Mapping switch statements to data classes

Question

This seems to come up alot in my code, i'm wondering if there is some way of removing the switch statement, or if there is a more elegant way of doing it?

    public class MetaData
{
    public string AlbumArtist { get; set; }
    public string AlbumTitle { get; set; }
    public string Year { get; set; }
    public string SongTitle { get; set; }


    public static MetaData CreateMetaDataFrom(IEnumerable<TextFrame> textFrames)
    {
        var metaData = new MetaData();

        foreach (var frame in textFrames)
        {
            switch (frame.Descriptor.ID)
            {
                case "TPE1":
                    metaData.AlbumArtist = frame.Content;
                    break;

                case "TALB":
                    metaData.AlbumTitle = frame.Content;
                    break;

                case "TIT2":
                    metaData.SongTitle = frame.Content;
                    break;

                case "TYER":
                    metaData.Year = frame.Content;
                    break;
            }
        }

        return metaData;
    }
}

Answer 1

From your code I conclude that IEnumerable<TextFrame> has always 4 members so you could just write (haven't tried it so check for the syntax):

public static MetaData CreateMetaDataFrom(IEnumerable<TextFrame> textFrames)
{
    return new MetaData()
    {
        metaData.AlbumArtist = textFrames.Where(frame => frame.Descriptor.ID = "TPE1").SingleOrDefault().Content,
        metaData.AlbumTitle = textFrames.Where(frame => frame.Descriptor.ID = "TALB").SingleOrDefault().Content, 
        metaData.SongTitle = textFrames.Where(frame => frame.Descriptor.ID = "TIT2").SingleOrDefault().Content;
        metaData.Year = textFrames.Where(frame => frame.Descriptor.ID = "TYER").SingleOrDefault().Content;
    };
}

Answer 2

This is related to an object-oriented approach. The usual method to get rid of if's or case's is to use a lookup table of criteria and effects. There are other techniques that use this same idea, such as data-directed programming (http://en.wikipedia.org/wiki/Data-directed_programming) and dispatch tables (http://en.wikipedia.org/wiki/Dispatch_table). Many language implementations use type dispatch tables to implement virtual method calls.

The lookup table could be a hashtable populated with lambda functions as so:

Dictionary<string, Func<MetaData, string, string>> lookup = new Dictionary<string, Func<MetaData, string, string>>();
lookup["TPE1"] = (m, v) => m.AlbumArtist = v;
lookup["TALB"] = (m, v) => m.AlbumTitle = v;
lookup["TIT2"] = (m, v) => m.SongTitle = v;
lookup["TYER"] = (m, v) => m.Year = v;

then you assign fields of metaData inside your loop as:

lookup[frame.Descriptor.ID](metaData, frame.Content);

Answer 3

You might want to look at implementing the Strategy Pattern. DimeCasts.Net have an excellent video tutorial that may help.

Answer 4

I was tempted to suggest the Strategy Pattern but you may need a slight variation. Consider writing a method in the TextFrame class, lets call it putContent(MetaData).

Then create subclasses of TextFrame each representing a different Type of Frame. Each subclass will override the putContent(Metadata) method and do its appropraite logic.

PseudoCode Example for TPE1:

 Metadata putContent(MetaData md){
       md.AlbumArtist = Content;
       return md;
 }

You MetaData code will then change to:

var metaData = new MetaData();

    foreach (var frame in textFrames)
    {
           metaData = frame.putContent(metaData);
    }
 return metaData;

Of course, to create the TextFrames them selves will then require a Factory so this is not the end of the story.

Answer 5

It seems like you know what the types will be before hand (using a switch) so why not just retrieve the values as required, without a for switch.

Examples are when using a hashtable, and you know what fields will be available, just use the fields.

ig you are not sure if the field is available, a simple test will be sufficiant if the list contains the value.

You can then even write a helper function to check and return the value if the list has the value.

Answer 6

What you really have here is a four-way setter. The canonical refactoring here is "Replace Parameter with Explicit Methods" (p285 of Refactoring by Martin Fowler). The Java example he gives is changing:

void setValue(String name, int value) {
  if (name.equals("height")) {
    _height = value;
    return;
  }
  if (name.equals("width")) {
    _width = value;
    return;
  }
}

to:

void setHeight(int arg) {
  _height = arg;
}

void setWidth(int arg) {
  _width = arg;
}

Assuming that the caller of CreateMetaDataFrom() knows what it's passing in, you could skip the switch/case and use the actual setters for those properties.