Table of Contents

Comparing objects

LINQ methods like Union, Distinct, Intersect, and Except require an IEqualityComparer<T> when you want to compare objects by something other than their default equality. The standard approach is to create a full class that implements IEqualityComparer<T> with both Equals and GetHashCode - which is a lot of ceremony for what's usually a one-line comparison.

GenericEqualityComparer<T> lets you define that comparison inline with a lambda expression.

Using GenericEqualityComparer

The comparer takes a function that extracts the value to compare. Two objects are considered equal when the function returns the same value for both.

using MADE.Collections.Compare;

var comparer = new GenericEqualityComparer<Product>(p => p.Id);

// Use with any LINQ method that accepts IEqualityComparer
var uniqueProducts = allProducts.Distinct(comparer);
var combined = listA.Union(listB, comparer);
var common = listA.Intersect(listB, comparer);

Practical example: merging permission sets

A common scenario is combining multiple collections while ensuring no duplicates. Without GenericEqualityComparer, you'd need a dedicated comparer class or resort to manual deduplication:

using MADE.Collections.Compare;

public static class ApplicationPermissions
{
    private static readonly GenericEqualityComparer<Permission> PermissionComparer =
        new(permission => permission.Id);

    private static readonly IEnumerable<Permission> AdminPermissions = new List<Permission>();
    private static readonly IEnumerable<Permission> UserPermissions = new List<Permission>();

    public static IEnumerable<Permission> GetAllPermissions()
    {
        return AdminPermissions.Union(UserPermissions, PermissionComparer);
    }
}

When to use this vs other approaches

Use GenericEqualityComparer when you need a quick equality comparison for LINQ operations and the comparison logic is simple (comparing by a single property or a computed value).

Use a full IEqualityComparer<T> implementation when you need the comparer in multiple places across your codebase, or when the equality logic is complex enough to warrant its own testable class.

Use records if you're on C# 9+ and can define your type as a record, which gives you value-based equality for free. But when you're working with existing classes or need to compare by a specific property rather than all properties, GenericEqualityComparer is the pragmatic choice.

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