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Using Objects Comparer to Compare Complex Objects in C#

Valerii Tereshchenko

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2 May 2019

Objects Comparer framework provides mechanism to recursively compare complex objects by properties (supports arrays, lists, different types of dynamic objects and more), allows to override comparison rules for specific properties and types.

Introduction

It is quite a 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). This small framework was developed to solve such kind of problems.

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 could be used in .NET Core projects (.NET Standard 1.6, 2.0 or above) and in .NET projects (version 4.5 or above).

Supported objects:

Primitive Types
Classes and structures
Enumerables (arrays, collections, lists, dictionaries, etc.)
Multidimensional arrays
Enumerations
Flags
Dynamic objects (ExpandoObject, DynamicObject and compiler generated dynamic objects)

Installation

Objects Comparer can be installed as NuGet package.

Install-Package ObjectsComparer

Source Code

Source code can be found on GitHub.

Important Notice!!!

If you have any issues with ObjectsComparer or ideas on how it could be improved, please create an issue on GitHub.

Reasons why:

Codeproject had and probably still has issues with sending notifications about new comments/replies. I don't check comments for this article manually every day.
There are almost the same articles on CSharpCorner, DZone and Medium. Those folks don't see your comments and you don't see theirs' comments. It would be much better to have all discussions in one place.
GitHub provides much more functionality to work with issues.

Basic Examples

To show how to use Objects Comparer, let's create two classes:

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

    public int IntProperty { get; set; }

    public SubClassA SubClass { get; set; }
}

public class SubClassA
{
    public bool BoolProperty { get; set; }
}

There are some examples below on how Objects Comparer can be used to compare instances of these classes.

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

//Compare objects
IEnumerable<Difference> differences;
var isEqual = comparer.Compare(a1, a2, out differences);

//Print results
Debug.WriteLine(isEqual ? "Objects are equal" : string.Join(Environment.NewLine, differenses));

Objects are equal

In the examples below, Compare objects and Print results blocks will be skipped for brevity except some cases.

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

Difference: DifferenceType=ValueMismatch, MemberPath='IntProperty', Value1='1', Value2='2'.

var a1 = new ClassA { SubClass = new SubClassA { BoolProperty = true } };
var a2 = new ClassA { SubClass = new SubClassA { BoolProperty = false } };
var comparer = new Comparer<ClassA>();

Difference: DifferenceType=ValueMismatch, MemberPath='SubClass.BoolProperty', 
Value1='True', Value2='False'.

var a1 = new StringBuilder("abc");
var a2 = new StringBuilder("abd");
var comparer = new Comparer<StringBuilder>();

Difference: DifferenceType=ValueMismatch, MemberPath='', Value1='abc', Value2='abd'.

Enumerables (Arrays, Collections, Lists, etc.)

In this case, enumerables can have different number of elements or some elements can have different values. Enumerables can be generic or non-generic. In case of non-generic enumerables, elements with the same index will be compared if types of this elements are equal, otherwise difference with DifferenceType=TypeMismatch will be added to the list of differences.

var a1 = new[] { 1, 2, 3 };
var a2 = new[] { 1, 2, 3 };
var comparer = new Comparer<int[]>();

Objects are equal

var a1 = new[] { 1, 2 };
var a2 = new[] { 1, 2, 3 };
var comparer = new Comparer<int[]>();

Difference: DifferenceType=ValueMismatch, MemberPath='Length', Value1='2', Value2='3'.

var a1 = new[] { 1, 2, 3 };
var a2 = new[] { 1, 4, 3 };
var comparer = new Comparer<int[]>();

Difference: DifferenceType=ValueMismatch, MemberPath='[1]', Value1='2', Value2='4'.

var a1 = new ArrayList { "Str1", "Str2" };
var a2 = new ArrayList { "Str1", 5 };
var comparer = new Comparer<ArrayList>();

Difference: DifferenceType=TypeMismatch, MemberPath='[1]', Value1='Str2', Value2='5'.

Sets

var a1 = new[] { 1, 2, 3 };
var a2 = new[] { 1, 2, 3 };
var comparer = new Comparer<int[]>();

Objects are equal

var a1 = new HashSet<int> { 1, 2, 3 };
var a2 = new HashSet<int> { 2, 1, 4 };
var comparer = new Comparer<HashSet<int>>();

Difference: DifferenceType=MissedElementInSecondObject, MemberPath='', 
Value1='3', Value2=''.
Difference: DifferenceType=MissedElementInFirstObject, MemberPath='', 
Value1='', Value2='4'.

Multidimensional Arrays

var a1 = new[] { new[] { 1, 2 } };
var a2 = new[] { new[] { 1, 3 } };
var comparer = new Comparer<int[][]>();

Difference: DifferenceType=ValueMismatch, MemberPath='[0][1]', Value1='2', Value2='3'.

var a1 = new[] { new[] { 1, 2 } };
var a2 = new[] { new[] { 2, 2 }, new[] { 3, 5 } };
var comparer = new Comparer<int[][]>();

Difference: DifferenceType=ValueMismatch, MemberPath='Length', Value1='1', Value2='2'.

var a1 = new[] { new[] { 1, 2 }, new[] { 3, 5 } };
var a2 = new[] { new[] { 1, 2 }, new[] { 3, 5, 6 } };
var comparer = new Comparer<int[][]>();

Difference: DifferenceType=ValueMismatch, MemberPath='[1].Length', Value1='2', Value2='3'.

var a1 = new[,] { { 1, 2 }, { 1, 3 } };
var a2 = new[,] { { 1, 3, 4 }, { 1, 3, 8 } };
var comparer = new Comparer<int[,]>();

Difference: DifferenceType=ValueMismatch, MemberPath='Dimension1', Value1='2', Value2='3'.

var a1 = new[,] { { 1, 2 } };
var a2 = new[,] { { 1, 3 } };
var comparer = new Comparer<int[,]>();

Difference: DifferenceType=ValueMismatch, MemberPath='[0,1]', Value1='2', Value2='3'.

Dynamic Objects

C# supports several types of objects, whose members can be dynamically added and removed at run time.

ExpandoObject

If you are not familiar with how to use ExpandoObject, you can read this or search for another example.

dynamic a1 = new ExpandoObject();
a1.Field1 = "A";
a1.Field2 = 5;
a1.Field4 = 4;
dynamic a2 = new ExpandoObject();
a2.Field1 = "B";
a2.Field3 = false;
a2.Field4 = "C";
var comparer = new Comparer();

Difference: DifferenceType=ValueMismatch, MemberPath='Field1', 
Value1='A', Value2='B'.

Difference: DifferenceType=MissedMemberInSecondObject, MemberPath='Field2', 
Value1='5', Value2=''.

Difference: DifferenceType=TypeMismatch, MemberPath='Field4', 
Value1='4', Value2='C'.

Difference: DifferenceType=MissedMemberInFirstObject, MemberPath='Field3', 
Value1='', Value2='False'.

dynamic a1 = new ExpandoObject();
a1.Field1 = "A";
a1.Field2 = 5;
dynamic a2 = new ExpandoObject();
a2.Field1 = "B";
a2.Field3 = false;
var comparer = new Comparer();

Difference: DifferenceType=ValueMismatch, MemberPath='Field1',

Difference: DifferenceType=MissedMemberInSecondObject, MemberPath='Field2', 
Value1='5', Value2=''.

Difference: DifferenceType=MissedMemberInFirstObject, MemberPath='Field3', 
Value1='', Value2='False'.

Behavior if member not exists could be changed by providing custom ComparisonSettings (see Comparison Settings below).

dynamic a1 = new ExpandoObject();
a1.Field1 = "A";
a1.Field2 = 0;
dynamic a2 = new ExpandoObject();
a2.Field1 = "B";
a2.Field3 = false;
a2.Field4 = "S";
var comparer = new Comparer(new ComparisonSettings { UseDefaultIfMemberNotExist = true });

Difference: DifferenceType=ValueMismatch, MemberPath='Field1', 
Value1='A', Value2='B'.

Difference: DifferenceType=ValueMismatch, MemberPath='Field4', 
Value1='', Value2='S'.

DynamicObject

DynamicObject is an abstract class and cannot be instantiated directly. Let’s assume that we have such implementation of the DynamicObject class. It is necessary to have a correct implementation of the method GetDynamicMemberNames, otherwise Objects Comparer wouldn't work in a right way.

If you are not familiar with how to use DynamicObject, you can read this or search for another example.

private class DynamicDictionary : DynamicObject
{
    public int IntProperty { get; set; }

    private readonly Dictionary<string, object> _dictionary = new Dictionary<string, object>();

    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        var name = binder.Name;

        return _dictionary.TryGetValue(name, out result);
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        _dictionary[binder.Name] = value;

        return true;
    }

    public override IEnumerable<string> GetDynamicMemberNames()
    {
        return _dictionary.Keys;
    }
}

dynamic a1 = new DynamicDictionary();
a1.Field1 = "A";
a1.Field3 = true;
dynamic a2 = new DynamicDictionary();
a2.Field1 = "B";
a2.Field2 = 8;
a2.Field3 = 1;
var comparer = new Comparer();

Difference: DifferenceType=ValueMismatch, MemberPath='Field1', 
Value1='A', Value2='B'.

Difference: DifferenceType=TypeMismatch, MemberPath='Field3', 
Value1='True', Value2='1'.

Difference: DifferenceType=MissedMemberInFirstObject, MemberPath='Field2', 
Value1='', Value2='8'.

Compiler Generated Objects

This type of dynamic objects is most popular and most easy to create.

dynamic a1 = new
{
    Field1 = "A",
    Field2 = 5,
    Field3 = true
};
dynamic a2 = new
{
    Field1 = "B",
    Field2 = 8
};
var comparer = new Comparer();

IEnumerable<Difference> differences;
var isEqual = comparer.Compare((object)a1, (object)a2, out differences);

Difference: DifferenceType=ValueMismatch, MemberPath='Field1', 
Value1='A', Value2='B'.

Difference: DifferenceType=TypeMismatch, MemberPath='Field2', 
Value1='5', Value2='8'.

Difference: DifferenceType=MissedMemberInSecondObject, MemberPath='Field3', 
Value1='True', Value2=''.

This example requires some additional explanations. Types of the objects a1 and a2 were generated by compiler and are considered as the same type if and only if objects a1 and a2 have same set of members (same name and same type). If casting to (object) is skipped in case of different set of members, RuntimeBinderException will be thrown.

Overriding Comparison Rules

Sometimes some of the members require custom comparison logic. To override comparison rule, we need to create custom value comparer or provide function how to compare objects and how to convert these objects to string(optional) and filter function(optional). Value Comparer should be inherited from AbstractValueComparer or should implement IValueComparer.

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

Override comparison rule for objects of particular type.

//Use MyComparer to compare all members of type string 
comparer.AddComparerOverride<string>(new MyValueComparer());
comparer.AddComparerOverride(typeof(string), new MyValueComparer());
//Use MyComparer to compare all members of type string except members which name starts with "Xyz"
comparer.AddComparerOverride(typeof(string), new MyValueComparer(), 
                                 member => !member.Name.StartsWith("Xyz"));
comparer.AddComparerOverride<string>(new MyValueComparer(), member => !member.Name.StartsWith("Xyz"));

Override comparison rule for particular member (Field or Property). If toStringFunction parameter is not provided, objects will be converted to string using ToString() method.

//Use MyValueComparer to compare StringProperty of ClassA
comparer.AddComparerOverride(() => new ClassA().StringProperty, new MyValueComparer());
comparer.AddComparerOverride(
    typeof(ClassA).GetTypeInfo().GetMember("StringProperty").First(),
    new MyValueComparer());
//Compare StringProperty of ClassA by length. If length equal consider that values are equal
comparer.AddComparerOverride(
    () => new ClassA().StringProperty,
    (s1, s2, parentSettings) => s1?.Length == s2?.Length,
    s => s?.ToString());
comparer.AddComparerOverride(
    () => new ClassA().StringProperty,
    (s1, s2, parentSettings) => s1?.Length == s2?.Length);

Override comparison rule for particular member(s) (Field or Property) by name.

//Use MyValueComparer to compare all members with name equal to "StringProperty"
comparer.AddComparerOverride("StringProperty", new MyValueComparer());

Overrides by type have highest priority, then overrides by member and overrides by member name have lowest priority. If more than one of value comparers of the same type (by type/by name/by member name) could be applied to the same member, exception AmbiguousComparerOverrideResolutionException will be thrown during comparison.

Example:

var a1 = new ClassA();
var a2 = new ClassA();
comparer.AddComparerOverride<string>(valueComparer1, member => member.Name.StartsWith("String"));
comparer.AddComparerOverride<string>(valueComparer2, member => member.Name.EndsWith("Property"));

var result = comparer.Compare(a1, a2);//Exception here

Comparison Settings

Comparer constructor has an optional settings parameter to configure some aspects of 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 using the same rules as root objects.

EmptyAndNullEnumerablesEqual

False by default. If true, empty enumerables (arrays, collections, lists, etc.) and null values will be considered as equal values.

UseDefaultIfMemberNotExist

If true and member does not exist, Objects Comparer will consider that this member is equal to default value of opposite member type. Applicable for dynamic types comparison only. False by default.

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 creation 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

This comparer creates generic implementation of comparer for each comparison.

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

Useful Value Comparers

Framework contains several custom comparers that can be useful in many cases.

DoNotCompareValueComparer

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

DynamicValueComparer

Receives comparison rule as a function.

NulableStringsValueComparer

Null and empty strings are considered as equal values. Has singleton implementation (NulableStringsValueComparer.Instance).

DefaultValueValueComparer

Allows to consider provided value and default value of specified type as equal values (see Example 3 below).

IgnoreCaseStringsValueComparer

Allows to compare string ignoring case. Has singleton implementation (IgnoreCaseStringsValueComparer.Instance).

UriComparer

Allows to compare Uri objects.

Examples

There are some more complex examples how Objects Comparer can be used.

Example 1: Expected Message

Challenge

Check if received message equal to the expected message.

Problems

DateCreated, DateSent and DateReceived properties need to be skipped
Auto generated Id property need to be skipped
Message property of Error class need to be skipped

Solution

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 DateTime DateSent { get; set; }

    public DateTime DateReceived { 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}, Type:{MessageType}, Status:{Status}";
    }
}

Configuring comparer.

_comparer = new Comparer<Message>(
    new ComparisonSettings
    {
        //Null and empty error lists are equal
        EmptyAndNullEnumerablesEqual = true
    });
//Do not compare Dates 
_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);

var expectedMessage = new Message
{
    MessageType = 1,
    Status = 0
};

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

IEnumerable<Difference> differences;
var isEqual = _comparer.Compare(expectedMessage, actualMessage, out differences);

Objects are equal

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,
    DateSent = DateTime.Now,
    DateReceived = 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" },
    }
};

IEnumerable<Difference> differences;
var isEqual = _comparer.Compare(expectedMessage, actualMessage, out differences);

Objects are equal

var expectedMessage = new Message
{
    MessageType = 1,
    Status = 1,
    Errors = new List<Error>
    {
        new Error { Id = 2, Messgae = "Some error #2" },
        new Error { Id = 8, Messgae = "Some error #8" }
    }
};

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

IEnumerable<Difference> differences;
var isEqual = _comparer.Compare(expectedMessage, actualMessage, out differences);

Difference: DifferenceType=ValueMismatch, MemberPath='Status', 
Value1='1', Value2='2'.

Difference: DifferenceType=ValueMismatch, MemberPath='Errors[1].Id', 
Value1='8', Value2='7'.

Example 2: Persons Comparison

Challenge

Compare persons from different sources.

Problems

PhoneNumber format can be in different. Example: "111-555-8888" and "(111) 555 8888"
MiddleName can exist in one source but does not exist in another source. It makes a sense to compare MiddleName only if it has value in both sources.
PersonId property need to be skipped

Solution

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);
    }
}

Configuring comparer.

_factory = new MyComparersFactory();
_comparer = _factory.GetObjectsComparer<Person>();

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> differences;
var isEqual = _comparer.Compare(person1, person2, out differences);

Objects are equal

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> differences;
var isEqual = _comparer.Compare(person1, person2, out differences);

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

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

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

Example 3: Comparing JSON Configuration Files

Challenge

There are files with settings with some differences that need to be found. Json.NET is used to deserialize JSON data.

Problems

URLs can be with or without http prefix.
DataCompression is Off by default
SmartMode1...3 disabled by default
ConnectionString, Email and Notifications need to be skipped
If ProcessTaskTimeout or TotalProcessTimeout settings skipped default values will be used, so if in one file setting does not exists and in another file this setting has default value, it is actually the same.

Files

Settings0

{
  "ConnectionString": "USER ID=superuser;PASSWORD=superpassword;DATA SOURCE=localhost:1111",
  "Email": {
    "Port": 25,
    "Host": "MyHost.com",
    "EmailAddress": "test@MyHost.com"
  },
  "Settings": {
    "DataCompression": "On",
    "DataSourceType": "MultiDataSource",
    "SomeUrl": "http://MyHost.com/VeryImportantData",
    "SomeOtherUrl": "http://MyHost.com/NotSoImportantData/",
    "CacheMode": "Memory",
    "MaxCacheSize": "1GB",
    "SuperModes": {
      "SmartMode1": "Enabled",
      "SmartMode2": "Disabled",
      "SmartMode3": "Enabled"
    }
  },
  "Timeouts": {
    "TotalProcessTimeout": 500,
    "ProcessTaskTimeout": 100
  },
  "BackupSettings": {
    "BackupIntervalUnit": "Day",
    "BackupInterval": 100
  },
  "Notifications": [
    {
      "Phone": "111-222-3333"
    },
    {
      "Phone": "111-222-4444"
    },
    {
      "EMail": "support@MyHost.com"
    }
  ],
  "Logging": {
    "Enabled": true,
    "Pattern": "Logs\\MyApplication.%data{yyyyMMdd}.log",
    "MaximumFileSize": "20MB",
    "Level": "ALL"
  }
}

Settings1

{
  "ConnectionString": "USER ID=admin;PASSWORD=*****;DATA SOURCE=localhost:22222",
  "Email": {
    "Port": 25,
    "Host": "MyHost.com",
    "EmailAddress": "test@MyHost.com"
  },
  "Settings": {
    "DataCompression": "On",
    "DataSourceType": "MultiDataSource",
    "SomeUrl": "MyHost.com/VeryImportantData",
    "SomeOtherUrl": "MyHost.com/NotSoImportantData/",
    "CacheMode": "Memory",
    "MaxCacheSize": "1GB",
    "SuperModes": {
      "SmartMode1": "enabled",
      "SmartMode3": "enabled"
    }
  },
  "BackupSettings": {
    "BackupIntervalUnit": "Day",
    "BackupInterval": 100
  },
  "Notifications": [
    {
      "Phone": "111-222-3333"
    },
    {
      "EMail": "support@MyHost.com"
    }
  ],
  "Logging": {
    "Enabled": true,
    "Pattern": "Logs\\MyApplication.%data{yyyyMMdd}.log",
    "MaximumFileSize": "20MB",
    "Level": "ALL"
  }
}

Settings2

{
  "ConnectionString": "USER ID=superuser;PASSWORD=superpassword;DATA SOURCE=localhost:1111",
  "Email": {
    "Port": 25,
    "Host": "MyHost.com",
    "EmailAddress": "test@MyHost.com"
  },
  "Settings": {
    "DataSourceType": "MultiDataSource",
    "SomeUrl": "http://MyHost.com/VeryImportantData",
    "SomeOtherUrl": "http://MyHost.com/NotSoImportantData/",
    "CacheMode": "Memory",
    "MaxCacheSize": "1GB",
    "SuperModes": {
      "SmartMode3": "Enabled"
    }
  },
  "Timeouts": {
    "TotalProcessTimeout": 500,
    "ProcessTaskTimeout": 200
  },
  "BackupSettings": {
    "BackupIntervalUnit": "Week",
    "BackupInterval": 2
  },
  "Notifications": [
    {
      "EMail": "support@MyHost.com"
    }
  ],
  "Logging": {
    "Enabled": false,
    "Pattern": "Logs\\MyApplication.%data{yyyyMMdd}.log",
    "MaximumFileSize": "40MB",
    "Level": "ERROR"
  }
}

Configuring comparer.

_comparer = new Comparer(new ComparisonSettings { UseDefaultIfMemberNotExist = true });
//Some fields should be ignored
_comparer.AddComparerOverride("ConnectionString", DoNotCompareValueComparer.Instance);
_comparer.AddComparerOverride("Email", DoNotCompareValueComparer.Instance);
_comparer.AddComparerOverride("Notifications", DoNotCompareValueComparer.Instance);
//Smart Modes are disabled by default. These fields are not case sensitive
var disabledByDefaultComparer = new DefaultValueValueComparer<string>
      ("Disabled", IgnoreCaseStringsValueComparer.Instance);
_comparer.AddComparerOverride("SmartMode1", disabledByDefaultComparer);
_comparer.AddComparerOverride("SmartMode2", disabledByDefaultComparer);
_comparer.AddComparerOverride("SmartMode3", disabledByDefaultComparer);
//http prefix in URLs should be ignored
var urlComparer = new DynamicValueComparer<string>(
    (url1, url2, settings) => url1.Trim('/').Replace(@"http://", 
    string.Empty) == url2.Trim('/').Replace(@"http://", string.Empty));
_comparer.AddComparerOverride("SomeUrl", urlComparer);
_comparer.AddComparerOverride("SomeOtherUrl", urlComparer);
//DataCompression is Off by default.
_comparer.AddComparerOverride("DataCompression", 
   new DefaultValueValueComparer<string>("Off", NulableStringsValueComparer.Instance));
//ProcessTaskTimeout and TotalProcessTimeout fields have default values.
_comparer.AddComparerOverride("ProcessTaskTimeout", 
   new DefaultValueValueComparer<long>(100, DefaultValueComparer.Instance));
_comparer.AddComparerOverride("TotalProcessTimeout", 
   new DefaultValueValueComparer<long>(500, DefaultValueComparer.Instance));

var settings0Json = LoadJson("Settings0.json");
var settings0 = JsonConvert.DeserializeObject<ExpandoObject>(settings0Json);
var settings1Json = LoadJson("Settings1.json");
var settings1 = JsonConvert.DeserializeObject<ExpandoObject>(settings1Json);

IEnumerable<Difference> differences;
var isEqual = _comparer.Compare(settings0, settings1, out differences);

Objects are equal

var settings0Json = LoadJson("Settings0.json");
var settings0 = JsonConvert.DeserializeObject<ExpandoObject>(settings0Json);
var settings2Json = LoadJson("Settings2.json");
var settings2 = JsonConvert.DeserializeObject<ExpandoObject>(settings2Json);

IEnumerable<Difference> differences;
var isEqual = _comparer.Compare(settings0, settings2, out differences);

Difference: DifferenceType=ValueMismatch, MemberPath='Settings.DataCompression', 
Value1='On', Value2='Off'.

Difference: DifferenceType=ValueMismatch, MemberPath='Settings.SuperModes.SmartMode1', 
Value1='Enabled', Value2='Disabled'.

Difference: DifferenceType=ValueMismatch, MemberPath='Timeouts.ProcessTaskTimeout', 
Value1='100', Value2='200'.

Difference: DifferenceType=ValueMismatch, MemberPath='BackupSettings.BackupIntervalUnit', 
Value1='Day', Value2='Week'.

Difference: DifferenceType=ValueMismatch, MemberPath='BackupSettings.BackupInterval', 
Value1='100', Value2='2'.

Difference: DifferenceType=ValueMismatch, MemberPath='Logging.Enabled', 
Value1='True', Value2='False'.

Difference: DifferenceType=ValueMismatch, MemberPath='Logging.MaximumFileSize', 
Value1='20MB', Value2='40MB'.

Difference: DifferenceType=ValueMismatch, MemberPath='Logging.Level', 
Value1='ALL', Value2='ERROR'.

Example 4: Custom Comparer for List

Challenge

This example came from one of the framework users, so it's 100% real life scenario. Compare list of items by content even if counts of items in the lists are different. Use Id property as an identifier.

Problems

By default, if counts of items in the lists are different Comparers, consider these lists as different and don't compare items.

Solution

Solution for this problem is to implement custom Comparer (it should be inherited from AbstractComparer<TypeOfTheList>) and implement comparison logic of comparing lists inside this Comparer. Then implement ComparersFactory (like in Example 2) to return custom Comparer if type equal to IList and use this factory.

Classes to compare.

public class FormulaItem
{
    public long Id { get; set; }
    public int Delay { get; set; }
    public string Name { get; set; }
    public string Instruction { get; set; }
}

public class Formula
{
    public long Id { get; set; }
    public string Name { get; set; }
    public IList<FormulaItem> Items { get; set; }
}

Custom Comparer implementation:

public class CustomFormulaItemsComparer: AbstractComparer<IList<FormulaItem>>
{
    public CustomFormulaItemsComparer(ComparisonSettings settings, 
    BaseComparer parentComparer, IComparersFactory factory) : base(settings, parentComparer, factory)
    {
    }

    public override IEnumerable<Difference> CalculateDifferences
                (IList<FormulaItem> obj1, IList<FormulaItem> obj2)
    {
        if (obj1 == null && obj2 == null)
        {
            yield break;
        }

        if (obj1 == null || obj2 == null)
        {
            yield return new Difference("", DefaultValueComparer.ToString(obj1) , 
                                        DefaultValueComparer.ToString(obj2));
            yield break;
        }

        if (obj1.Count != obj2.Count)
        {
            yield return new Difference("Count", obj1.Count.ToString(), obj2.Count.ToString(),
                    DifferenceTypes.NumberOfElementsMismatch);
        }

        foreach (var formulaItem in obj1)
        {
            var formulaItem2 = obj2.FirstOrDefault(fi => fi.Id == formulaItem.Id);

            if (formulaItem2 != null)
            {
                var comparer = Factory.GetObjectsComparer<FormulaItem>();

                foreach (var difference in comparer.CalculateDifferences(formulaItem, formulaItem2))
                {
                    yield return difference.InsertPath($"[Id={formulaItem.Id}]");
                }
            }
        }
    }
}

Factory:

public class MyComparersFactory : ComparersFactory
{
    public override IComparer<T> GetObjectsComparer<T>(ComparisonSettings settings = null,
        BaseComparer parentComparer = null)
    {
        if (typeof(T) != typeof(IList<FormulaItem>))
        {
            return base.GetObjectsComparer<T>(settings, parentComparer);
        }

        var comparer = new CustomFormulaItemsComparer(settings, parentComparer, this);

        return (IComparer<T>) comparer;

    }
}

Configuring Comparer:

_factory = new MyComparersFactory();
_comparer = _factory.GetObjectsComparer<Formula>();

var formula1 = new Formula
{
    Id = 1,
    Name = "Formula 1",
    Items = new List<FormulaItem>
    {
        new FormulaItem
        {
            Id = 1,
            Delay = 60,
            Name = "Item 1",
            Instruction = "Instruction 1"
        }
    }
};

var formula2 = new Formula
{
    Id = 1,
    Name = "Formula 1",
    Items = new List<FormulaItem>
    {
        new FormulaItem
        {
            Id = 1,
            Delay = 80,
            Name = "Item One",
            Instruction = "Instruction One"
        }
    }
};

var isEqual = _comparer.Compare(formula1, formula2, out var differences);

Difference: DifferenceType=ValueMismatch, MemberPath='Items[Id=1].Delay', 
Value1='60', Value2='80'. 
Difference: DifferenceType=ValueMismatch, MemberPath='Items[Id=1].Name', 
Value1='Item 1', Value2='Item One'. 
Difference: DifferenceType=ValueMismatch, MemberPath='Items[Id=1].Instruction', 
Value1='Instruction 1', Value2='Instruction One'.

var formula1 = new Formula
{
    Id = 1,
    Name = "Formula 1",
    Items = new List<FormulaItem>
    {
        new FormulaItem
        {
            Id = 1,
            Delay = 60,
            Name = "Item 1",
            Instruction = "Instruction 1"
        }
    }
};

var formula2 = new Formula
{
    Id = 1,
    Name = "Formula 1",
    Items = new List<FormulaItem>
    {
        new FormulaItem
        {
            Id = 1,
            Delay = 80,
            Name = "Item One",
            Instruction = "Instruction One"
        },
        new FormulaItem
        {
            Id = 2,
            Delay = 30,
            Name = "Item Two",
            Instruction = "Instruction Two"
        }
    }
};

var isEqual = _comparer.Compare(formula1, formula2, out var differences);

Difference: DifferenceType=NumberOfElementsMismatch, MemberPath='Items.Count', 
Value1='1', Value2='2'. Difference: DifferenceType=ValueMismatch, 
MemberPath='Items[Id=1].Delay', Value1='60', Value2='80'. 
Difference: DifferenceType=ValueMismatch, MemberPath='Items[Id=1].Name', 
Value1='Item 1', Value2='Item One'. 
Difference: DifferenceType=ValueMismatch, MemberPath='Items[Id=1].Instruction', 
Value1='Instruction 1', Value2='Instruction One'.

That's it. Enjoy!

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