Question

I’m relatively new to MVC and EF Code first and cannot find a solution to this problem.

I have used EF code first to create 2 entities, Invoice and InvoiceItem. One Invoice can have many instances of InvoiceItem. InvoiceItem can only have one instance of Invoice.

Code snippets as follows:

public class Invoice
{
    [ScaffoldColumn(false)]
    public int InvoiceId { get; set; }
    public string Description { get; set; }

    public virtual ICollection<InvoiceItem> InvoiceItems { get; set; }
}

public class InvoiceItem
{
    [ScaffoldColumn(false)]
    public int InvoiceItemId { get; set; }

    [DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:C}")]
    public decimal Amount { get; set; }

    public virtual Invoice Invoice { get; set; }
}

---

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Invoice>().HasMany(i => i.InvoiceItems);
    modelBuilder.Entity<InvoiceItem>().HasRequired(i => i.Invoice);
}

As a test, I’m just trying to create one InvoiceItem for an Invoice with the following code

if (ModelState.IsValid)
{
    _repository.Add(invoice);
    _repository.Save();

    invoice.InvoiceItems.Add(new InvoiceItem {Amount = 99});

    return RedirectToAction("Index");
}

When it gets to the invoice.InvoiceItems.Add() line, it throws an “Object reference not set to an instance of an object.” error.

What am I missing?

Answer 1

My initial guess here is that the ‘InvoiceItems’ property hasn’t been initialised. You could do a null check before you add to the collection:

if(invoice.InvoiceItems == null) 
{ 
   invoice.InvoiceItems = new List<InvoiceItem>();
}
invoice.InvoiceItems.Add(new InvoiceItem {Amount = 99});

Alternatively make InvoiceItems a readonly property by creating a private field and initalise in the get {} if it’s null:

private ICollection<InvoiceItem> invoiceItems;

public ICollection<InvoiceItem> InvoiceItems
{
   get
   {
      return this.invoiceItems ?? (this.invoiceItems = new List<InvoiceItem>());
   }
}