Add list to object in Entity Framework

Question

I have two Models called "Client" and "Software". Clients can have the same Software and Software can be on multiple clients. I want to add a List of Software to a specific Client.

Client Model:

public class Client
    {
        [Key]
        public int id { get; set; }
        [Required]
        public ICollection<Software>? Softwares { get; set; }
    }

Software Model:

public class Software
    {
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int id { get; set; }
        public ICollection<Client>? Clients { get; set; }
    }

Why does this throw a NullReferenceException?

public void submit()
        {
            using (var repo = new ClientRepository(contextFactory.CreateDbContext()))
            {
                client = repo.getClientByID(Convert.ToInt32(clientid));

                foreach (var software in toAddList)
                {
                    client.Softwares.Add(software); //Error occurs here
                }

            }               
        }

Repo Code

public Client getClientByID(int id)
        {
            return _context.Clients.Find(id);
        }

add repo.getClientByID(Convert.ToInt32(clientid)); code

Mohammad Aghazadeh
– Mohammad Aghazadeh

2022-10-17 07:27:04 +00:00
Commented Oct 17, 2022 at 7:27 — Mohammad Aghazadeh
– Mohammad Aghazadeh, Commented Oct 17, 2022 at 7:27

Fitri Halim · Accepted Answer · 2022-11-11 11:13:15Z

2

Because you need to include Softwares when fetching from Clients table in database

In your repo, change your code to be like this :

public Client getClientByID(int id)
{
     return _context.Clients.Include(k => k.Softwares).Single(i => i.id == id);
}

edited Nov 11, 2022 at 11:13

answered Oct 17, 2022 at 7:34

Fitri Halim

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2 Comments

Fitri Halim Over a year ago

@HenkHolterman, For me since it is already an Id I would just use First(), because I am confident that there is only data anyway by the virtue of database constrain itself, no need for database to scan throught entire table to check for any possible duplicate during this data retrieval.

Fitri Halim Over a year ago

Alright, I will edit my code

Mohammad Aghazadeh · Accepted Answer · 2022-10-17 07:51:40Z

1

Here, two objects can be null, both the client itself and the software list, But your error occurs because the list of software is null, to solve this problem you need to load the list of software when receiving Client information from the database,But before that, you must make sure of the existence of the Client. solve it as follows:

public Client getClientByID(int id)
{
    return _context.Clients.Include(s=>s.Softwares).FirstOrDefault(u => u.id == id);
}

client = repo.getClientByID(Convert.ToInt32(clientid));

if(client is null)
{
   //Do somthing Like Return or throw new Exception
}

foreach (var software in toAddList)
{
  client.Softwares.Add(software);
}

edited Oct 17, 2022 at 7:51

answered Oct 17, 2022 at 7:35

Mohammad Aghazadeh

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