Question

Is there a default IEqualityComparer<T> implementation that uses ReferenceEquals?

EqualityComparer<T>.Default uses ObjectComparer, which uses object.Equals(). In my case, the objects already implement IEquatable<T>, which I need to ignore and compare by object’s reference only.

Answer 1

Just in case there is no default implementation, this is my own:

Edit by 280Z28: Rationale for using RuntimeHelpers.GetHashCode(object), which many of you probably haven’t seen before. 🙂 This method has two effects that make it the correct call for this implementation:

1. It returns 0 when the object is null. Since ReferenceEquals works for null parameters, so should the comparer’s implementation of GetHashCode().
2. It calls Object.GetHashCode() non-virtually. ReferenceEquals specifically ignores any overrides of Equals, so the implementation of GetHashCode() should use a special method that matches the effect of ReferenceEquals, which is exactly what RuntimeHelpers.GetHashCode is for.

[end 280Z28]

using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;

/// <summary>
/// A generic object comparerer that would only use object's reference, 
/// ignoring any <see cref="IEquatable{T}"/> or <see cref="object.Equals(object)"/>  overrides.
/// </summary>
public class ObjectReferenceEqualityComparer<T> : EqualityComparer<T>
    where T : class
{
    private static IEqualityComparer<T> _defaultComparer;

    public new static IEqualityComparer<T> Default
    {
        get { return _defaultComparer ?? (_defaultComparer = new ObjectReferenceEqualityComparer<T>()); }
    }

    #region IEqualityComparer<T> Members

    public override bool Equals(T x, T y)
    {
        return ReferenceEquals(x, y);
    }

    public override int GetHashCode(T obj)
    {
        return RuntimeHelpers.GetHashCode(obj);
    }

    #endregion
}