IEqualityComparer中令人费解的行为 <T> ，未命中Equals（）方法中的断点

Question

VS2013, .Net 4.5.2.

Given this code

            var opCollectionA = new List<Option>() 
            {
              ...some elements added here..
            };

            var opCollectionB = new List<Option>() 
            {
              ...some elements added here..
            };

            var InAandB = opCollectionA.Except(opCollectionB, new OptionComparer()).ToArray();

and this comparer class:

public class OptionComparer : EqualityComparer<Option>
{

    public override bool Equals(Option x, Option y)
    {
        //Check whether the objects are the same object. 
        if (Object.ReferenceEquals(x, y)) return true;
        //Check whether the options have the same Entry value. 
        return x != null && y != null && x.OptionEntry.Equals(y.OptionEntry);
    }

    public override int GetHashCode(Option obj)
    {
        //Get hash code for the OptionSection field if it is not null. 
        int hashOptionSection = obj.OptionSection == null ? 0 : obj.OptionSection.GetHashCode();

        //Get hash code for the OptionEntry field. 
        int hashOptionEntry = obj.OptionEntry.GetHashCode();

        //Calculate the hash code for the Option. 
        return hashOptionSection ^ hashOptionEntry;
    }
}

I am finding in Debug mode that GetHashcode is being called for each comparison but never Equals. Consequently my comparer doesn't give the desired outcome.

Can anyone explain this?

TIA.

Answer 1

A hash code is used to speed up comparisons. So a GetHashCode() implementation should be faster than Equals .

GetHashCode() must return equal hash codes for equal objects (where "equal" means that Equals() would return true ).

Since there are not enough int values to have a different hash code for each different object (for example strings), there will of course be different objects sharing the same hash code.

If GetHashCode() returns the same hash code for two objects, Equals() must be called to decide if they are really equal. But if GetHashCode() returns different values, you already know that these objects cannot be equal and don't have to call the - normally much more expensive - Equals() method.

CONCLUSION:

In your code you seem to expect two Option s to be equal if their OptionEntry is equal. So the easiest way to fix your problem is to change your GetHashCode() so that it simply returns the hash code of OptionEntry :

public override int GetHashCode(Option obj)
{
    return obj.OptionEntry.GetHashCode();
}