c# Linq List - handle null

Question

which in the second list im trying to create a relationship, however if it cant a match, how do I ignore and not add an item?

var clientData = File.ReadAllLines(txtClients.Text)
                    .Skip(1)
                    .Select(x => x.Split(','))
                    .Select(x => new Client()
                    {
                        ClientTempId = x[0],
                        Email = x[1],
                        FirstName = x[2],
                        LastName = x[3],
                        AccountId = accountId
                    });

var orderData = File.ReadAllLines(txtOrders.Text)
                  .Skip(1)
                  .Select(x => x.Split(','))
                  .Select(x => new Order()
                  {
                      OrderTempId = x[0],
                     ClientId = clientData.FirstOrDefault(c=>c.ClientTempId == x[1]).Id ==string.Empty?"Error here!!":x[1],
                     //How do I handle errors, if client does not exist, or row is in wrong format? dont want to break code just want a list or issues
                      Name = x[3],
                      AccountId = accountId
                  });

Answer 1

You can return null instead and then filter those out:

var orderData = File.ReadAllLines(txtOrders.Text)
    .Skip(1)
    .Select(x => x.Split(','))
    .Select(x =>
    {
        // do your check here, and return null
        if (clientData.FirstOrDefault(c => c.ClientTempId == x[1]) == null)
            return null;

        // otherwise return the normal Order object
        return new Order()
        {
            OrderTempId = x[0],
            ClientId = x[1],
            Name = x[3],
            AccountId = accountId
        };
    })
    // then filter out null values
    .Where(x => x != null);

Once that is covered, as EZI pointed out in the comments, your actual check is quite expensive. You can make it more efficient by turning your clientData into a dictionary:

var clientDataDictionary = clientData.ToDictionary(c => c.ClientTempId);

Then, you can do the lookup above in constant time:

if (clientDataDictionary.ContainsKey(x[1]))
    return null;