Using LINQ to group social networking links for each user

In my opinion, the current solution is not effective because every time you call FirstOrDefault method it creates enumerator and enumerates until element meets predicate. The SocialLink collection is not expected to be big (0-20 elements) but the collection is users may be big. And creation such a model for 100 users may create some overhead as well as pressure to GC.

The solution with dictionary is more effective than FirstOrDefault, however you have to still deal with possible duplicates and population of dictionary is relatively expensive operation.

I would propose another solution, that seems a little more effective than dictionary, but not so elegant.

public class UserViewModel
{
   public UserViewModel(String name, IEnumerable<SocialLink> links)
   {
      this.Name = name;
      if (links != null){
         foreach(var link in links)
         {
            switch (link.SocialType)
            {
                case SocialType.FaceBook:
                    FaceBook = link.Url;
                    break;
                case SocialType.Twitter:
                    Twitter = link.Url;
                    break;
                case SocialType.Youtube:
                    Youtube = link.Url;
                    break;
                case SocialType.Linkedin:
                    Linkedin = link.Url;
                    break;
                case SocialType.GooglePlus:
                    GooglePlus = link.Url;
                    break;
                case SocialType.Telegram:
                    Telegram = link.Url;
                    break;
                case SocialType.Instagram:
                    Instagram = link.Url;
                    break;
            }
         }
      }
   }

   public string Name { get; set; }
   public string Facebook { get; set; }
   public string Twitter { get; set; }
   public string Youtube { get; set; }
   public string Linkedin { get; set; }
   public string GooglePlus { get; set; }
   public string Telegram { get; set; }
   public string Instagram { get; set; }
}

In this case you create enumerator object and scan collection only once. You can improve to not assign property if it has already a value.

PS. I would change your enum to start from 1, and leave 0 for exceptional cases, because 0 is a default value you may end up with some awkward bugs.

I think you shouldn't use hard-coded social links as properties on the UserViewModel but instead simplify it to

public class UserViewModel
{
    public string Name { get; set; }
    public IEnumerable<SocialLink> SocialLinks { get; set; }
}

and let the view render a dynamic list of links and format the Url properly for display. The code-behind should not do it.

With this simplification the query becomes

var result = users.Select(p => new UserViewModel
{
    Name = p.Name,
    SocialLinks = 
        p.SocialLinks
        .GroupBy(x => x.Type)
        .Select(g => g.FirstOrDefault())
        .ToList()   
}).ToList();

Nothing is hard-coded anymore. The only thing you need to ever adjust in case of more social link types is probably the view but even this should render just fine without any additional maintenance.

public class SocialLink
{
    public string Url { get; set; }
    public SocialType Type { get; set; }
    public User User { get; set; }
    public int UserId { get; set; }
}

Why repeat User, and UserId?
Why repeat UserId if User has an Id?

I don't like this

public string Facebook { get; set; }

People are to have and expectation they can revise SocialLinks and it does not.

I would do the work in UserViewModel and derive from User
If you have other selects you would need to repeats the code

List<User> users = new List<User>();
//pop users
List<UserViewModel> usersVM = users.Select(p => new UserViewModel(p)).ToList();

public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<SocialLink> SocialLinks { get; set; }
    public User(int id, string name, ICollection<SocialLink> socialLinks)
    {
        Id = id;
        Name = name;
        SocialLinks = socialLinks;
    }
    public User(User user)
    {
        Id = user.Id;
        Name = user.Name;
        SocialLinks = user.SocialLinks;
    }
}
public class SocialLink
{
    public string Url { get; set; }
    public SocialType Type { get; set; }
    public SocialLink(string url, SocialType type)
    {
        Url = url;
        Type = type;
    }
}
public class UserViewModel : User
{
    public string Facebook { get; set; }
    public string Twitter { get; set; }
    public string Youtube { get; set; }
    public string Linkedin { get; set; }
    public string GooglePlus { get; set; }
    public string Telegram { get; set; }
    public string Instagram { get; set; }
    public UserViewModel (User user) : base(user)
    {
        PopLinks();
    }
    public UserViewModel(int id, string name, ICollection<SocialLink> socialLinks) : base(id, name, socialLinks)
    {
        PopLinks();
    }
    public void PopLinks()
    {
        if (SocialLinks != null)
        {
            foreach (var link in SocialLinks)
            {   // borrowed from another answer +1
                switch (link.Type)
                {
                    case SocialType.FaceBook:
                        Facebook = link.Url;
                        break;
                    case SocialType.Twitter:
                        Twitter = link.Url;
                        break;
                    case SocialType.Youtube:
                        Youtube = link.Url;
                        break;
                    case SocialType.Linkedin:
                        Linkedin = link.Url;
                        break;
                    case SocialType.GooglePlus:
                        GooglePlus = link.Url;
                        break;
                    case SocialType.Telegram:
                        Telegram = link.Url;
                        break;
                    case SocialType.Instagram:
                        Instagram = link.Url;
                        break;
                }
            }
        }
    }
}