Objects Comparer

• ObjectsComparer.zip
• GitHub
• Nuget

Introduction

It is quite common situation when complex objects should be compared. Sometimes objects can contain nested elements, or some members should be excluded from comparison (auto generated identifiers, create/update date etc.), or some members can have custom comparison rules (same data in different formats, like phone numbers). To solve such kind of problems I have developed small framework to compare objects.

Briefly, Objects Comparer is an object-to-object comparer, which allows to compare objects recursively member by member and define custom comparison rules for certain properties, fields or types.

Objects comparer can be considered as ready to use framework or as an idea for similar solutions. This article is mostly focused on using framework than on implementation. If you will be interested in implementation, modification or you will have any ideas how to make this framework better feel free to contact me in any way.

Installation

Install Objects Comparer as NuGet package or download code from CodeProject or GitHub and add Objects Comparer project to your solution.

Install-Package ObjectsComparer

Basic Example

public class ClassA 
{ 
    public string StringProperty { get; set; } 
    public int IntProperty { get; set; } 
}

There are two examples below how Objects Comparer can be used to compare instances of this class.

var a1 = new ClassA { StringProperty = "String", IntProperty = 1 };
var a2 = new ClassA { StringProperty = "String", IntProperty = 1 };

var comparer = new Comparer<ClassA>();
var isEqual = comparer.Compare(a1, a2);

Debug.WriteLine("a1 and a2 are " + (isEqual ? "equal" : "not equal"));

a1 and a2 are equal

var a1 = new ClassA { StringProperty = "String", IntProperty = 1 };
var a2 = new ClassA { StringProperty = "String", IntProperty = 2 };
var comparer = new Comparer<ClassA>();

IEnumerable<Difference> differenses;
var isEqual = comparer.Compare(a1, a2, out differenses);
var differensesList = differenses.ToList();

Debug.WriteLine("a1 and a2 are " + (isEqual ? "equal" : "not equal"));

if (!isEqual)
{
    Debug.WriteLine("Differences:");
    Debug.WriteLine(string.Join(Environment.NewLine, differensesList));
}

a1 and a2 are not equal

Differences:

Difference: MemberPath='IntProperty', Value1='1', Value2='2'

Overriding comparison rules

To override comparison rules we need to create custom value comparer. This class should be inherited from AbstractValueComparer<T> or should implement IValueComparer<T>.

public class MyComparer: AbstractValueComparer<T>
{
    public override bool Compare(string obj1, string obj2, ComparisonSettings settings)
    {
        return obj1 == obj2; //Implement comparison logic here
    }
}

Type comparison rule override.

comparer.AddComparerOverride<string>(new MyComparer());

Field comparison rule override.

comparer.AddComparerOverride(() => new ClassA().StringProperty, new MyComparer());

comparer.AddComparerOverride(
    () => new ClassA().StringProperty, 
    (s1, s2, parentSettings) => s1 == s2,
    s => s.ToString());

comparer.AddComparerOverride(
    () => new ClassA().StringProperty, 
    (s1, s2, parentSettings) => s1 == s2);

Comparison Settings

Comparer has an optional settings parameter to configure comparison.

RecursiveComparison.

True by default. If true, all members which are not primitive types, do not have custom comparison rule and do not implement ICompareble will be compared as separate objects using the same rules as current objects.

EmptyAndNullEnumerablesEqual.

False by default. If true, empty enumerables and null values will be considered as equal values.

Comparison Settings class allows to store custom values that can be used in custom comparers.

SetCustomSetting<T>(T value, string key = null)

GetCustomSetting<T>(string key = null)

Factory

Factory provides a way to encapsulate comparers creeation and configuration. Factory should implement IComparersFactory or should be inherited from ComparersFactory.

public class MyComparersFactory: ComparersFactory
{
    public override IComparer<T> GetObjectsComparer<T>(ComparisonSettings settings = null, IBaseComparer parentComparer = null)
    {
        if (typeof(T) == typeof(ClassA))
        {
            var comparer = new Comparer<ClassA>(settings, parentComparer, this);
            comparer.AddComparerOverride<Guid>(new MyCustomGuidComparer());

            return (IComparer<T>)comparer;
        }

        return base.GetObjectsComparer<T>(settings, parentComparer);
    }
}

Non-generic comparer

var comparer = new Comparer();

var isEqual = comparer.Compare(a1, a2);

This comparer creates generic implementation of comparer for each comparison. Generic comparer works faster.

Useful Value Comparers

Framework contains several custom comparers that can be useful.

DoNotCompareValueComparer.

Allows to to skip some fields/types. Has singleton implementation (DoNotCompareValueComparer.Instance).

DynamicValueComparer<T>.

Receives comparison rule as a function.

NulableStringsValueComparer.

Null and empty strings considered as equal values.

Examples

There are some examples how Objects Comparer can be used.

NUnit is used for developing unit tests to show how examples work.

Example 1: Expected Message

Challenge: Check if received message equal to the expected message.

public class Error
{
    public int Id { get; set; }

    public string Messgae { get; set; }
}

public class Message
{
    public string Id { get; set; }

    public DateTime DateCreated { get; set; }

    public int MessageType { get; set; }

    public int Status { get; set; }

    public List<Error> Errors { get; set; }

    public override string ToString()
    {
        return $"Id:{Id}, Date:{DateCreated}, Type:{MessageType}, Status:{Status}";
    }
}

[TestFixture]
public class Example1Tests
{
    private IComparer<Message> _comparer;

    [SetUp]
    public void SetUp()
    {
        _comparer = new Comparer<Message>(
            new ComparisonSettings
            {
                //Null and empty error lists are equal
                EmptyAndNullEnumerablesEqual = true
            });

        //Do not compare DateCreated
        _comparer.AddComparerOverride<DateTime>(DoNotCompareValueComparer.Instance);
        //Do not compare Id
        _comparer.AddComparerOverride(() => new Message().Id, DoNotCompareValueComparer.Instance);
        //Do not compare Message Text
        _comparer.AddComparerOverride(() => new Error().Messgae, DoNotCompareValueComparer.Instance);
    }

    [Test]
    public void EqualMessagesWithoutErrorsTest()
    {
        var expectedMessage = new Message
        {
            MessageType = 1,
            Status = 0,
        };

        var actualMessage = new Message
        {
            Id = "M12345",
            DateCreated = DateTime.Now,
            MessageType = 1,
            Status = 0,
        };

        var isEqual = _comparer.Compare(expectedMessage, actualMessage);

        Assert.IsTrue(isEqual);
    }

    [Test]
    public void EqualMessagesWithErrorsTest()
    {
        var expectedMessage = new Message
        {
            MessageType = 1,
            Status = 1,
            Errors = new List<Error>
            {
                new Error { Id = 2 },
                new Error { Id = 7 }
            }
        };

        var actualMessage = new Message
        {
            Id = "M12345",
            DateCreated = DateTime.Now,
            MessageType = 1,
            Status = 1,
            Errors = new List<Error>
            {
                new Error { Id = 2, Messgae = "Some error #2" },
                new Error { Id = 7, Messgae = "Some error #7" },
            }
        };

        var isEqual = _comparer.Compare(expectedMessage, actualMessage);

        Assert.IsTrue(isEqual);
    }
}

Example 2: Persons comparison

Challenge: Compare persons from different sources.

public class Person
{
    public Guid PersonId { get; set; }

    public string FirstName { get; set; }

    public string LastName { get; set; }

    public string MiddleName { get; set; }

    public string PhoneNumber { get; set; }

    public override string ToString()
    {
        return $"{FirstName} {MiddleName} {LastName} ({PhoneNumber})";
    }
}

Phone number can have different formats. Let’s compare only digits.

public class PhoneNumberComparer: AbstractValueComparer<string>
{
    public override bool Compare(string obj1, string obj2, ComparisonSettings settings)
    {
        return ExtractDigits(obj1) == ExtractDigits(obj2);
    }

    private string ExtractDigits(string str)
    {
        return string.Join(
            string.Empty,
            (str ?? string.Empty)
                .ToCharArray()
                .Where(char.IsDigit));
    }
}

Factory allows not to configure comparer every time we need to create it.

public class MyComparersFactory: ComparersFactory
{
    public override IComparer<T> GetObjectsComparer<T>(ComparisonSettings settings = null, IBaseComparer parentComparer = null)
    {
        if (typeof(T) == typeof(Person))
        {
            var comparer = new Comparer<Person>(settings, parentComparer, this);
            //Do not compare PersonId
            comparer.AddComparerOverride<Guid>(DoNotCompareValueComparer.Instance);
            //Sometimes MiddleName can be skipped. Compare only if property has value.
            comparer.AddComparerOverride(
                () => new Person().MiddleName,
                (s1, s2, parentSettings) => string.IsNullOrWhiteSpace(s1) || string.IsNullOrWhiteSpace(s2) || s1 == s2);
            comparer.AddComparerOverride(
                () => new Person().PhoneNumber,
                new PhoneNumberComparer());

            return (IComparer<T>)comparer;
        }

        return base.GetObjectsComparer<T>(settings, parentComparer);
    }
}

[TestFixture]
public class Example2Tests
{
    private MyComparersFactory _factory;
    private IComparer<Person> _comparer;

    [SetUp]
    public void SetUp()
    {
        _factory = new MyComparersFactory();
        _comparer = _factory.GetObjectsComparer<Person>();
    }

    [Test]
    public void EqualPersonsTest()
    {
        var person1 = new Person
        {
            PersonId = Guid.NewGuid(),
            FirstName = "John",
            LastName = "Doe",
            MiddleName = "F",
            PhoneNumber = "111-555-8888"
        };

        var person2 = new Person
        {
            PersonId = Guid.NewGuid(),
            FirstName = "John",
            LastName = "Doe",
            PhoneNumber = "(111) 555 8888"
        };

        IEnumerable<Difference> differenses;
        var isEqual = _comparer.Compare(person1, person2, out differenses);

        Assert.IsTrue(isEqual);

        Debug.WriteLine($"Persons {person1} and {person2} are equal");
    }

    [Test]
    public void DifferentPersonsTest()
    {
        var person1 = new Person
        {
            PersonId = Guid.NewGuid(),
            FirstName = "Jack",
            LastName = "Doe",
            MiddleName = "F",
            PhoneNumber = "111-555-8888"
        };

        var person2 = new Person
        {
            PersonId = Guid.NewGuid(),
            FirstName = "John",
            LastName = "Doe",
            MiddleName = "L",
            PhoneNumber = "222-555-9999"
        };

        IEnumerable<Difference> differenses;
        var isEqual = _comparer.Compare(person1, person2, out differenses);

        var differensesList = differenses.ToList();
        Assert.IsFalse(isEqual);

        Assert.AreEqual(3, differensesList.Count);
        Assert.IsTrue(differensesList.Any(d => d.MemberPath == "FirstName" && d.Value1 == "Jack" && d.Value2 == "John"));
        Assert.IsTrue(differensesList.Any(d => d.MemberPath == "MiddleName" && d.Value1 == "F" && d.Value2 == "L"));
        Assert.IsTrue(differensesList.Any(d => d.MemberPath == "PhoneNumber" && d.Value1 == "111-555-8888" && d.Value2 == "222-555-9999"));

        Debug.WriteLine($"Persons {person1} and {person2}");
        Debug.WriteLine("Differences:");
        Debug.WriteLine(string.Join(Environment.NewLine, differensesList));
    }
}

Persons John F Doe (111-555-8888) and John Doe ((111) 555 8888) are equal

Persons Jack F Doe (111-555-8888) and John L Doe (222-555-9999)

Differences:

Difference: MemberPath='FirstName', Value1='Jack', Value2='John'.

Difference: MemberPath='MiddleName', Value1='F', Value2='L'.

Difference: MemberPath='PhoneNumber', Value1='111-555-8888', Value2='222-555-9999'.