Question

I want to remove duplicates in a list using following code, but it does not work. Anyone could enlighten me? Thanks.

public sealed class Pairing
{
    public int Index { get; private set; }
    public int Length { get; private set; }
    public int Offset { get; private set; }

    public Pairing(int index, int length, int offset)
    {
        Index = index;
        Length = length;
        Offset = offset;
    }
}


class MyComparer : IEqualityComparer<Pairing>
{
    public bool Equals(Pairing x, Pairing y)
    {
        return ((x.Index == y.Index) && (x.Length == y.Length) && (x.Offset == y.Offset));
    }

    public int GetHashCode(Pairing obj)
    {
        return obj.GetHashCode();
    }
}
class Program
{
    static void Main(string[] args)
    {
        List<Pairing> ps = new List<Pairing>();
        ps.Add(new Pairing(2, 4, 14));
        ps.Add(new Pairing(1, 2, 4));
        ps.Add(new Pairing(2, 4, 14));

        var unique = ps.Distinct(new MyComparer());
        foreach (Pairing p in unique)
        {
            Console.WriteLine("{0}\t{1}\t{2}", p.Index, p.Length, p.Offset);
        }
        Console.ReadLine();
    }
}

Answer 1

According to the example on the IEnumerable.Distinct page you will need to implement GetHashCode() so that the equal objects return the same hashcode. If you do not override GetHashCode() in your object it is not guaranteed to return the same hashcode.

// If Equals() returns true for a pair of objects 
// then GetHashCode() must return the same value for these objects.

public int GetHashCode(Product product)
{
    //Check whether the object is null
    if (Object.ReferenceEquals(product, null)) return 0;

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

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

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