Check if a date range is within a date range

Question

I have the following class:

public class Membership
{
    public DateTime StartDate { get; set; }
    public DateTime? EndDate { get; set; } // If null then          


        

Answer 1

I came up with the following method to check if dates overlap, it may not be the most effecient way but i hope this helps..

public static bool DateRangesOverlap(DateTime startDateA, DateTime endDateA, DateTime startDateB, DateTime endDateB)
{
    var allDatesA = new List<DateTime>();
    var allDatesB = new List<DateTime>();

    for (DateTime date = startDateA; date <= endDateA; date = date.AddDays(1))
    {
        allDatesA.Add(date);
    }

    for (DateTime date = startDateB; date <= endDateB; date = date.AddDays(1))
    {
        allDatesB.Add(date);
    }

    var isInRange = false;
    foreach (var date in allDatesA)
    {
        var existsInAllDatesB = allDatesB.Any(x => x == date);
        if (existsInAllDatesB)
        {
            isInRange = true;
            break;
        }
    }

    return isInRange;
}

Answer 2

Basically, a date range overlaps another if any of its endings are within the other range, or vice versa.

static bool AllowedToAdd(List<Membership> membershipList, Membership newItem)
{
    return !membershipList.Any(m =>
        (m.StartDate < newItem.StartDate &&
         newItem.StartDate < (m.EndDate ?? DateTime.MaxValue))
        ||
        (m.StartDate < (newItem.EndDate ?? DateTime.MaxValue) &&
         (newItem.EndDate ?? DateTime.MaxValue) <= (m.EndDate ?? DateTime.MaxValue))
        ||
        (newItem.StartDate < m.StartDate &&
         m.StartDate < (newItem.EndDate ?? DateTime.MaxValue))
        ||
        (newItem.StartDate < (m.EndDate ?? DateTime.MaxValue) &&
         (m.EndDate ?? DateTime.MaxValue) <= (newItem.EndDate ?? DateTime.MaxValue))
        );
}

With the usage:

if (AllowedToAdd(membershipList, newItem))
    membershipList.Add(newItem);