YetAnotherEqualityComparer

Yet Another member Equality Comparer for .NET

Problem

Do you often have to override Equals and GetHashCode in order to compare objects based on its members?

public class MyOldType
{
    public int? ImportantNullableInt { get; set; }

    public DateTime ImportantDate { get; set; }

    public string ImportantString { get; set; }

    public string NotImportantString { get; set; }

    public override bool Equals(object obj)
    {
        if (object.ReferenceEquals(this, obj))
        {
            return true;
        }

        var other = obj as MyOldType;

        if (other == null)
        {
            return false;
        }

        return this.ImportantNullableInt == other.ImportantNullableInt
            && this.ImportantDate == other.ImportantDate
            && string.Equals(this.ImportantString, other.ImportantString, StringComparison.InvariantCultureIgnoreCase);
    }

    public override int GetHashCode()
    {
        return ImportantNullableInt.GetHashCode()
            ^ ImportantDate.GetHashCode()
            ^ StringComparer.InvariantCultureIgnoreCase.GetHashCode(this.ImportantString);
    }
}

If you've done this a number of times you might feel that enumerating the same fields in both methods is not the right thing. Maintaining the same list of members in both methods as type grows may be a bit of pain. And imagine consequences if in GetHashCode you've mistaken and there's used hash code of NotImportantString instead of ImportantString or you forgot to take case-invariant hash code.

Solution

This small library allows you to setup members list only once per type and have Equals and GetHashCode implemented in terms of this setup.

public class MyNewType
{
    private static readonly MemberEqualityComparer<MyNewType> Comparer = new MemberEqualityComparer<MyNewType>()
        .Add(o => o.ImportantNullableInt)
        .Add(o => o.ImportantDate)
        .Add(o => o.ImportantString, StringComparer.InvariantCultureIgnoreCase);

    public int? ImportantNullableInt { get; set; }

    public DateTime ImportantDate { get; set; }

    public string ImportantString { get; set; }

    public string NotImportantString { get; set; }

    public override bool Equals(object obj)
    {
        return Comparer.Equals(this, obj);
    }

    public override int GetHashCode()
    {
        return Comparer.GetHashCode(this);
    }
}

That's all you need to do, MemberEqualityComparer takes care about the rest.

Additionally MemberEqualityComparer<T> acts as IEqualityComparer<T> so you may use it to compare types you can't change, value types (DateTime, TimeSpan, nullables and your own structs) and anonymous types.

Alternatives

There also exist more advanced libraries able to provide not only IEqualityComparer<T> but IComparer<T> also. Be sure to check those too:

1. ComparerExtensions
2. Comparers (by Stephen Cleary)

Installation

The package is available as NuGet https://www.nuget.org/packages/YetAnotherEqualityComparer/