Understanding Why Distinct() Doesn't Call Equals in .NET 8

Debugging C# Distinct() Issues: Why Equals Isn't Invoked

Imagine you’re building a robust C# library and suddenly, a simple call to doesn’t behave as expected. You’ve implemented and even an , but breakpoints never trigger. 🤔

This is a common frustration when working with collections in .NET, especially when dealing with complex objects like lists. You expect .NET to compare objects using your defined method, yet it seems to ignore it completely. What’s going wrong?

In your case, the key issue revolves around how C# handles equality comparison for collections. When objects contain properties, their default hashing and equality mechanisms behave differently from simple properties like strings or integers.

Let’s dive into the details, break down the problem step by step, and find out why isn’t working as expected. More importantly, we’ll explore how to fix it so your collection filtering behaves correctly. 🚀

Command	Example of use
SequenceEqual()	Compares two collections element by element, ensuring they are equal in both order and content. Used here to compare lists of identifiers in ClaimPath.
HashCode.Combine()	Combines multiple hash codes into a single hash, improving performance and reducing collisions when implementing GetHashCode().
IEquatable<T>	Defines a type-specific method for checking equality, ensuring that Equals() is used correctly when objects of type ClaimPath are compared.
IEqualityComparer<T>	Provides a way to define custom equality comparison for collections, ensuring Distinct() works as expected by explicitly defining equality logic.
DisallowNull]	Ensures that null values cannot be passed to a method parameter, reducing potential runtime errors in GetHashCode().
Distinct()	Removes duplicate elements from a collection, but relies on GetHashCode() and Equals() implementation to determine uniqueness.
Assert.AreEqual()	Used in unit tests to validate that expected and actual values match, ensuring the correctness of the distinct filtering logic.
ToList()	Converts an IEnumerable to a List, ensuring materialization of query results before further processing.
[TestClass] / [TestMethod]	Annotations from MSTest framework used to define and execute unit tests for verifying expected behavior of the implemented logic.

Understanding Why Distinct() Doesn't Call Equals

In the scripts provided, the primary issue revolves around how C#'s method determines uniqueness within collections. Normally, when working with custom objects, you expect and to be called. However, as seen in the example, these methods are not triggered when the class contains an IEnumerable property like a list. The reason is that lists themselves do not override equality comparison, which results in distinct treating every object as unique, even when their contents are identical. This is a subtle but crucial detail that affects many developers.

To address this, we implemented and a custom . The IEquatable interface allows for a more precise equality comparison at the object level, but since the method relies on GetHashCode(), we needed to manually construct a stable hash. The first implementation relied on , which is useful for simple properties like strings or integers. However, when dealing with lists, we iterated over each element and generated a cumulative hash value, ensuring consistent equality comparison across instances.

To verify that our changes worked, we wrote unit tests using MSTest. The test checks whether duplicates are properly removed when calling . Initially, calling Distinct() without an explicit comparer failed, proving that the default behavior does not handle complex equality scenarios well. However, when we passed our custom , the method correctly filtered out duplicates. This reinforces the importance of writing explicit comparers when working with objects containing nested collections.

A real-world analogy would be comparing two shopping lists. If you compare them as objects, they might be seen as different even if they contain the exact same items. But if you compare their contents explicitly, you can determine whether they are truly identical. By implementing within and a structured hashing strategy, we ensured that objects with the same identifiers are correctly recognized as duplicates. This small yet critical adjustment can save hours of debugging in complex C# applications. 🚀

using System;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Linq;

public class ClaimPath : IEquatable<ClaimPath>
{
    public List<ClaimIdentifier> Identifiers { get; private set; } = new();
    public bool Starred { get; private set; }

    public ClaimPath(IEnumerable<ClaimIdentifier> identifiers, bool starred)
    {
        Identifiers = identifiers.ToList();
        Starred = starred;
    }

    public override bool Equals(object obj)
    {
        return Equals(obj as ClaimPath);
    }

    public bool Equals(ClaimPath other)
    {
        if (other == null) return false;
        return Identifiers.SequenceEqual(other.Identifiers) && Starred == other.Starred;
    }

    public override int GetHashCode()
    {
        int hash = 17;
        foreach (var id in Identifiers)
        {
            hash = hash * 23 + (id?.GetHashCode() ?? 0);
        }
        return hash * 23 + Starred.GetHashCode();
    }
}

public class ClaimPathComparer : IEqualityComparer<ClaimPath>
{
    public bool Equals(ClaimPath x, ClaimPath y)
    {
        if (x == null || y == null) return false;
        return x.Equals(y);
    }

    public int GetHashCode([DisallowNull] ClaimPath obj)
    {
        return obj.GetHashCode();
    }
}

using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Collections.Generic;
using System.Linq;

[TestClass]
public class ClaimPathTests
{
    [TestMethod]
    public void Distinct_ShouldRemoveDuplicates()
    {
        var listOfClaimPaths = new List<ClaimPath>
        {
            new ClaimPath(new List<ClaimIdentifier> { new ClaimIdentifier("A") }, false),
            new ClaimPath(new List<ClaimIdentifier> { new ClaimIdentifier("B") }, false),
            new ClaimPath(new List<ClaimIdentifier> { new ClaimIdentifier("A") }, false)
        };

        var distinctList = listOfClaimPaths.Distinct(new ClaimPathComparer()).ToList();

        Assert.AreEqual(2, distinctList.Count);
    }
}

Why Distinct() Doesn't Work With Complex Objects in C#

Another crucial aspect of in C# is how it handles complex objects with nested properties. When an object contains a list, as in our case, C# does not automatically compare the contents of that list. Instead, it checks object references, which means two different instances of a list—even with identical elements—are treated as separate. This explains why isn't called and why Distinct() fails to remove duplicates.

To work around this, a deep comparison strategy must be used. This can be done by overriding to compare individual elements inside the list using . However, for performance reasons, lists should be sorted before comparison to ensure they are checked in a consistent order. Additionally, should iterate through the list and compute a hash for each item, combining them into a single value. Without this, two objects with identical lists could still produce different hash codes, leading to unexpected results.

In practical applications, developers working with API responses, database records, or configurations stored in JSON often face similar challenges. If you’re handling collections in a LINQ query and find that isn’t working as expected, verifying how equality is determined is essential. Consider using a custom when working with lists, dictionaries, or complex nested objects. This ensures consistency and prevents unnecessary duplicates from creeping into your data. 🚀

1. Why doesn’t call my method?
2. By default, relies on . If your class contains lists, the hash code calculation may not be consistent, causing C# to treat all objects as unique.
3. How can I make recognize duplicates correctly?
4. Implement and ensure correctly compares list elements using . Also, override GetHashCode() to generate stable hash values.
5. Does using a custom help?
6. Yes! Passing a custom comparer to ensures that equality checks work as expected, even for complex objects.
7. What’s the difference between and ?
8. is used for direct comparisons between objects, while allows custom equality logic when working with collections.
9. Are there performance concerns with overriding ?
10. Yes. A poorly optimized can slow down lookups. Using or manually iterating through list items for hashing can improve performance.

Understanding how works with complex objects is crucial for developers working with collections. When dealing with lists inside an object, C# does not automatically compare their contents, leading to unexpected behavior. Implementing and customizing ensures that objects with identical lists are treated as duplicates.

By applying these techniques, you can avoid data inconsistencies in scenarios like API processing, database filtering, or configuration management. Mastering these equality comparison methods will make your C# applications more efficient and reliable. Don't let hidden distinct issues slow you down—debug smarter and keep your collections clean! ✅