Question

Many of my application service layer methods go something like this:

    public class Command
    {
        public int Id { get; private set; }
    }

    public class Repository
    {
        public Entity Load(int id)
        {
            // the usual stuff here
        }
    }

    public class AppService
    {
        public void Execute(Command command)
        {
            var entity = new Repository().Load(command.Id);
            if (entity == null)
            {
                // what type of exception do I throw here?
            }
        }
    }

What type of exception should I throw if the client has issued a command for an entity that cannot be found? The InvalidOperationException spec on MSDN refers to “object state” not being valid. Seems like that’s not really applicable here – it’s really just a bad command.

Any suggestions?

Answer 1

I would favor InvalidOperationException as this makes semantic sense. The state of the Command is bad as its Id is not valid.

Also, I find providing parity with the .Net Framework is helpful for developers who join your project later. Looking to how Microsoft handles similar situations, such as Enumerable.Single<TSource> or EntityReference<TEntity>.Load we find they use InvalidOperationException.