How to Best Compare Reference Types in .NET?

Best Practices for .NET Reference Type Comparison

When comparing reference types in .NET, it is important to understand the difference between reference equality and value equality. Reference equality checks whether two references point to the same object, while value equality checks whether the objects have the same data.

Override the equality operator (==) and the Equals method

It is not recommended to override the equality operator (==) and/or the Equals method to compare reference types unless the type expresses value semantics (i.e., immutable objects that are considered equal based on their data). In this case, consider implementing the System.IEquatable interface and three operators: Equals, GetHashCode, and ==/!=.

IEquatable interface

If your reference type represents value semantics, implement System.IEquatable. This ensures correct and efficient implementation of value equality, including handling object identity and value equality separately.

IComparable interface

IComparable is primarily designed to be used with value types, not reference types. Avoid using it with reference types.

Custom comparison

Instead of overriding the equality operator or implementing IComparable, consider creating a custom method to compare reference types. Use the Equals method to check the object identity and override it to compare related properties.

Example of equal values

The following is an example of implementing value equality for the Point class:

<code class="language-csharp">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() => HashCode.Combine(X, Y);
}</code>

Conclusion

For reference types with value semantics, implement System.IEquatable and all three equality operators (Equals, GetHashCode, ==/!=). For reference classes that represent mutable values, do not override the ==/!= operator. Use their default behavior to check object identity.

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