List.except on custom class

Question

lets say I have a custom class:

public class WineCellar
{

    public string year;
    public string wine;
    public double nrbottles;
}

Lets say I now have a List of this custom class:

List<WineCellar> orignialwinecellar = List<WineCellar>();

containing these items:

2012 Chianti 12

2011 Chianti 6

2012 Chardonay 12

2011 Chardonay 6

I know that if I want to compare two list and return a new list that has only items that are not in the other list I would do:

var newlist = list1.Except(list2);

How can I extend this to a custom class? Lets say I have:

string[] exceptionwinelist = {"Chardonay", "Riesling"};

I would like this to be returned:

List<WineCellar> result = originalwinecellar.wine.Except(exceptionwinelist);

This pseudocode obviously doesnt work but hopefully illustrates what I m trying to do. This shoudl then return a List of the custom class winecellar with following items:

2012 Chianti 12

2011 Chianti 6

Thanks.

Answer 1

You don't really want to use Except here, as you don't have a collection of WineCellar objects to use as a blacklist. What you have is a collection of rules: "I don't want objects with such and such wine names".

Therefore it's better to simply use Where:

List<WineCellar> result = originalwinecellar
    .Where(w => !exceptionwinelist.Contains(w.wine))
    .ToList();

In human-readable form:

I want all WineCellars where the wine name is not present in the list of exceptions.

As an aside, the WineCellar class name is a bit misleading; those objects are not cellars, they are inventory items.

Answer 2

One solution is with an extension method:

public static class WineCellarExtensions
{
    public static IEnumerable<WineCellar> Except(this List<WineCellar> cellar, IEnumerable<string> wines)
    {
        foreach (var wineCellar in cellar)
        {
            if (!wines.Contains(wineCellar.wine))
            {
                yield return wineCellar;
            }
        }
    }
}

And then use it like this:

List<WineCellar> result = originalwinecellar.Except(exceptionwinelist).ToList();

Answer 3

exceptionWineList is a string[] but originalWineCellar is a List<WineCellar>, WineCellar is not a string, so it does not make sense to perform an Except between these.

You could just as easily do,

// use HashSet for look up performance.
var exceptionWineSet = new HashSet<string>(exceptionWineList);
var result = orginalWineCellar.Where(w => !exceptionWineSet.Contains(w.Wine));

---

What I think you are alluding to in your question is something like

WineCellar : IEquatable<string>
{
    ...
    public bool Equals(string other)
    {
        return other.Equals(this.wine, StringComparison.Ordinal);
    }
}

which allows you to equate WineCellars to strings.

---

However, if I were to rework your model I'd come up with something like,

enum WineColour
{
    Red,
    White,
    Rose
}

enum WineRegion
{
    Bordeaux,
    Rioja,
    Alsace,
    ...
}

enum GrapeVariety
{
    Cabernet Sauvignon,
    Merlot,
    Ugni Blanc,
    Carmenere,
    ...
}

class Wine
{
    public string Name { get; set; }
    public string Vineyard { get; set; }
    public WineColour Colour { get; set; }
    public WineRegion Region { get; set; }
    public GrapeVariety Variety { get; set; }
}

class WineBottle
{
    public Wine Contents { get; set; }
    public int Millilitres { get; set; }
    public int? vintage { get; set; }
}

class Bin : WineBottle
{
    int Number { get; set; }
    int Quantity { get; set; }
}

class Cellar : ICollection<WineBottle> 
{
    ...
}

Then, you can see that there are several ways to compare Wine and I may want to filter a Cellar on one or more of Wine's properties. Therefore I might be temtpted to give myself some flexibility,

class WineComparer : EqualityComparer<Wine>
{
    [Flags]
    public Enum WineComparison
    {
        Name = 1,
        Vineyard= 2,
        Colour = 4,
        Region = 8,
        Variety = 16,
        All = 31
    }

    private readonly WineComparison comparison;

    public WineComparer()
        : this WineComparer(WineComparison.All)
    {
    }

    public WineComparer(WineComparison comparison)
    {
        this.comparison = comparison;
    }

    public override bool Equals(Wine x, Wine y)
    {
        if ((this.comparison & WineComparison.Name) != 0
            && !x.Name.Equals(y.Name))
        {
            return false;
        }

        if ((this.comparison & WineComparison.Vineyard) != 0
            && !x.Vineyard.Equals(y.Vineyard))
        {
            return false;
        }

        if ((this.comparison & WineComparison.Region) != 0
            && !x.Region.Equals(y.Region))
        {
            return false;
        }

        if ((this.comparison & WineComparison.Colour) != 0
            && !x.Colour.Equals(y.Colour))
        {
            return false;
        }

        if ((this.comparison & WineComparison.Variety) != 0
            && !x.Variety.Equals(y.Variety))
        {
            return false;
        }

        return true;
    }

    public override bool GetHashCode(Wine obj)
    {
        var code = 0;
        if ((this.comparison & WineComparison.Name) != 0)
        {
            code = obj.Name.GetHashCode();
        }

        if ((this.comparison & WineComparison.Vineyard) != 0)
        {
            code = (code * 17) + obj.Vineyard.GetHashCode();
        }

        if ((this.comparison & WineComparison.Region) != 0)
        {
            code = (code * 17) + obj.Region.GetHashCode();
        }

        if ((this.comparison & WineComparison.Colour) != 0)
        {
            code = (code * 17) + obj.Colour.GetHashCode();
        }

        if ((this.comparison & WineComparison.Variety) != 0)
        {
            code = (code * 17) + obj.Variety.GetHashCode();
        }

        return code;
    }
}

this probably looks like a lot of effort but it has some use. Lets say we wanted all the wine except the Red Rioja in your cellar, you could do something like,

var comparison = new WineComparer(
    WineComparison.Colour + WineComparison.Region);

var exception = new Wine { Colour = WineColour.Red, Region = WineRegion.Rioja }; 

var allButRedRioja = cellar.Where(c => 
    !comparison.Equals(c.Wine, exception));