抛出异常的最佳方法

Question

Do you know a better way (more pretty) than below to throw an exception?

public long GetPlaylistId(long songInPlaylistId)
{
    var songInPlaylist = service.GetById(songInPlaylistId);
    return songInPlaylist
            .With(x => x.Playlist)
            .ReturnValueOrException(x => x.Id, 
                                         new ArgumentException(
                                             "Bad argument 'songInPlaylistId'"));
}

Monadic extension methods:

public static TResult With<TInput, TResult>(this TInput obj, 
                                            Func<TInput, TResult> evaluator)
    where TInput : class
    where TResult : class
{
    return obj == null ? null : evaluator(obj);
}

public static TResult ReturnValueOrException<TInput, TResult>(
    this TInput obj, Func<TInput, TResult> evaluator, Exception exception)
    where TInput : class
{
    if (obj != null)
    {
        return evaluator(obj);
    }

    throw exception;
}

Answer 1

If it is valid to try to get the playlist for something that doesn't have a playlist, then you should not throw an exception but should just return a special value that means "not found" instead (for example, 0 or -1 depending on how your playlist IDs work).

Alternatively you could write a TryGetPlaylistId() method which works in a similar way to Microsoft's TryXXX() methods (eg SortedList.TryGetValue() ), for example:

public bool TryGetPlaylistId(long songInPlaylistId, out long result)
{
    result = 0;
    var songInPlaylist = service.GetById(songInPlaylistId);

    if (songInPlaylist == null)
        return false;

    if (songInPlaylist.Playlist == null)
        return false;

    result = songInPlaylist.Playlist.Id;
    return true;
}

A small problem with this approach is that you are obscuring information that might be of use when trying to diagnose issues. Perhaps adding Debug.WriteLine() or some other form of logging would be of use. The point being, you can't differentiate between the case where the playlist ID is not found, and the case where it is found but doesn't contain a playlist.

Otherwise, you could throw an exception which has a more informative message, for example:

public long GetPlaylistId(long songInPlaylistId)
{
    var songInPlaylist = service.GetById(songInPlaylistId);

    if (songInPlaylist == null)
        throw new InvalidOperationException("songInPlaylistId not found: " + songInPlaylistId);

    if (songInPlaylist.Playlist == null)
        throw new InvalidOperationException("Playlist for ID " + songInPlaylistId " has no playlist: ");

    return songInPlaylist.Playlist.Id;
}

It might be the case that it is valid to not find the song in the playlist, but it is NOT valid to find one which does not have a playlist, in which case you would return a special value in the first case and throw an exception in the second case, for example:

public long GetPlaylistId(long songInPlaylistId)
{
    var songInPlaylist = service.GetById(songInPlaylistId);

    if (songInPlaylist == null)
        return -1; // -1 means "playlist not found".

    if (songInPlaylist.Playlist == null)
        throw new InvalidOperationException("Playlist for ID " + songInPlaylistId " has no playlist: ");

    return songInPlaylist.Playlist.Id;
}

In any case, I personally think that your extension methods are just obscuring the code.

Answer 2

try{
if (obj != null)
{
    return evaluator(obj);
}
}
catch(Exception ex)
{
throw;
}

return obj;

You should not throw error unless caught in to some. Better return null in the given case and handle it in your calling code:

Answer 3

And what will happen if I have more than one such ambiguous methods in my class? It's very difficult to invent different rules for any method. You will be confused in the end. What do you think about this solution?

public class ApplicationResponse
{
    public IList<string> Errors { get; set; }
    public dynamic Data { get; set; }

    public bool HasErrors()
    {
        return Errors != null && Errors.Any();
    }
}

public ApplicationResponse GetPlaylistId(long songInPlaylistId)
{
    var songInPlaylist = service.GetById(songInPlaylistId);
    if (songInPlaylist == null)
    {
        return new ApplicationResponse { Errors = new[] { "Song was not found." } };
    }

    if (songInPlaylist.Playlist == null)
    {
        return new ApplicationResponse { Errors = new[] { "Playlist was not found." } };
    }

    return new ApplicationResponse { Data = songInPlaylist.Playlist.Id };
}

public HttpResponseMessage SomeRequest([FromUri] songInPlaylistId)
{
    var response = appService.GetPlaylistId(long songInPlaylistId);
    if (response.HasErrors())
    {
        // reply with error status
    }

    // reply with ok status
}

In such case I can send all the errors to a client.