Why Must You Override `GetHashCode` When Overriding `Equals`?

Overriding GetHashCode and Equals for Consistent Object Comparison

When you redefine how objects of a class are compared for equality by overriding the Equals method, it's essential to also override the GetHashCode method. This is particularly important when your objects are used as keys in hash-based collections like dictionaries or hash sets.

Consider a class Foo:

public override bool Equals(object obj)
{
    // ... comparison logic based on FooId ...
}

If you customize Equals to compare Foo objects based on a FooId property, the default GetHashCode (inherited from Object) will still generate hash codes based on the object's memory address. This inconsistency leads to unpredictable behavior when using Foo objects as keys in hash tables.

Why Consistent Hashing Matters

Overriding GetHashCode is crucial for:

• Consistent Hashing: An object's hash code determines its location in a hash table. Equal objects must have the same hash code to ensure they're consistently placed in the same bucket.
• Avoiding Hash Collisions: If two objects have the same hash code (a collision), hash-based collections might incorrectly assume they're identical without calling Equals to verify. A well-implemented GetHashCode minimizes collisions, ensuring Equals is used to determine true equality.

Implementing GetHashCode Effectively

The GetHashCode implementation should align with the Equals method's logic:

• Equal objects must have identical hash codes.
• Identical hash codes don't guarantee equality, but they trigger the Equals method for a definitive comparison.

For the Foo class, a suitable GetHashCode override is:

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

This generates hash codes based on FooId, mirroring the equality check in Equals.

Using Custom Comparers for Enhanced Clarity

When overriding Equals and GetHashCode, consider adding custom equality operators (== and !=) for improved code readability and maintainability.

The above is the detailed content of Why Must You Override `GetHashCode` When Overriding `Equals`?. For more information, please follow other related articles on the PHP Chinese website!