🚵🏻 👨🏽‍🏫 🤙🏿 Enum and switch, and what's wrong with them 🤘🏿 👨🏾 🤾🏻

How often have you had a situation where you add a new value to an enum and then spend hours trying to find all the places where it is used, and then add a new case so as not to get an ArgumentOutOfRangeException at runtime?

Idea

If the only problem is the switch statement and keeping track of new types, then let's get rid of them!

The idea is to replace the use of switch with the visitor pattern.

Example 1

- API , , , .

DocumentType.cs:

public enum DocumentType
{
    Invoice,
    PrepaymentAccount
}

public interface IDocumentVisitor<out T>
{
    T VisitInvoice();
    T VisitPrepaymentAccount();
}

public static class DocumentTypeExt
{
    public static T Accept<T>(this DocumentType self, IDocumentVisitor<T> visitor)
    {
        switch (self)
        {
            case DocumentType.Invoice:
                return visitor.VisitInvoice();
            case DocumentType.PrepaymentAccount:
                return visitor.VisitPrepaymentAccount();
            default:
                throw new ArgumentOutOfRangeException(nameof(self), self, null);
        }
    }
}

, , .Net . .

visitor DatabaseSearchVisitor.cs:

public class DatabaseSearchVisitor : IDocumentVisitor<IDocument>
{
    private ApiId _id;
    private Database _db;

    public DatabaseSearchVisitor(ApiId id, Database db)
    {
        _id = id;
        _db = db;
    }

    public IDocument VisitInvoice() => _db.SearchInvoice(_id);
    public IDocument VisitPrepaymentAccount() => _db.SearchPrepaymentAccount(_id);
}

public void UpdateStatus(ApiDoc doc)
{
    var searchVisitor = new DatabaseSearchVisitor(doc.Id, _db);

    var databaseDocument = doc.Type.Accept(searchVisitor);

    databaseDocument.Status = doc.Status;

    _db.SaveChanges();
}

2

, :

public enum PurseEventType
{
    Increase,
    Decrease,
    Block,
    Unlock
}

public sealed class PurseEvent
{
    public PurseEventType Type { get; }
    public string Json { get; }

    public PurseEvent(PurseEventType type, string json)
    {
        Type = type;
        Json = json;
    }
}

. visitor:

public interface IPurseEventTypeVisitor<out T>
{
    T VisitIncrease();
    T VisitDecrease();
    T VisitBlock();
    T VisitUnlock();
}

public sealed class PurseEventTypeNotificationVisitor : IPurseEventTypeVisitor<Missing>
{
    private readonly INotificationManager _notificationManager;
    private readonly PurseEventParser _eventParser;
    private readonly PurseEvent _event;

    public PurseEventTypeNotificationVisitor(PurseEvent @event, PurseEventParser eventParser, INotificationManager notificationManager)
    {
        _notificationManager = notificationManager;
        _event = @event;
        _eventParser = eventParser;
    }

    public Missing VisitIncrease() => Missing.Value;

    public Missing VisitDecrease() => Missing.Value;

    public Missing VisitBlock()
    {
        var blockEvent = _eventParser.ParseBlock(_event);
        _notificationManager.NotifyBlockPurseEvent(blockEvent);
        return Missing.Value;
    }

    public Missing VisitUnlock()
    {
        var blockEvent = _eventParser.ParseUnlock(_event);
        _notificationManager.NotifyUnlockPurseEvent(blockEvent);
        return Missing.Value;
    }
}

. Missing System.Reflection Unit. Result, , , .

public void SendNotification(PurseEvent @event)
{
    var notificationVisitor = new PurseEventTypeNotificationVisitor(@event, _eventParser, _notificationManager);
    @event.Type.Accept(notificationVisitor);
}

, , visitor . .

public static T Accept<TVisitor, T>(this DocumentType self, in TVisitor visitor)
    where TVisitor : IDocumentVisitor<T>
    {
        switch (self)
        {
            case DocumentType.Invoice:
                return visitor.VisitInvoice();
            case DocumentType.PrepaymentAccount:
                return visitor.VisitPrepaymentAccount();
            default:
                throw new ArgumentOutOfRangeException(nameof(self), self, null);
        }
    }

visitor , class struct.

, :

public void UpdateStatus(ApiDoc doc)
{
    var searchVisitor = new DatabaseSearchVisitor(doc.Id, _db);

    var databaseDocument = doc.Type.Accept<DatabaseSearchVisitor, IDocument>(searchVisitor);

    databaseDocument.Status = doc.Status;

    _db.SaveChanges();
}

generic, , .

in-place

, visitor — . match.

public static T Match<T>(this DocumentType self, Func<T> invoiceCase, Func<T> prepaymentAccountCase)
{
    var visitor = new FuncVisitor<T>(invoiceCase, prepaymentCase);
    return self.Accept<FuncVisitor<T>, T>(visitor);
}

FuncVisitor:

public readonly struct FuncVisitor<T> : IDocumentVisitor<T>
{
    private readonly Func<T> _invoiceCase;
    private readonly Func<T> _prepaymentAccountCase;

    public FuncVisitor(Func<T> invoiceCase, Func<T> prepaymentAccountCase)
    {
        _invoiceCase = invoiceCase;
        _prepaymentAccountCase = prepaymentAccountCase;
    }

    public T VisitInvoice() => _invoiceCase();
    public T VisitPrepaymentAccount() => _prepaymentAccountCase();
}

match:

public void UpdateStatus(ApiDoc doc)
{
    var databaseDocument = doc.Type.Match(
        () => _db.SearchInvoice(doc.Id),
        () => _db.SearchPrepaymentAccount(doc.Id)
    );

    databaseDocument.Status = doc.Status;

    _db.SaveChanges();
}

enum :

, .

case switch.

, enum.

Enum and switch, and what's wrong with them

Idea

Example 1

2

in-place

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