Select on empty list's group item gives non-empty list-CodePudding

This is my sample code:

var users = GetUsers();
var userGroups = users.GroupBy(x => x.Id);

foreach (var userGroup in userGroups)
{
    var user = userGroup.First();
    var carriedItems = userGroup.Select(x => x.CarriedItemId).ToList();
    Console.WriteLine($"UserId={user.Id}, CarriedItemsCount={carriedItems.Count}");
}

static IEnumerable<User> GetUsers()
{
    return new List<User>()
    {
        new User() { Id = 1, CarriedItemId = new List<int> { 6 }},
        new User() { Id = 1, CarriedItemId = new List<int> { 12 }},
        new User() { Id = 2, CarriedItemId = new List<int> { 2 }},
        new User() { Id = 3, CarriedItemId = new List<int>() }
    };
}

class User
{
    public int Id { get; set; }
    public List<int> CarriedItemId { get; set; }
}

Output of this is:

UserId=1, CarriedItemsCount=2
UserId=2, CarriedItemsCount=1
UserId=3, CarriedItemsCount=1

Why UserId=3 has 1 carried item, but not 0? After debugging I can see, that his carriedItems list contains default int32 value, which is 0. Why? I expected his carriedItems list to be empty. Can someone spot a bug?

CodePudding user response：

  var carriedItems = userGroup.Select(x => x.CarriedItemId).ToList();
  Console.WriteLine($"UserId={user.Id}, CarriedItemsCount={carriedItems.Count}");

carriedItems is not the count of CarriedItemId, its a list of each entry of the usergroup with that ID. The one for id = 3 has 1 entry that contains 0.

CodePudding user response：

You've messed up types and ranges. Your code as it is does not Count the things you think it does. Right now, it counts Collections. Not Items. See comments below:

class User
{
    public int Id { get; set; }

    // this 'CarriedItemId' is a LIST, so it's rather a "CarriedItemIds"
    public List<int> CarriedItemId { get; set; }
}


var users = GetUsers();
var userGroups = users.GroupBy(x => x.Id);

foreach (var userGroup in userGroups)
{
    var user = userGroup.First();

    // watch carefully what you select here
    // CarriedItemId is a LIST, not a single item
    // 'carriedItems' is a List<List<int>>
    var carriedItems = userGroup.Select(x => x.CarriedItemId).ToList();
    Console.WriteLine($"UserId={user.Id}, CarriedItemsCount={carriedItems.Count}");
}

Therefore, for a userId=3, there's one record, that record has an item list, which, although empty, is still a list, and record exists, so you get a 1.

To get a merged set of all itemIds, use SelectMany instead:

var carriedItems = userGroup.SelectMany(x => x.CarriedItemId).ToList();

This will merge all CarriedItemId from all User records, and you will end up with expected 2/1/0 counts.