C# implementation of deep/recursive object comparison in .net 3.5

Question

I am looking for a C# specific , open source (or source code available) implementation of recursive, or deep, object comparison.

I currently have two graphs of live

Answer 1

Here's an NUnit 2.4.6 custom constraint we use for comparing complex graphs. It supports embedded collections, parent references, setting tolerance for numeric comparisons, identifying field names to ignore (even deep within the hierarchy), and decorating types to be always ignored.

I'm sure this code can be adapted to be used outside NUnit, the bulk of the code isn't dependent on NUnit.

We use this in thousands of unit tests.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Reflection;
using System.Text;
using NUnit.Framework;
using NUnit.Framework.Constraints;

namespace Tests
{
    public class ContentsEqualConstraint : Constraint
    {
        private readonly object expected;
        private Constraint failedEquality;
        private string expectedDescription;
        private string actualDescription;

        private readonly Stack<string> typePath = new Stack<string>();
        private string typePathExpanded;

        private readonly HashSet<string> _ignoredNames = new HashSet<string>();
        private readonly HashSet<Type> _ignoredTypes = new HashSet<Type>();
        private readonly LinkedList<Type> _ignoredInterfaces = new LinkedList<Type>();
        private readonly LinkedList<string> _ignoredSuffixes = new LinkedList<string>();
        private readonly IDictionary<Type, Func<object, object, bool>> _predicates = new Dictionary<Type, Func<object, object, bool>>();

        private bool _withoutSort;
        private int _maxRecursion = int.MaxValue;

        private readonly HashSet<VisitedComparison> _visitedObjects = new HashSet<VisitedComparison>();

        private static readonly HashSet<string> _globallyIgnoredNames = new HashSet<string>();
        private static readonly HashSet<Type> _globallyIgnoredTypes = new HashSet<Type>();
        private static readonly LinkedList<Type> _globallyIgnoredInterfaces = new LinkedList<Type>();

        private static object _regionalTolerance;

        public ContentsEqualConstraint(object expectedValue)
        {
            expected = expectedValue;
        }

        public ContentsEqualConstraint Comparing<T>(Func<T, T, bool> predicate)
        {
            Type t = typeof (T);

            if (predicate == null)
            {
                _predicates.Remove(t);
            }
            else
            {
                _predicates[t] = (x, y) => predicate((T) x, (T) y);
            }
            return this;
        }

        public ContentsEqualConstraint Ignoring(string fieldName)
        {
            _ignoredNames.Add(fieldName);
            return this;
        }

        public ContentsEqualConstraint Ignoring(Type fieldType)
        {
            if (fieldType.IsInterface)
            {
                _ignoredInterfaces.AddFirst(fieldType);
            }
            else
            {
                _ignoredTypes.Add(fieldType);
            }
            return this;
        }

        public ContentsEqualConstraint IgnoringSuffix(string suffix)
        {
            if (string.IsNullOrEmpty(suffix))
            {
                throw new ArgumentNullException("suffix");
            }
            _ignoredSuffixes.AddLast(suffix);
            return this;
        }

        public ContentsEqualConstraint WithoutSort()
        {
            _withoutSort = true;
            return this;
        }

        public ContentsEqualConstraint RecursingOnly(int levels)
        {
            _maxRecursion = levels;
            return this;
        }

        public static void GlobalIgnore(string fieldName)
        {
            _globallyIgnoredNames.Add(fieldName);
        }

        public static void GlobalIgnore(Type fieldType)
        {
            if (fieldType.IsInterface)
            {
                _globallyIgnoredInterfaces.AddFirst(fieldType);
            }
            else
            {
                _globallyIgnoredTypes.Add(fieldType);
            }
        }

        public static IDisposable RegionalIgnore(string fieldName)
        {
            return new RegionalIgnoreTracker(fieldName);
        }

        public static IDisposable RegionalIgnore(Type fieldType)
        {
            return new RegionalIgnoreTracker(fieldType);
        }

        public static IDisposable RegionalWithin(object tolerance)
        {
            return new RegionalWithinTracker(tolerance);
        }

        public override bool Matches(object actualValue)
        {
            typePathExpanded = null;
            actual = actualValue;
            return Matches(expected, actualValue);
        }

        private bool Matches(object expectedValue, object actualValue)
        {

            bool matches = true;

            if (!MatchesNull(expectedValue, actualValue, ref matches))
            {
                return matches;
            }
            // DatesEqualConstraint supports tolerance in dates but works as equal constraint for everything else
            Constraint eq = new DatesEqualConstraint(expectedValue).Within(tolerance ?? _regionalTolerance);
            if (eq.Matches(actualValue))
            {
                return true;
            }

            if (MatchesVisited(expectedValue, actualValue, ref matches))
            {
                if (MatchesDictionary(expectedValue, actualValue, ref matches) &&
                    MatchesList(expectedValue, actualValue, ref matches) &&
                    MatchesType(expectedValue, actualValue, ref matches) &&
                    MatchesPredicate(expectedValue, actualValue, ref matches))
                {
                    MatchesFields(expectedValue, actualValue, eq, ref matches);
                }
            }

            return matches;
        }

        private bool MatchesNull(object expectedValue, object actualValue, ref bool matches)
        {
            if (IsNullEquivalent(expectedValue))
            {
                expectedValue = null;
            }

            if (IsNullEquivalent(actualValue))
            {
                actualValue = null;
            }

            if (expectedValue == null && actualValue == null)
            {
                matches = true;
                return false;
            }

            if (expectedValue == null)
            {
                expectedDescription = "null";
                actualDescription = "NOT null";
                matches = Failure;
                return false;
            }

            if (actualValue == null)
            {
                expectedDescription = "not null";
                actualDescription = "null";
                matches = Failure;
                return false;
            }

            return true;
        }

        private bool MatchesType(object expectedValue, object actualValue, ref bool matches)
        {
            Type expectedType = expectedValue.GetType();
            Type actualType = actualValue.GetType();

            if (expectedType != actualType)
            {
                try
                {
                    Convert.ChangeType(actualValue, expectedType);
                }
                catch(InvalidCastException)             
                {
                    expectedDescription = expectedType.FullName;
                    actualDescription = actualType.FullName;
                    matches = Failure;
                    return false;
                }

            }
            return true;
        }

        private bool MatchesPredicate(object expectedValue, object actualValue, ref bool matches)
        {
            Type t = expectedValue.GetType();
            Func<object, object, bool> predicate;

            if (_predicates.TryGetValue(t, out predicate))
            {
                matches = predicate(expectedValue, actualValue);
                return false;
            }
            return true;
        }

        private bool MatchesVisited(object expectedValue, object actualValue, ref bool matches)
        {
            var c = new VisitedComparison(expectedValue, actualValue);

            if (_visitedObjects.Contains(c))
            {
                matches = true;
                return false;
            }

            _visitedObjects.Add(c);

            return true;
        }

        private bool MatchesDictionary(object expectedValue, object actualValue, ref bool matches)
        {
            if (expectedValue is IDictionary && actualValue is IDictionary)
            {
                var expectedDictionary = (IDictionary)expectedValue;
                var actualDictionary = (IDictionary)actualValue;

                if (expectedDictionary.Count != actualDictionary.Count)
                {
                    expectedDescription = expectedDictionary.Count + " item dictionary";
                    actualDescription = actualDictionary.Count + " item dictionary";
                    matches = Failure;
                    return false;
                }

                foreach (DictionaryEntry expectedEntry in expectedDictionary)
                {
                    if (!actualDictionary.Contains(expectedEntry.Key))
                    {
                        expectedDescription = expectedEntry.Key + " exists";
                        actualDescription = expectedEntry.Key + " does not exist";
                        matches = Failure;
                        return false;
                    }
                    if (CanRecurseFurther)
                    {
                        typePath.Push(expectedEntry.Key.ToString());
                        if (!Matches(expectedEntry.Value, actualDictionary[expectedEntry.Key]))
                        {
                            matches = Failure;
                            return false;
                        }
                        typePath.Pop();
                    }
                }
                matches = true;
                return false;
            }
            return true;
        }

        private bool MatchesList(object expectedValue, object actualValue, ref bool matches)
        {
            if (!(expectedValue is IList && actualValue is IList))
            {
                return true;
            }

            var expectedList = (IList) expectedValue;
            var actualList = (IList) actualValue;

            if (!Matches(expectedList.Count, actualList.Count))
            {
                matches = false;
            }
            else
            {
                if (CanRecurseFurther)
                {
                    int max = expectedList.Count;

                    if (max != 0 && !_withoutSort)
                    {
                        SafeSort(expectedList);
                        SafeSort(actualList);
                    }

                    for (int i = 0; i < max; i++)
                    {
                        typePath.Push(i.ToString());

                        if (!Matches(expectedList[i], actualList[i]))
                        {
                            matches = false;
                            return false;
                        }
                        typePath.Pop();
                    }
                }
                matches = true;
            }
            return false;
        }

        private void MatchesFields(object expectedValue, object actualValue, Constraint equalConstraint, ref bool matches)
        {
            Type expectedType = expectedValue.GetType();

            FieldInfo[] fields = expectedType.GetFields(BindingFlags.Instance | BindingFlags.NonPublic);

            // should have passed the EqualConstraint check
            if (expectedType.IsPrimitive ||
                expectedType == typeof(string) ||
                expectedType == typeof(Guid) ||
                fields.Length == 0)
            {
                failedEquality = equalConstraint;
                matches = Failure;
                return;
            }

            if (expectedType == typeof(DateTime))
            {
                var expectedDate = (DateTime)expectedValue;
                var actualDate = (DateTime)actualValue;

                if (Math.Abs((expectedDate - actualDate).TotalSeconds) > 3.0)
                {
                    failedEquality = equalConstraint;
                    matches = Failure;
                    return;
                }
                matches = true;
                return;
            }

            if (CanRecurseFurther)
            {
                while(true)
                {
                    foreach (FieldInfo field in fields)
                    {
                        if (!Ignore(field))
                        {
                            typePath.Push(field.Name);
                            if (!Matches(GetValue(field, expectedValue), GetValue(field, actualValue)))
                            {
                                matches = Failure;
                                return;
                            }
                            typePath.Pop();
                        }
                    }
                    expectedType = expectedType.BaseType;
                    if (expectedType == null)
                    {
                        break;
                    }
                    fields = expectedType.GetFields(BindingFlags.Instance | BindingFlags.NonPublic);
                }
            }
            matches = true;
            return;
        }

        private bool Ignore(FieldInfo field)
        {
            if (_ignoredNames.Contains(field.Name) ||
                _ignoredTypes.Contains(field.FieldType) ||
                _globallyIgnoredNames.Contains(field.Name) ||
                _globallyIgnoredTypes.Contains(field.FieldType) ||
                field.GetCustomAttributes(typeof (IgnoreContentsAttribute), false).Length != 0)
            {
                return true;
            }

            foreach(string ignoreSuffix in _ignoredSuffixes)
            {
                if (field.Name.EndsWith(ignoreSuffix))
                {
                    return true;
                }
            }

            foreach (Type ignoredInterface in _ignoredInterfaces)
            {
                if (ignoredInterface.IsAssignableFrom(field.FieldType))
                {
                    return true;
                }
            }
            return false;
        }

        private static bool Failure
        {
            get
            {
                return false;
            }
        }

        private static bool IsNullEquivalent(object value)
        {
            return value == null ||
                    value == DBNull.Value ||
                   (value is int && (int) value == int.MinValue) ||
                   (value is double && (double) value == double.MinValue) ||
                   (value is DateTime && (DateTime) value == DateTime.MinValue) ||
                   (value is Guid && (Guid) value == Guid.Empty) ||
                   (value is IList && ((IList)value).Count == 0);
        }

        private static object GetValue(FieldInfo field, object source)
        {
            try
            {
                return field.GetValue(source);
            }
            catch(Exception ex)
            {
                return ex;
            }
        }

        public override void WriteMessageTo(MessageWriter writer)
        {
            if (TypePath.Length != 0)
            {
                writer.WriteLine("Failure on " + TypePath);
            }

            if (failedEquality != null)
            {
                failedEquality.WriteMessageTo(writer);
            }
            else
            {
                base.WriteMessageTo(writer);
            }
        }
        public override void WriteDescriptionTo(MessageWriter writer)
        {
            writer.Write(expectedDescription);
        }

        public override void WriteActualValueTo(MessageWriter writer)
        {
            writer.Write(actualDescription);
        }

        private string TypePath
        {
            get
            {
                if (typePathExpanded == null)
                {
                    string[] p = typePath.ToArray();
                    Array.Reverse(p);
                    var text = new StringBuilder(128);
                    bool isFirst = true;
                    foreach(string part in p)
                    {
                        if (isFirst)
                        {
                            text.Append(part);
                            isFirst = false;
                        }
                        else
                        {
                            int i;
                            if (int.TryParse(part, out i))
                            {
                                text.Append("[" + part + "]");
                            }
                            else
                            {
                                text.Append("." + part);
                            }
                        }
                    }
                    typePathExpanded = text.ToString();
                }
                return typePathExpanded;
            }
        }

        private bool CanRecurseFurther
        {
            get
            {
                return typePath.Count < _maxRecursion;
            }
        }

        private static bool SafeSort(IList list)
        {
            if (list == null)
            {
                return false;
            }

            if (list.Count < 2)
            {
                return true;
            }

            try
            {
                object first = FirstNonNull(list) as IComparable;
                if (first == null)
                {
                    return false;
                }

                if (list is Array)
                {
                    Array.Sort((Array)list);
                    return true;
                }
                return CallIfExists(list, "Sort");
            }
            catch
            {
                return false;
            }
        }

        private static object FirstNonNull(IEnumerable enumerable)
        {
            if (enumerable == null)
            {
                throw new ArgumentNullException("enumerable");
            }
            foreach (object item in enumerable)
            {
                if (item != null)
                {
                    return item;
                }
            }
            return null;
        }

        private static bool CallIfExists(object instance, string method)
        {
            if (instance == null)
            {
                throw new ArgumentNullException("instance");
            }
            if (String.IsNullOrEmpty(method))
            {
                throw new ArgumentNullException("method");
            }
            Type target = instance.GetType();
            MethodInfo m = target.GetMethod(method, new Type[0]);
            if (m != null)
            {
                m.Invoke(instance, null);
                return true;
            }
            return false;
        }

        #region VisitedComparison Helper

        private class VisitedComparison
        {
            private readonly object _expected;
            private readonly object _actual;

            public VisitedComparison(object expected, object actual)
            {
                _expected = expected;
                _actual = actual;
            }

            public override int GetHashCode()
            {
                return GetHashCode(_expected) ^ GetHashCode(_actual);
            }

            private static int GetHashCode(object o)
            {
                if (o == null)
                {
                    return 0;
                }
                return o.GetHashCode();
            }

            public override bool Equals(object obj)
            {
                if (obj == null)
                {
                    return false;
                }

                if (obj.GetType() != typeof(VisitedComparison))
                {
                    return false;
                }

                var other = (VisitedComparison) obj;
                return _expected == other._expected &&
                       _actual == other._actual;
            }
        }

        #endregion

        #region RegionalIgnoreTracker Helper

        private class RegionalIgnoreTracker : IDisposable
        {
            private readonly string _fieldName;
            private readonly Type _fieldType;

            public RegionalIgnoreTracker(string fieldName)
            {
                if (!_globallyIgnoredNames.Add(fieldName))
                {
                    _globallyIgnoredNames.Add(fieldName);
                    _fieldName = fieldName;
                }
            }

            public RegionalIgnoreTracker(Type fieldType)
            {
                if (!_globallyIgnoredTypes.Add(fieldType))
                {
                    _globallyIgnoredTypes.Add(fieldType);
                    _fieldType = fieldType;
                }
            }

            public void Dispose()
            {
                if (_fieldName != null)
                {
                    _globallyIgnoredNames.Remove(_fieldName);
                }
                if (_fieldType != null)
                {
                    _globallyIgnoredTypes.Remove(_fieldType);
                }
            }
        }

        #endregion

        #region RegionalWithinTracker Helper

        private class RegionalWithinTracker : IDisposable
        {
            public RegionalWithinTracker(object tolerance)
            {
                _regionalTolerance = tolerance;
            }

            public void Dispose()
            {
                _regionalTolerance = null;
            }
        }

        #endregion

        #region IgnoreContentsAttribute

        [AttributeUsage(AttributeTargets.Field)]
        public sealed class IgnoreContentsAttribute : Attribute
        {
        }

        #endregion
    }
    public class DatesEqualConstraint : EqualConstraint
    {
        private readonly object _expected;

        public DatesEqualConstraint(object expectedValue) : base(expectedValue)
        {
            _expected = expectedValue;
        }

        public override bool Matches(object actualValue)
        {
            if (tolerance != null && tolerance is TimeSpan)
            {
                if (_expected is DateTime && actualValue is DateTime)
                {
                    var expectedDate = (DateTime) _expected;
                    var actualDate = (DateTime) actualValue;
                    var toleranceSpan = (TimeSpan) tolerance;

                    if ((actualDate - expectedDate).Duration() <= toleranceSpan)
                    {
                        return true;
                    }
                }
                tolerance = null;
            }
            return base.Matches(actualValue);
        }
    }
}

Answer 2

Here is a simple comparer that we've used with unit testing to assert that two objects have equal properties. It's a mash-up of ideas found in various articles, and handles circular references.

public static class TestHelper
{
    public static void AssertAreEqual(Object expected, Object actual, String name)
    {
        // Start a new check with an empty list of actual objects checked
        // The list of actual objects checked is used to ensure that circular references don't result in infinite recursion
        List<Object> actualObjectsChecked = new List<Object>();
        AssertAreEqual(expected, actual, name, actualObjectsChecked);
    }

    private static void AssertAreEqual(Object expected, Object actual, String name, List<Object> actualObjectsChecked)
    {
        // Just return if already checked the actual object
        if (actualObjectsChecked.Contains(actual))
        {
            return;
        }

        actualObjectsChecked.Add(actual);

        // If both expected and actual are null, they are considered equal
        if (expected == null && actual == null)
        {
            return;
        }

        if (expected == null && actual != null)
        {
            Assert.Fail(String.Format("The actual value of {0} was not null when null was expected.", name));
        }

        if (expected != null && actual == null)
        {
            Assert.Fail(String.Format("The actual value of {0} was null when an instance was expected.", name));
        }

        // Get / check type info
        // Note: GetType always returns instantiated (i.e. most derived) type
        Type expectedType = expected.GetType();
        Type actualType = actual.GetType();
        if (expectedType != actualType)
        {
            Assert.Fail(String.Format("The actual type of {0} was not the same as the expected type.", name));
        }

        // If expected is a Primitive, Value, or IEquatable type, assume Equals is sufficient to check
        // Note: Every IEquatable type should have also overridden Object.Equals
        if (expectedType.IsPrimitive || expectedType.IsValueType || expectedType.IsIEquatable())
        {
            Assert.IsTrue(expected.Equals(actual), "The actual {0} is not equal to the expected.", name);
            return;
        }

        // If expected is an IEnumerable type, assume comparing enumerated items is sufficient to check
        IEnumerable<Object> expectedEnumerable = expected as IEnumerable<Object>;
        IEnumerable<Object> actualEnumerable = actual as IEnumerable<Object>;
        if ((expectedEnumerable != null) && (actualEnumerable != null))
        {
            Int32 actualEnumerableCount = actualEnumerable.Count();
            Int32 expectedEnumerableCount = expectedEnumerable.Count();

            // Check size first
            if (actualEnumerableCount != expectedEnumerableCount)
            {
                Assert.Fail(String.Format("The actual number of enumerable items in {0} did not match the expected number.", name));
            }

            // Check items in order, assuming order is the same
            for (int i = 0; i < actualEnumerableCount; ++i)
            {
                AssertAreEqual(expectedEnumerable.ElementAt(i), actualEnumerable.ElementAt(i), String.Format("{0}[{1}]", name, i), actualObjectsChecked);
            }

            return;
        }

        // If expected is not a Primitive, Value, IEquatable, or Ienumerable type, assume comparing properties is sufficient to check
        // Iterate through properties
        foreach (PropertyInfo propertyInfo in actualType.GetProperties(BindingFlags.Instance | BindingFlags.Public))
        {
            // Skip properties that can't be read or require parameters
            if ((!propertyInfo.CanRead) || (propertyInfo.GetIndexParameters().Length != 0))
            {
                continue;
            }

            // Get properties from both
            Object actualProperty = propertyInfo.GetValue(actual, null);
            Object expectedProperty = propertyInfo.GetValue(expected, null);

            AssertAreEqual(expectedProperty, actualProperty, String.Format("{0}.{1}", name, propertyInfo.Name), actualObjectsChecked);
        }
    }

    public static Boolean IsIEquatable(this Type type)
    {
        return type.GetInterfaces().Any(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(IEquatable<>));
    }

}

Answer 3

This is a complex area; I've done some things like this at runtime, and it quickly gets messy. If possible, you might find that the simplest way to do this is to serialize the objects and compare the serialized form (perhaps xml-diff and XmlSerializer). This is complicated a little by the types not being known until runtime, but not hugely (you can always use new XmlSerializer(obj.GetType()) etc).

That would be my default approach, anyway.

Answer 4

Using the Nuget suggested by Jesse and this code I managed to compare two objects with great results.

using KellermanSoftware.CompareNetObjects;
using System;

namespace MyProgram.UnitTestHelper
{
  public class ObjectComparer
  {

    public static bool ObjectsHaveSameValues(object first, object second)
    {
      CompareLogic cl = new CompareLogic();
      ComparisonResult result = cl.Compare(first, second);

      if (!result.AreEqual)
        Console.WriteLine(result.DifferencesString);

      return result.AreEqual;
    }
  }
}

Answer 5

I found a really nice, free implementation at www.kellermansoftware.com called Compare .NET Objects which can be found here. Highly recommended.

---

Appears to have relocated to github - most recent version is available here

Answer 6

This is actually a simple process. Using reflection you can compare each field on the object.

public static Boolean ObjectMatches(Object x, Object y)
{
    if (x == null && y == null)
        return true;
    else if ((x == null && y != null) || (x != null && y == null))
        return false;       

    Type tx = x.GetType();
    Type ty = y.GetType();

    if (tx != ty)
        return false;

    foreach(FieldInfo field in tx.GetFields(BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance | BindingFlags.DeclaredOnly))
    {
        if (field.FieldType.IsValueType && (field.GetValue(x).ToString() != field.GetValue(y).ToString()))
            return false;
        else if (field.FieldType.IsClass && !ObjectMatches(field.GetValue(x), field.GetValue(y)))
            return false;               
    }

    return true;
}