c# winform , How to get only available Computer accounts

Question

I have account table with some accounts. Some of them are already assigned to a computer. Some of them are not - they are available.

Here is my class:

public class ComputerAccount
{
    public int AccountId { get; set; }
    public string AccountName { get; set; }
    public int DepId { get; set; } 
    public int ComputerId { get; set; }
}

and my computer class:

public class Computer
    {
        public int ComputerId { get; set; }
        public string ComputerName { get; set; }
        public int DepId { get; set; } 
        public string AccountName { get; set; }
    }

If, in the Account table, ComputerId == null, then I know this account is available.

I have a computer list in my DataGridView. When I want to update, I just CellClick to load another Update form.

I have Loadaccounts funtion in update form that displays all accounts in ComboBox.

What I want in ComboBox is to populate only those available accounts, plus current account, if the computer has an account. Otherwise only those which are available.

I can populate all computer ids where ComputerId == null, but this should include the current account. It does not display the current account. As previously mentioned, if the computer has an account I want to display it, plus the computer ids where ComputerId == null.

Here is what I am trying:

using (db)
{

   var account = (from u in db.ComputerAccounts
                  where u.DepId == DepId  && u.ComputerId == null
                  select new { Name = u.AccountName, Value = u.AccountId }).ToList();

                    account.Insert(0, new { Name = "Choose account", Value = -1 });

                    CmbAccounts.DisplayMember = "Name";
                    CmbAccounts.ValueMember = "Value";
                    CmbAccounts.DataSource = account;
                }

But this does not populate current account because I'm using u.ComputerId == null. I have this because some Accounts do not have any computer bound. Thank you in advance.