C#

results = (from u in dc.Users
            select new UserEntity
            {
                .
                .
                .
                ModifiedDT = u.ModifiedDT.GetValueOrDefault()  //<==THROWS
            }).ToList();

results = (from u in dc.Users
            select new UserEntity
            {
                .
                .
                .
                ModifiedDT = u.ModifiedDT.HasValue ? u.ModifiedDT : null //<== WORKS
            }).ToList();

public T GetValueOrDefault(T defaultValue)
{
    return HasValue ? value : defaultValue;
}

private async void AddUser()
{
    UserEntity user = new UserEntity
    {
        FirstName = "Jack",
        LastName = "Stone",
        UserName = "jstone",
        Password = ""
    };

    await Task.Factory.StartNew(() =>
    {
        IBizObj bo = new BizObj(connString);
        bo.AddUser(user);

    }).ContinueWith(task =>
    {
        UserList.Add(user);
    });
}

public class BizObj : IBizObj
{
    IRepository _repoisitory;

<pre>
public BizObj(string connectionString)
{
    _repoisitory = new FalconRepository(connectionString);
}

public int AddUser(UserEntity entity)
{
    return _repoisitory.AddUser(entity);
}

public int AddUser(UserEntity entity)
{
    using (var dc = GetDataContext())
    {
        int results = 0;

        try
        {
            User user = new User
            {
                FirstName = entity.FirstName,
                LastName = entity.LastName,
                UserName = entity.UserName,
                Password = entity.Password,
                CanLogIn = entity.CanLogIn,
                PasswordChangeDate = entity.PasswordChangeDate,
                CreatedByUserId = 1,
                CreatedDT = DateTime.Now,
            };

            dc.Users.InsertOnSubmit(user);
            dc.SubmitChanges();

            results = user.Id;
        }
        catch (Exception e)
        {
             //<===================================== HANDLE EXCEPTION HERE
        }

        return results;
    }
}