Is there a built-in method to compare collections?

Question

I would like to compare the contents of a couple of collections in my Equals method. I have a Dictionary and an IList. Is there a built-in method to do this?

Edited: I want to compare two Dictionaries and two ILists, so I think what equality means is clear - if the two dictionaries contain the same keys mapped to the same values, then they're equal.

Answer 1

Enumerable.SequenceEqual

Determines whether two sequences are equal by comparing their elements by using a specified IEqualityComparer(T).

You can't directly compare the list & the dictionary, but you could compare the list of values from the Dictionary with the list

Answer 2

As others have suggested and have noted, SequenceEqual is order-sensitive. To solve that, you can sort the dictionary by key (which is unique, and thus the sort is always stable) and then use SequenceEqual. The following expression checks if two dictionaries are equal regardless of their internal order:

dictionary1.OrderBy(kvp => kvp.Key).SequenceEqual(dictionary2.OrderBy(kvp => kvp.Key))

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EDIT: As pointed out by Jeppe Stig Nielsen, some object have an IComparer<T> that is incompatible with their IEqualityComparer<T>, yielding incorrect results. When using keys with such an object, you must specify a correct IComparer<T> for those keys. For example, with string keys (which exhibit this issue), you must do the following in order to get correct results:

dictionary1.OrderBy(kvp => kvp.Key, StringComparer.Ordinal).SequenceEqual(dictionary2.OrderBy(kvp => kvp.Key, StringComparer.Ordinal))

Answer 3

In addition to the mentioned SequenceEqual, which

is true if two lists are of equal length and their corresponding elements compare equal according to a comparer

(which may be the default comparer, i.e. an overriden Equals())

it is worth mentioning that in .Net4 there is SetEquals on ISet objects, which

ignores the order of elements and any duplicate elements.

So if you want to have a list of objects, but they don't need to be in a specific order, consider that an ISet (like a HashSet) may be the right choice.

Answer 4

Take a look at the Enumerable.SequenceEqual method

var dictionary = new Dictionary<int, string>() {{1, "a"}, {2, "b"}};
var intList = new List<int> {1, 2};
var stringList = new List<string> {"a", "b"};
var test1 = dictionary.Keys.SequenceEqual(intList);
var test2 = dictionary.Values.SequenceEqual(stringList);

Answer 5

This is not directly answering your questions, but both the MS' TestTools and NUnit provide

 CollectionAssert.AreEquivalent

which does pretty much what you want.

Answer 6

.NET Lacks any powerful tools for comparing collections. I've developed a simple solution you can find at the link below:

http://robertbouillon.com/2010/04/29/comparing-collections-in-net/

This will perform an equality comparison regardless of order:

var list1 = new[] { "Bill", "Bob", "Sally" };
var list2 = new[] { "Bob", "Bill", "Sally" };
bool isequal = list1.Compare(list2).IsSame;

This will check to see if items were added / removed:

var list1 = new[] { "Billy", "Bob" };
var list2 = new[] { "Bob", "Sally" };
var diff = list1.Compare(list2);
var onlyinlist1 = diff.Removed; //Billy
var onlyinlist2 = diff.Added;   //Sally
var inbothlists = diff.Equal;   //Bob

This will see what items in the dictionary changed:

var original = new Dictionary<int, string>() { { 1, "a" }, { 2, "b" } };
var changed = new Dictionary<int, string>() { { 1, "aaa" }, { 2, "b" } };
var diff = original.Compare(changed, (x, y) => x.Value == y.Value, (x, y) => x.Value == y.Value);
foreach (var item in diff.Different)
  Console.Write("{0} changed to {1}", item.Key.Value, item.Value.Value);
//Will output: a changed to aaa

Answer 7

I didn't know about Enumerable.SequenceEqual method (you learn something every day....), but I was going to suggest using an extension method; something like this:

    public static bool IsEqual(this List<int> InternalList, List<int> ExternalList)
    {
        if (InternalList.Count != ExternalList.Count)
        {
            return false;
        }
        else
        {
            for (int i = 0; i < InternalList.Count; i++)
            {
                if (InternalList[i] != ExternalList[i])
                    return false;
            }
        }

        return true;

    }

Interestingly enough, after taking 2 seconds to read about SequenceEqual, it looks like Microsoft has built the function I described for you.

Answer 8

To compare collections you can also use LINQ. Enumerable.Intersect returns all pairs that are equal. You can comparse two dictionaries like this:

(dict1.Count == dict2.Count) && dict1.Intersect(dict2).Count() == dict1.Count

The first comparison is needed because dict2 can contain all the keys from dict1 and more.

You can also use think of variations using Enumerable.Except and Enumerable.Union that lead to similar results. But can be used to determine the exact differences between sets.

Answer 9

How about this example:

 static void Main()
{
    // Create a dictionary and add several elements to it.
    var dict = new Dictionary<string, int>();
    dict.Add("cat", 2);
    dict.Add("dog", 3);
    dict.Add("x", 4);

    // Create another dictionary.
    var dict2 = new Dictionary<string, int>();
    dict2.Add("cat", 2);
    dict2.Add("dog", 3);
    dict2.Add("x", 4);

    // Test for equality.
    bool equal = false;
    if (dict.Count == dict2.Count) // Require equal count.
    {
        equal = true;
        foreach (var pair in dict)
        {
            int value;
            if (dict2.TryGetValue(pair.Key, out value))
            {
                // Require value be equal.
                if (value != pair.Value)
                {
                    equal = false;
                    break;
                }
            }
            else
            {
                // Require key be present.
                equal = false;
                break;
            }
        }
    }
    Console.WriteLine(equal);
}

Courtesy : https://www.dotnetperls.com/dictionary-equals

Answer 10

For ordered collections (List, Array) use SequenceEqual

for HashSet use SetEquals

for Dictionary you can do:

namespace System.Collections.Generic {
  public static class ExtensionMethods {
    public static bool DictionaryEquals<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> d1, IReadOnlyDictionary<TKey, TValue> d2) {
      if (object.ReferenceEquals(d1, d2)) return true; 
      if (d2 is null || d1.Count != d2.Count) return false;
      foreach (var (d1key, d1value) in d1) {
        if (!d2.TryGetValue(d1key, out TValue d2value)) return false;
        if (!d1value.Equals(d2value)) return false;
      }
      return true;
    }
  }
}

(A more optimized solution will use sorting but that will require IComparable<TValue>)

Answer 11

public bool CompareStringLists(List<string> list1, List<string> list2)
{
    if (list1.Count != list2.Count) return false;

    foreach(string item in list1)
    {
        if (!list2.Contains(item)) return false;
    }

    return true;
}

Answer 12

No, because the framework doesn't know how to compare the contents of your lists.

Have a look at this:

http://blogs.msdn.com/abhinaba/archive/2005/10/11/479537.aspx

Answer 13

There wasn't, isn't and might not be, at least I would believe so. The reason behind is collection equality is probably an user defined behavior.

Elements in collections are not supposed to be in a particular order though they do have an ordering naturally, it's not what the comparing algorithms should rely on. Say you have two collections of:

{1, 2, 3, 4}
{4, 3, 2, 1}

Are they equal or not? You must know but I don't know what's your point of view.

Collections are conceptually unordered by default, until the algorithms provide the sorting rules. The same thing SQL server will bring to your attention is when you trying to do pagination, it requires you to provide sorting rules:

https://learn.microsoft.com/en-US/sql/t-sql/queries/select-order-by-clause-transact-sql?view=sql-server-2017

Yet another two collections:

{1, 2, 3, 4}
{1, 1, 1, 2, 2, 3, 4}

Again, are they equal or not? You tell me ..

Element repeatability of a collection plays its role in different scenarios and some collections like Dictionary<TKey, TValue> don't even allow repeated elements.

I believe these kinds of equality are application defined and the framework therefore did not provide all of the possible implementations.

Well, in general cases Enumerable.SequenceEqual is good enough but it returns false in the following case:

var a = new Dictionary<String, int> { { "2", 2 }, { "1", 1 }, };
var b = new Dictionary<String, int> { { "1", 1 }, { "2", 2 }, };
Debug.Print("{0}", a.SequenceEqual(b)); // false

I read some answers to questions like this(you may google for them) and what I would use, in general:

public static class CollectionExtensions {
    public static bool Represents<T>(this IEnumerable<T> first, IEnumerable<T> second) {
        if(object.ReferenceEquals(first, second)) {
            return true;
        }

        if(first is IOrderedEnumerable<T> && second is IOrderedEnumerable<T>) {
            return Enumerable.SequenceEqual(first, second);
        }

        if(first is ICollection<T> && second is ICollection<T>) {
            if(first.Count()!=second.Count()) {
                return false;
            }
        }

        first=first.OrderBy(x => x.GetHashCode());
        second=second.OrderBy(x => x.GetHashCode());
        return CollectionExtensions.Represents(first, second);
    }
}

That means one collection represents the other in their elements including repeated times without taking the original ordering into account. Some notes of the implementation:

• GetHashCode() is just for the ordering not for equality; I think it's enough in this case

• Count() will not really enumerates the collection and directly fall into the property implementation of ICollection<T>.Count

• If the references are equal, it's just Boris

Answer 14

I've made my own compare method. It returns common, missing, and extra values.

private static void Compare<T>(IEnumerable<T> actual, IEnumerable<T> expected, out IList<T> common, out IList<T> missing, out IList<T> extra) {
    common = new List<T>();
    missing = new List<T>();
    extra = new List<T>();

    var expected_ = new LinkedList<T>( expected );
    foreach (var item in actual) {
        if (expected_.Remove( item )) {
            common.Add( item );
        } else {
            extra.Add( item );
        }
    }
    foreach (var item in expected_) {
        missing.Add( item );
    }
}

Answer 15

Comparing dictionaries' contents:

To compare two Dictionary<K, V> objects, we can assume that the keys are unique for every value, thus if two sets of keys are equal, then the two dictionaries' contents are equal.

Dictionary<K, V> dictionaryA, dictionaryB;
bool areDictionaryContentsEqual = new HashSet<K>(dictionaryA.Keys).SetEquals(dictionaryB.Keys);

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Comparing collections' contents:

To compare two ICollection<T> objects, we need to check:

1. If they are of the same length.
2. If every T value that appears in the first collection appears an equal number of times in the second.

public static bool AreCollectionContentsEqual<T>(ICollection<T> collectionA, ICollection<T> collectionB)
    where T : notnull
{
    if (collectionA.Count != collectionB.Count)
    {
        return false;
    }
    Dictionary<T, int> countByValueDictionary = new(collectionA.Count);
    foreach(T item in collectionA)
    {
        countByValueDictionary[item] = countByValueDictionary.TryGetValue(item, out int count) 
            ? count + 1 
            : 1;
    }
    foreach (T item in collectionB)
    {
        if (!countByValueDictionary.TryGetValue(item, out int count) || count < 1)
        {
            return false;
        }
        countByValueDictionary[item] = count - 1;
    }
    return true;
}

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These solutions should be optimal since their time and memory complexities are O(n), while the solutions that use ordering/sorting have time and memory complexities greater than O(n).