Generator.Equals

A source code generator for automatically implementing IEquatable<T> using only attributes.

Requirements

In order to use this library, you must:

• Use a target framework that supports .NET Standard >= 2.0
• Set your project's C# LangVersion property to 9.0 or higher.

Installation

Simply add the package Generator.Equals to your project. Keep reading to learn how to add the attributes to your types.

Migrating from version 3

Inherited Equality Attributes

Version 4 introduces support for inherited equality attributes on overridden properties, making repeating attributes unnecessary. When a child class overrides a virtual property from a parent class, it now automatically inherits the equality attribute (e.g., [OrderedEquality]) from the parent. You no longer need to redeclare attributes on overriding properties.

[Equatable]
public partial class Parent
{
    [OrderedEquality]
    public virtual int[] Values { get; set; }
}

[Equatable]
public partial class Child : Parent
{
    // Automatically inherits [OrderedEquality] from Parent
    public override int[] Values { get; set; }
}

Improved Inheritance Chain Detection

Version 4 improves how base.Equals() is called in inheritance hierarchies. Previously, generated code would only call base.Equals() if the immediate base class had [Equatable]. Now, the generator walks the entire inheritance chain and calls base.Equals() if:

1. Any ancestor has the [Equatable] attribute, OR
2. Any ancestor has manually overridden Equals(object)

This fixes scenarios where equality was incorrectly skipped in multi-level inheritance:

[Equatable]
public partial class GrandParent
{
    public string Name { get; set; }
}

// No [Equatable] - inherits GrandParent's Equals
public class Parent : GrandParent
{
    public int Age { get; set; }
}

[Equatable]
public partial class Child : Parent
{
    public string School { get; set; }
}

Before (v3): Child.Equals() did NOT call base.Equals() because Parent lacks [Equatable]. Only School was compared, ignoring Name.

After (v4): Child.Equals() calls base.Equals() because GrandParent has [Equatable]. Both School and Name are compared correctly.

Ignore Inherited Members

The IgnoreInheritedMembers property controls how inherited members are handled:

[Equatable(IgnoreInheritedMembers = true)]
public partial class Child : Parent
{
    // Will NOT call base.Equals() even if Parent has [Equatable]
    // Only properties defined in Child are compared
    public string School { get; set; }
}

Migrating from version 2

Migrating to version 3 is very straightforward.

1. Ensure projects are targeting C# 9.0 or latter using the MSBuild property LangVersion.
2. Be aware that IEquatable<T> for classes is now implemented explicitly in order to support deep equality. As a result, the method Equals(T) method is no longer marked as public. Most code should still work, requiring only to be recompiled as the ABI has changed.

If you have an existing project using Generator.Equals and don't need any of the new features, you can still use version 2.x. The differences are minimal between both major versions.

Usage

The below sample shows how to use Generator.Equals to override the default equality implementation for a C# record, enhancing it with the ability to determine the equality between the array contents of the record.

using Generator.Equals;

[Equatable]
partial record MyRecord(
    [property: OrderedEquality] string[] Fruits
);

class Program
{
    static void Main(string[] args)
    {
        var record1 = new MyRecord(new[] {"banana", "apple"});
        var record2 = new MyRecord(new[] {"banana", "apple"});

        Console.WriteLine(record1 == record2);
    }
}

Need more than records? Generator.Equals supports properties (and fields) also across classes, structs and record structs.

using Generator.Equals;

[Equatable]
partial class MyClass
{
    [DefaultEquality] 
    private int _secretNumber = 42;

    [OrderedEquality] 
    public string[] Fruits { get; set; }
}

[Equatable]
partial struct MyStruct
{
    [OrderedEquality] 
    public string[] Fruits { get; set; }
}

[Equatable]
partial record struct MyRecordStruct(
    [property: OrderedEquality] string[] Fruits
);

Supported Comparers

Below is a list of all supported comparers. Would you like something else added? Let me know by raising an issue or sending a PR!

Default

This is the comparer that's used when a property has no attributes indicating otherwise. The generated code will use EqualityComparer<T>.Default for both equals and hashing operation.

Fields are not used in comparison unless explicitly annotated. To enable the default comparison for a field, annotate it with the DefaultEquality attribute.

IgnoreEquality

[IgnoreEquality] 
public string Name { get; set; }

As the name implies, the property is ignored during Equals and GetHashCode calls!

OrderedEquality

[OrderedEquality]
public string[] Fruits { get; set; } // Fruits have to be in the same order for the array to be considered equal.

This equality comparer will compare properties as a sequence instead of a reference. This works just like Enumerable.SequenceEqual, which assumes both lists are of the same size and same sort.

Bear in mind that the property has to implement IEnumerable and the that the items themselves implement equality (you can use Generator.Equals in the items too!).

You can also specify a custom comparer for the elements:

// Using StringComparison for string collections
[OrderedEquality(StringComparison.OrdinalIgnoreCase)]
public string[] Tags { get; set; }

// Using a custom comparer type
[OrderedEquality(typeof(StringComparer), nameof(StringComparer.OrdinalIgnoreCase))]
public string[] Names { get; set; }

// Using a custom IEqualityComparer<T> with a Default static member
[OrderedEquality(typeof(MyCustomComparer))]
public string[] Values { get; set; }

UnorderedEquality

[UnorderedEquality]
public string[] Fruits { get; set; } // Does not care about the order of the fruits!

[UnorderedEquality]
public IDictionary<string, object> Properties { get; set; } // Works with dictionaries too!

This equality comparer will compare properties as an unordered sequence instead of a reference. This works just like Enumerable.SequenceEqual, but it does not care about the order as long as the all values (including the repetitions) are present.

As with OrderedEquality, bear in mind that the property (or key and values if using a dictionary) has to implement IEnumerable and the that the items themselves implement equality (you can use Generator.Equals in the items too!).

You can also specify a custom comparer for the elements:

// Using StringComparison for string collections
[UnorderedEquality(StringComparison.OrdinalIgnoreCase)]
public List<string> Tags { get; set; }

// Using a custom comparer type
[UnorderedEquality(typeof(StringComparer), nameof(StringComparer.OrdinalIgnoreCase))]
public List<string> Names { get; set; }

SetEquality

[SetEquality]
public HashSet<string> Fruits { get; set; } // Fruits can be in any order and duplicates are ignored

This equality comparer will do a set comparison, using SetEquals whenever the underlying collection implements ISet<T>, otherwise falling back to manually comparing both collections, which can be expensive for large collections.

You can also specify a custom comparer for the elements:

// Using StringComparison for string collections
[SetEquality(StringComparison.OrdinalIgnoreCase)]
public HashSet<string> Tags { get; set; }

// Using a custom comparer type
[SetEquality(typeof(StringComparer), nameof(StringComparer.OrdinalIgnoreCase))]
public HashSet<string> Names { get; set; }

Hashing always returns 0 for this type of equality.

ReferenceEquality

[ReferenceEquality] 
public string Name { get; set; } // Will only return true if strings are the same reference (eg. when used with string.Intern)

This will ignore whatever equality is implemented for a particular object and compare references instead.

StringEquality

[StringEquality(StringComparison.CurrentCulture | CurrentCultureIgnoreCase | InvariantCulture | InvariantCultureIgnoreCase | Ordinal | OrdinalIgnoreCase)]
public string Title { get; set; } // Will use the StringComparison set in constructor when comparing strings

CustomEquality

class LengthEqualityComparer : IEqualityComparer<string>
{
    public static readonly LengthEqualityComparer Default = new();

    public bool Equals(string? x, string? y) => x?.Length == y?.Length;

    public int GetHashCode(string obj) => obj.Length.GetHashCode();
}

class NameEqualityComparer 
{
    public static readonly IEqualityComparer<string> Default = new SomeCustomComparer();
}


[CustomEquality(typeof(LengthEqualityComparer))] 
public string Name1 { get; set; } // Will use LengthEqualityComparer to compare the values of Name1.

[CustomEquality(typeof(NameEqualityComparer))] 
public string Name2 { get; set; } // Will use NameEqualityComparer.Default to compare values of Name2.

[CustomEquality(typeof(StringComparer), nameof(StringComparer.OrdinalIgnoreCase))] 
public string Name2 { get; set; } // Will use StringComparer.OrdinalIgnoreCase to compare values of Name2.

This attribute allows you to specify a custom comparer for a particular property. For it to work, the type passed as an argument to CustomEqualityAttribute should fulfill AT LEAST one of the following:

• Have a static field/property named Default returning a valid IEqualityComparer instance for the target type;
• Have a static field/property with the same name passed to the CustomComparerAttribute returning a valid IEqualityComparer instance for the target type;
• Implement IEqualityComparer and expose a parameterless constructor.

Advanced Options

Explicit Mode

The generator allows you to explicitly specify which properties are used to generate the IEquatable.

To do this, set the Explicit property of EquatableAttribute to true and specify the required properties using DefaultEqualityAttribute or other attributes.

using Generator.Equals;

[Equatable(Explicit = true)]
partial class MyClass
{
    // Only this property will be used for equality!
    [DefaultEquality] 
    public string Name { get; set; } = "Konstantin"; 
    
    public string Description { get; set; } = "";
}

Ignore Inherited Members

By default (IgnoreInheritedMembers = false), the generated code handles inherited members as follows:

• If any ancestor has [Equatable] or a custom Equals override, base.Equals() is called to delegate equality
• If NO ancestor has [Equatable], all inherited properties from the entire chain are explicitly compared

Set IgnoreInheritedMembers = true to skip calling base.Equals() and ignore all inherited properties. This is useful when you want to completely redefine equality for a derived class without considering the base class's properties.

using Generator.Equals;

[Equatable]
partial class Person
{
    public string Name { get; set; }
}

[Equatable(IgnoreInheritedMembers = true)]
partial class Doctor : Person
{
    // Will NOT call base.Equals(), so Person.Name is not compared.
    // Only Id and Specialization are used for equality.
    public string Id { get; set; }
    public string Specialization { get; set; }
}