Searching for an item in a list that's nested inside another list based on a property value in C# using LINQ?

Is there a way to search for an item in a list that’s nested inside another list based on a property value using LINQ?

Given the follow models below, for a given Order (variable customerOrder), I want to return the earliest order date (Date) where the Day is "Sunday".

models:

public class Order
{
    public int Id { get; set; }
    public List<OrderLine> OrderLines { get; set; }
}

public class OrderLine
{
    public string Description { get; set; }
    public List<OrderDate> OrderDates { get; set; }
}

public class OrderDate
{
    public DateTime Date { get; set; }
    public string Day { get; set; }
}

code:

var dates = new List<DateTime>();
foreach(var a in customerOrder.OrderLines)
{
    var orderDate = a.OrderDates.Where(x => x.DateTypeId.Equals("Sunday")).FirstOrDefault();
    dates.Add(orderDate.ActualDate);
}
dates.OrderBy(d => d.Date);

return dates.FirstOrDefault();

>Solution :

You can use Linq to achieve your result:

customerOrder.OrderLines
    .Select(ol => ol.OrderDates
        .Where(x => x.Day.Equals("Sunday"))
        .FirstOrDefault())
    .Where(d => d != null)
    .OrderBy(d => d.Date)
    .FirstOrDefault();

Here is a Linqpad query with some test data and dump for you to try.
Simply copy and paste in Linqpad.

void Main()
{
    var customerOrder = new Order
    {
        Id = 1,
        OrderLines = Enumerable
            .Range(0, 10)
            .Select(i => new OrderLine
            {
                Description = $"Line Description {i}",
                OrderDates = Enumerable.Range(0, 10)
                    .Select(j => new OrderDate
                    {
                        Date = DateTime.Now.AddDays(i+j),
                        Day = DateTime.Now.AddDays(i+j).DayOfWeek.ToString()
                    })
                    .ToList()
            })
            .ToList()
        }
        .Dump();

    customerOrder.OrderLines
            .Select(ol => ol.OrderDates
                .Where(x => x.Day.Equals("Sunday"))
                .FirstOrDefault())
            .Where(d => d != null)
            .OrderBy(d => d.Date)
            .Dump()
            .FirstOrDefault()
            .Dump();
}

// You can define other methods, fields, classes and namespaces here

public class Order
{
    public int Id { get; set; }
    public List<OrderLine> OrderLines { get; set; }
}

public class OrderLine
{
    public string Description { get; set; }
    public List<OrderDate> OrderDates { get; set; }
}

public class OrderDate
{
    public DateTime Date { get; set; }
    public string Day { get; set; }
}