Implementing IEqualityComparer<T> on an object with two properties in C#

Question

I have a case where I need to grab a bunch of items on distinct, but my source is a collection of objects with two properties, like this:

public class SkillRequirement
{
    public string Skill { get; set; }
    public string Requirement { get; set; }
}

I try to get a collection as follows:

SkillRequirementComparer sCom = new SkillRequirementComparer();

var distinct_list = source.Distinct(sCom);

I tried to implement an IEqualityComparer<T> for this, but I fell stumped at the GetHashCode() method.

The class for the Comparer:

public class SkillRequirementComparer : IEqualityComparer<SkillRequirement>
{
    public bool Equals(SkillRequirement x, SkillRequirement y)
    {
        if (x.Skill.Equals(y.Skill) && x.Requirement.Equals(y.Requirement))
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    public int GetHashCode(SkillRequirement obj)
    {
        //?????
    }
}

Normally I would just use GetHashCode() on a property, but because I am comparing on two properties, I'm a bit at a loss of what to do. Am I doing anything wrong, or missing something really obvious?

Answer 1

You can implement GetHashCode in the following way:

public int GetHashCode(SkillRequirement obj)
{
    unchecked
    {
        int hash = 17;
        hash = hash * 23 + obj.Skill.GetHashCode();
        hash = hash * 23 + obj.Requirement.GetHashCode();
        return hash;
    }
}

_{originally from J.Skeet}

If the properties can be null you should avoid a NullReferenceException, e.g.:

int hash = 17;
hash = hash * 23 + (obj.Skill ?? "").GetHashCode();
hash = hash * 23 + (obj.Requirement ?? "").GetHashCode();
return hash;

Answer 2

I would like to link the following stack overflow posts too though the question is already answered..

GetHashCode -

Why is it important to override GetHashCode when Equals method is overridden?

Also, in the above answer Tim Schmelter says the properties can be null you should avoid a NullReferenceException

int hash = 17;
hash = hash * 23 + (obj.Skill ?? "").GetHashCode();
hash = hash * 23 + (obj.Requirement ?? "").GetHashCode();
return hash;

IEqualityComparer -

1. What is the difference between using IEqualityComparer and Equals/GethashCode Override
2. What's the role of GetHashCode in the IEqualityComparer in .NET?
3. How and when to use IEqualityComparer in C#

IEquatable - What's the difference between IEquatable and just overriding Object.Equals()?

Equals - Guidelines for Overloading Equals()

class TwoDPoint : System.Object
{
    public readonly int x, y;

    public TwoDPoint(int x, int y)  //constructor
    {
        this.x = x;
        this.y = y;
    }

    public override bool Equals(System.Object obj)
    {
        // If parameter is null return false.
        if (obj == null)
        {
            return false;
        }

        // If parameter cannot be cast to Point return false.
        TwoDPoint p = obj as TwoDPoint;
        if ((System.Object)p == null)
        {
            return false;
        }

        // Return true if the fields match:
        return (x == p.x) && (y == p.y);
    }

    public bool Equals(TwoDPoint p)
    {
        // If parameter is null return false:
        if ((object)p == null)
        {
            return false;
        }

        // Return true if the fields match:
        return (x == p.x) && (y == p.y);
    }

    public override int GetHashCode()
    {
        //return x ^ y;
    }
}