How Should I Compare Two Instances of a Reference Type in C#?

Best Practice: Comparing Two Reference Type Instances in C#

When comparing reference types, be sure to clarify the expected behavior: value equality or reference identity.

Override the equality operator ("==") and the Equals method to achieve reference identity

In the past, developers would override the "==" and Equals methods to check for referential identity. However, coding standards advise against this as it may lead to ambiguity in certain situations.

Option 1: Implement the IEquatable interface

for value equality

For reference types with value semantics (immutable types where equality is treated as equality), the recommended approach is to implement the System.IEquatable interface. This allows you to define a custom equality comparison that focuses on values ​​rather than references.

Example:

class Point : IEquatable<Point> {
    // ...
    public bool Equals(Point other) {
        return X.Equals(other.X) && Y.Equals(other.Y);
    }
    // ...
}

Option 2: Use a custom Equals method with type checking to achieve value equality

If you prefer a custom Equals method, make sure it checks both reference equality and the type of the object being compared. This prevents unexpected behavior when comparing derived class objects.

Example:

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);
}

When to use referential identity

Do not implement the "==" and "!=" operators for reference classes that do not represent immutable values. Instead, rely on their default behavior of comparing objects for identity.

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