.First<User>().Name,
               Hobby = g.Select(u => u.Hobby)
           };
  
  
  foreach (var user in groupedUsers)
  {
      Console.WriteLine("Name: {0}", user.Name);
      foreach (var hobby in user.Hobby)
      {
          Console.WriteLine("Hobby: {0}", hobby);
      }
  }
  

• re the _concat aspect of your question, using:

  static class EnumerableExtensions 
  {  
      public static String AsJoined( this IEnumerable<String> enumerable )
      {
          return AsJoined( enumerable, "," );
      }
  
      public static String AsJoined( this IEnumerable<String> enumerable, String separator )
      {
          return String.Join( separator, enumerable.ToArray() );
      }
  }
  

  The outputting foreach in bruno conde and Jon Skeet's answers can become:

  Console.WriteLine( "User:\tHobbies");
  foreach ( var group in groupedUsers )
      Console.WriteLine( "{0}:\t{1}", group.Key, group.Select( g => g.Hobby ).AsJoined( ", " ) );
  

  ... and you'll get the precise result output format you asked for (yes, I know the others have already solved your problem, but its hard to resist!)

• To do it in one Linq Statement. There is no way I'd recommend the code, but it shows that it could be done.

              var groupedUsers = from user in users
                             group user by user.User into userGroup
                             select new
                             {
                                 User = userGroup.Key,
                                 userHobies =
                                     userGroup.Aggregate((a, b) => 
                                         new { User = a.User, Hobby = (a.Hobby + ", " + b.Hobby) }).Hobby
                             }
                              ;
          foreach (var x in groupedUsers)
          {
              Debug.WriteLine(String.Format("{0} {1}", x.User, x.userHobies));
          }