Example class layout for a domain aggregate in a command model

SampleWriteAggregate.cs

public class Employee : Aggregate
{
	private readonly IAccountService _accountService;
	
	// all mutable properties are private there is no reason to expose these as properties
	// in the write model
	private string _firstName;
	private string _lastName;
	
	public Employee(IAccountService accountService)
	{
		_accountService = accountService;
	}
	
	// public interface exposes semantic operations, using the ubiquitous language
	public void CreateNew(Guid accountId, string firstName, string lastName)
	{
		// always do validation in the public method. This is the chance for the aggregate to refuse the 
		// request
		
		var accountIsActive = _accountService.AccountIsActive(accountId);
		if(!accountIsActive)
			throw new InvalidOperationException("Cannot create employee on an inactive account.")
		
		if(Id != null)
			throw new InvalidOperationException("This instance appears to have already been created.");
		
		if(string.IsNullOrEmpty(firstName))
			throw new ArgumentNullException(nameof(firstName));
		
		if(string.IsNullOrEmpty(lastName))
			throw new ArgumentNullException(nameof(lastName));
		
		/***********************************************************************************************	
		 *	Don't actually set the state in the public method, let the event handler take care of that
		 * 	this ensures the event only fires if the aggregate approves the operation and also prevents
		 * 	us for having state set in two different places since Apply is how the event store restores
		 * 	the state of the object
		 ***********************************************************************************************/
		
		// Dynamic method on the base class
		ApplyEvent(new CreateEmployee(Guid.NewGuid(), firstName, lastName));
	}
	
	// private Apply methods simply modify the objects state. This method is called dynamically
	// from the Aggregate base class
	private void Apply(CreateEmployee e)
	{
		Id = e.Id; // exists on base class 
		_firstName = e.FirstName;
		_lastName = e.LastName;
	}
	
	// snapshots are how we get state out of this protected object
	public override EmployeeSnapshot GetSnapshot()
	{
		return new EmployeeSnapshot
		{
			Id = Id,
			FirstName = _firstName,
			LastName = _lastName
		};
	}
}