title: Flatten IEnumerable<string> - SelectMany issue
body: <p>I am trying to pull the <code>UserId's</code> from a table but when I pull it it comes in the form of a string array containing a string array. </p>
  
  <p>How do I flatten this to just get a string array? I have tried using <code>SelectMany(i =&gt; i.UserId)</code> but I get the error </p>
  
  <blockquote>
    <p>Cannot implicitly convert type'System.Collections.Generic.IEnumerable&lt; char>' to'System.Collections.Generic.IEnumerable&lt; string>'</p>
  </blockquote>
  
  <p>Why is this? Why does it think that the result is of type char?</p>
  
  <pre><code>[DataContract]
  public class AdminDashboardData
  {
      [DataMember]
      public IEnumerable&lt;string&gt; Users { get; set; }
  }
  
  public IEnumerable&lt;string&gt; GetUserIds(IEnumerable&lt;int&gt; groupIds)
  {
      using(GameDbContext entityContext = new GameDbContext())
      {
          return entityContext.InvestigatorGroupUsers
              .Include(i =&gt; i.InvestigatorGroup)
              .Where(i =&gt; i.InvestigatorGroup.IsTrashed == false)
              .Where(i =&gt; groupIds.Contains(i.InvestigatorGroupId))
              .Select(i =&gt; i.UserId).Distinct().ToFullyLoaded();
      }
  }
  
  public class InvestigatorGroupUser
  {
      public int InvestigatorGroupUserId { get; set; }
      public int InvestigatorGroupId { get; set; }
      public string UserId { get; set; }
      public InvestigatorGroup InvestigatorGroup { get; set; }
  }
  
  public AdminDashboardData GetDashboardData(string userId)
  {
      IGamePlayedRepository GamePlayedRepo = _GamePlayedRepo ?? new GamePlayedRepository();
  
      // Get the investigator groups from the userid
      IEnumerable&lt;int&gt; groupIds = GroupUserRepo.GetGroupIds(userId);
  
      AdminDashboardData data = new AdminDashboardData();
  
      if (!groupIds.IsNullOrEmpty())
      {
          data.Users = GroupUserRepo.GetUserIds(groupIds);
      }
  
      return data;
  }
  
  public static IEnumerable&lt;T&gt; ToFullyLoaded&lt;T&gt;(this IEnumerable&lt;T&gt; enumerable)
  {
      return enumerable.ToList();
  }
  </code></pre>