Why is LINQ's Distinct Method Inefficient with Custom Objects, and How Can I Fix It?

Addressing LINQ's Distinct Method Inefficiency for Custom Objects

LINQ's Distinct() method can be inefficient when used with custom objects. This is because, by default, it relies on reference equality—meaning it only considers two objects distinct if they occupy different memory locations. Identical objects (with the same property values) will be treated as separate entities, leading to duplicated results.

The solution lies in implementing the IEquatable<T> interface within your custom object class. This allows you to define how equality is determined for your objects, based on their properties rather than their memory addresses.

Here's an improved Author class implementing IEquatable<Author>:

public class Author : IEquatable<Author>
{
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public bool Equals(Author other)
    {
        if (other is null) return false;
        return FirstName == other.FirstName && LastName == other.LastName;
    }

    public override int GetHashCode()
    {
        return HashCode.Combine(FirstName, LastName);
    }
}

By overriding the Equals() method and implementing GetHashCode(), you provide Distinct() with the logic it needs to correctly identify and remove duplicate Author objects based on their FirstName and LastName properties. Note the use of HashCode.Combine for a more robust hash code generation. This ensures that objects considered equal by Equals() also produce the same hash code. Using this approach, Distinct() will now accurately identify and remove duplicate author objects based on their names.

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