LINQ recursive query to return hierarchical set of groups

Question

Given a list of the following models

public class Team
{
    public int TeamId { get; set; }
    public int ParentTeamId { get; set; }
}

I

Answer 1

Firstly you will need an object like this, so the Team object might be:

public class Team
{
    public int? ParentId { get; set; }
    public IEnumerable ChildTeams { get; set; }
}

Then a recursive function:

private IEnumerable BuildTeams(IEnumerable allTeams, int? parentId)
{
    var teamTree = new List();
    var childTeams = allTeams.Where(o => o.ParentId == parentId).ToList();

    foreach (var team in childTeams)
    {
        var t = new Team();
        var children = BuildTeams(allTeams, team.TeamID);
        t.ChildTeams = children;
        teamTree.Add(t);
    }

    return teamTree ;
}

The first call passes a null for parent, and will pull all the teams that have a null parent :), though I notice your teams don't have a null for parent, so not sure how you identify the top level ones currently?