两点的C＃集合不返回任何结果

Question

Class:

public class Point : IEqualityComparer<Point>
{               
    public char HorizontalPosition { get; set; }
    public int VerticalPosition { get; set; }

    public Point(char horizontalPosition, int verticalPosition)
    {
        HorizontalPosition = char.ToUpper(horizontalPosition);
        VerticalPosition = verticalPosition;           
    }   

    public bool Equals(Point x, Point y)
    {
        return (x.VerticalPosition == y.VerticalPosition && x.HorizontalPosition == y.HorizontalPosition);
    }

    public int GetHashCode(Point obj)
    {
        return (obj.HorizontalPosition.GetHashCode() + obj.VerticalPosition.GetHashCode());
    }
}

I am trying to find common Points (intersection) in two collections, but the result is empty collection - two elements should be in it. Why? I've implemented IEqualityComparer. Did I did something wrong?

Example collections:

  List<Point> first = new List<Point> { new Point('a', 1), new Point('b', 2) };
  List<Point> second = new List<Point> { new Point('a', 1), new Point('b', 2) };
  List<Point> intersection = first.Intersect(second).ToList();

Intersection is empty list, but two elements should be in it.

Answer 1

IEqualityComparer is an interface you can give to the Intersect method to compare items. It is not used by default to compare anything. So your code is just using the built-in Equals in Object which will return false unless the objects are the same object.

You have to either override the default Equal and GetHashCode methods in the class or tell the intersection to use your implementation of the comparer. But you shouldn't implement the comparer in a data storage class.

Answer 2

You should override Equals and GetHashCode from object:

public class Point
{
    public char HorizontalPosition { get; set; }
    public int VerticalPosition { get; set; }

    public Point(char horizontalPosition, int verticalPosition)
    {
        HorizontalPosition = char.ToUpper(horizontalPosition);
        VerticalPosition = verticalPosition;
    }

    public override int GetHashCode()
    {
        unchecked
        { 
            return (HorizontalPosition * 397) ^ VerticalPosition;
        }
    }

    protected bool Equals(Point other)
    {
        return Equals(HorizontalPosition, other.HorizontalPosition) && Equals(VerticalPosition, other.VerticalPosition);
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj)) return false;
        if (ReferenceEquals(this, obj)) return true;
        if (obj.GetType() != this.GetType()) return false;
        return Equals((Point)obj);
    }
}

You could also implement a custom IEqualityComparer and pass it to intersect :

public class PointComparer : IEqualityComparer<Point>
{
    public bool Equals(Point a, Point b)
    {
        return a.HorizontalPosition == b.HorizontalPosition && a.VerticalPosition == b.VerticalPosition;
    }

    public int GetHashCode(Point p)
    {
        unchecked
        { 
            return (p.HorizontalPosition * 397) ^ p.VerticalPosition;
        }
    }
}

// ...

List<Point> intersection = first.Intersect(second, new PointComparer()).ToList();

As mentioned in the comments by @decPL, you should also reconsider your hash code implementation.

Answer 3

Unless specified, Intersect uses EqualityComparer<Point>.Default which will use the object.Equals and object.GetHashCode methods for comparison (they will just check if the reference is the same);

to make it work, pass the comparer to the method:

  List<Point> first = new List<Point> { new Point('a', 1), new Point('b', 2) };
  List<Point> second = new List<Point> { new Point('a', 1), new Point('b', 2) };
  List<Point> intersection = first.Intersect(second, new Point('a', 0)).ToList();

Although, ideally for the SRP you shouldn't have the comparer on the Point class itself as it would look hacky as it looks above creating a Point just as logic class for comparison.

From MSDN:

EqualityComparer

Intersect

Answer 4

List<Point> first = new List<Point> { new Point('a', 1), new Point('b', 2) };
            List<Point> second = new List<Point> { new Point('a', 1), new Point('b', 2) };
            List<Point> intersection = first.Intersect(second, new PointComparer()).ToList();


public class Point 
{
    public char HorizontalPosition { get; set; }
    public int VerticalPosition { get; set; }

    public Point(char horizontalPosition, int verticalPosition)
    {
        HorizontalPosition = char.ToUpper(horizontalPosition);
        VerticalPosition = verticalPosition;
    }
}

public class PointComparer : IEqualityComparer<Point>
{
    public bool Equals(Point x, Point y)
    {
        return (x.VerticalPosition == y.VerticalPosition && x.HorizontalPosition == y.HorizontalPosition);
    }

    public int GetHashCode(Point obj)
    {
        return (obj.HorizontalPosition.GetHashCode() + obj.VerticalPosition.GetHashCode());
    }
}

Try above example

Answer 5

You should separate your Point and PointComparer classes.

The manual has good example:

public class ProductA
{ 
    public string Name { get; set; }
    public int Code { get; set; }
}

public class ProductComparer : IEqualityComparer<ProductA>
{

    public bool Equals(ProductA x, ProductA y)
    {
        //Check whether the objects are the same object. 
        if (Object.ReferenceEquals(x, y)) return true;

        //Check whether the products' properties are equal. 
        return x != null && y != null && x.Code.Equals(y.Code) && x.Name.Equals(y.Name);
    }

    public int GetHashCode(ProductA obj)
    {
        //Get hash code for the Name field if it is not null. 
        int hashProductName = obj.Name == null ? 0 : obj.Name.GetHashCode();

        //Get hash code for the Code field. 
        int hashProductCode = obj.Code.GetHashCode();

        //Calculate the hash code for the product. 
        return hashProductName ^ hashProductCode;
    }
}

https://msdn.microsoft.com/en-us/library/bb460136(v=vs.110).aspx

Answer 6

As you can find on reference https://referencesource.microsoft.com/

System\\Linq\\Enumerable.cs

    public static IEnumerable<TSource> Intersect<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second) {
                if (first == null) throw Error.ArgumentNull("first");
                if (second == null) throw Error.ArgumentNull("second");
                return IntersectIterator<TSource>(first, second, null);
            }

...
    static IEnumerable<TSource> IntersectIterator<TSource>(IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
        {
            Set<TSource> set = new Set<TSource>(comparer);
            foreach (TSource element in second) set.Add(element);
            foreach (TSource element in first)
                if (set.Remove(element)) yield return element;
        }
...
// If value is in set, remove it and return true; otherwise return false
        public bool Remove(TElement value) {
            int hashCode = InternalGetHashCode(value);
            int bucket = hashCode % buckets.Length;
            int last = -1;
            for (int i = buckets[bucket] - 1; i >= 0; last = i, i = slots[i].next) {
                if (slots[i].hashCode == hashCode && comparer.Equals(slots[i].value, value)) {
                    if (last < 0) {
                        buckets[bucket] = slots[i].next + 1;
                    }
                    else {
                        slots[last].next = slots[i].next;
                    }
                    slots[i].hashCode = -1;
                    slots[i].value = default(TElement);
                    slots[i].next = freeList;
                    freeList = i;
                    return true;
                }
            }
            return false;
        }

Your comparer actually doesn't used