I find the following explanation quite good. It's based on the book by Vaughn Vernon and 'injects' the domain service in the domain model through the method call that actually needs this service.
public class PurchaseOrder
{
public string Id { get; private set; }
public string VendorId { get; private set; }
public string PONumber { get; private set; }
public string Description { get; private set; }
public decimal Total { get; private set; }
public DateTime SubmissionDate { get; private set; }
public ICollection<Invoice> Invoices { get; private set; }
public decimal InvoiceTotal
{
get { return this.Invoices.Select(x => x.Amount).Sum(); }
}
public bool IsFullyInvoiced
{
get { return this.Total <= this.InvoiceTotal; }
}
bool ContainsInvoice(string vendorInvoiceNumber)
{
return this.Invoices.Any(x => x.VendorInvoiceNumber.Equals(
vendorInvoiceNumber, StringComparison.OrdinalIgnoreCase));
}
public Invoice Invoice(IInvoiceNumberGenerator generator,
string vendorInvoiceNumber, DateTime date, decimal amount)
{
// These guards maintain business integrity of the PO.
if (this.IsFullyInvoiced)
throw new Exception("The PO is fully invoiced.");
if (ContainsInvoice(vendorInvoiceNumber))
throw new Exception("Duplicate invoice!");
var invoiceNumber = generator.GenerateInvoiceNumber(
this.VendorId, vendorInvoiceNumber, date);
var invoice = new Invoice(invoiceNumber, vendorInvoiceNumber, date, amount);
this.Invoices.Add(invoice);
DomainEvents.Raise(new PurchaseOrderInvoicedEvent(this.Id, invoice.InvoiceNumber));
return invoice;
}
}
public class PurchaseOrderService
{
public PurchaseOrderService(IPurchaseOrderRepository repository,
IInvoiceNumberGenerator invoiceNumberGenerator)
{
this.repository = repository;
this.invoiceNumberGenerator = invoiceNumberGenerator;
}
readonly IPurchaseOrderRepository repository;
readonly IInvoiceNumberGenerator invoiceNumberGenerator;
public void Invoice(string purchaseOrderId,
string vendorInvoiceNumber, DateTime date, decimal amount)
{
// Transaction management, along with committing the unit of work
// can be moved to ambient infrastructure.
using (var ts = new TransactionScope())
{
var purchaseOrder = this.repository.Get(purchaseOrderId);
if (purchaseOrder == null)
throw new Exception("PO not found!");
purchaseOrder.Invoice(this.invoiceNumberGenerator,
vendorInvoiceNumber, date, amount);
this.repository.Commit();
ts.Complete();
}
}
}