What is “Best Practice” For Comparing Two Instances of a Reference Type?

Implementing equality in .NET correctly, efficiently and without code duplication is hard. Specifically, for reference types with value semantics (i.e. immutable types that treat equvialence as equality), you should implement the System.IEquatable<T> interface, and you should implement all the different operations (Equals, GetHashCode and ==, !=).

As an example, here’s a class implementing value equality:

class Point : IEquatable<Point> {
    public int X { get; }
    public int Y { get; }

    public Point(int x = 0, int y = 0) { X = x; Y = y; }

    public bool Equals(Point other) {
        if (other is null) return false;
        return X.Equals(other.X) && Y.Equals(other.Y);
    }

    public override bool Equals(object obj) => Equals(obj as Point);

    public static bool operator ==(Point lhs, Point rhs) => object.Equals(lhs, rhs);

    public static bool operator !=(Point lhs, Point rhs) => ! (lhs == rhs);

    public override int GetHashCode() => X.GetHashCode() ^ Y.GetHashCode();
}

The only movable parts in the above code are the bolded parts: the second line in Equals(Point other) and the GetHashCode() method. The other code should remain unchanged.

For reference classes that do not represent immutable values, do not implement the operators == and !=. Instead, use their default meaning, which is to compare object identity.

The code intentionally equates even objects of a derived class type. Often, this might not be desirable because equality between the base class and derived classes is not well-defined. Unfortunately, .NET and the coding guidelines are not very clear here. The code that Resharper creates, posted in another answer, is susceptible to undesired behaviour in such cases because Equals(object x) and Equals(SecurableResourcePermission x) will treat this case differently.

In order to change this behaviour, an additional type check has to be inserted in the strongly-typed Equals method above:

public bool Equals(Point other) {
    if (other is null) return false;
    if (other.GetType() != GetType()) return false;
    return X.Equals(other.X) && Y.Equals(other.Y);
}