C＃经典错误。对象需要被实例化

Question

这是一个经典问题，但我不知道如何在我的代码中弄明白。

对象引用未设置为对象的实例。

我知道我必须写一些像new IList<SubInvoice>();这样的东西，但我不知道在哪里。

private void GetSub()
{
    var invoice = Selected;
    var invs = (from i in ctx.Invoices
                join p in ctx.products on i.refSupp equals p.refsup
                join u in ctx.units on p.unit equals u.idunit
                where i.suppInvNumber == invoice.suppInvNumber
                select new SubInvoice
                { 
                    Invoice = i,
                    description = p.description,
                    unit1 = u.unit1
                });

    if(Products != null)
    Products.Clear();

    if (invs != null)
        foreach(var inv in invs)
        {
            SubInvoice sub = new SubInvoice(){Invoice=inv.Invoice, description = inv.description, unit1 = inv.unit1};
            Products.Add(sub); //error
        }

}

private IList<SubInvoice> _products;

public IList<SubInvoice> Products
{
    get
    {
        return _products;
    }
    set
    {
        _products = value;
        OnPropertyChanged("Products");
    }
}

public class SubInvoice
{
    public Invoice Invoice { get; set; }
    public string description { get; set; }
    public string unit1 { get; set; }
}

Answer 1

private IList<SubInvoice> _products = new List<SubInvoice>()

Answer 2

替换如下：

if(Products != null)
    Products.Clear();

与

if(Products != null)
    Products.Clear();
else
    Products = new List<SubInvoice>();