## C#/.NET Little Wonders: The SequenceEqual() Method

*Once again, in this series of posts I look at the parts of the .NET Framework that may seem trivial, but can help improve your code by making it easier to write and maintain. The index of all my past little wonders posts can be found here.*

First of all, thanks for all the well-wishers for me and my family, things are starting to settle down a bit, so I hope to be able to continue to blog at least on a bi-weekly basis – and I’m hoping to sprinkle in some C++ blog entries as well.

But for today, we’re going to look at a handy extension method defined in the **Enumerable** class that let’s us check two sequences for equality.

### Introduction - Comparing Sequences

There are times, we want to check to sequences to see if they are equal. The basic definition of two equal sequences is if they have:

- The same number of elements, and
- The same values at each position in both sequences.

That is, two sequences are equal if the count of items are the same and the first elements have the same value, the second elements have the same value, etc.

So, for example, if we were comparing two arrays to see if they were equal, we could do *something* like this:

1: public static bool EqualArrays<T>(T[] first, T[] second)

` 2: {`

3: // first check to make sure length same

4: if (first.Length != second.Length)

` 5: {`

6: return false;

` 7: }`

` 8: `

9: // then examine item by item, stopping at the point not equal

10: for (int i = 0; i < first.Length; i++)

` 11: {`

12: if (!Equals(first[i], second[i]))

` 13: {`

14: return false;

` 15: }`

` 16: }`

` 17: `

18: return true;

` 19: }`

But that only works for arrays, what if we wanted to make it more generic to cover any **IEnumerable<T>** sequences? Well, we can use the **GetEnumerator()** and modify the method to take **IEnumerable<T>** instance instead, like:

1: public static bool EqualEnumerables<T>(IEnumerable<T> first, IEnumerable<T> second)

` 2: {`

3: // get enumerators

4: using (var firstPos = first.GetEnumerator())

5: using (var secondPos = second.GetEnumerator())

` 6: {`

7: // move to next item

` 8: var hasFirst = firstPos.MoveNext();`

` 9: var hasSecond = secondPos.MoveNext();`

` 10: `

11: // as long as both have an item

12: while (hasFirst && hasSecond)

` 13: {`

14: // compare em

15: if(!Equals(firstPos.Current, secondPos.Current))

` 16: {`

17: return false;

` 18: }`

` 19: `

20: // advance to next items

` 21: hasFirst = firstPos.MoveNext();`

` 22: hasSecond = secondPos.MoveNext();`

` 23: }`

` 24: `

25: // if we are done, at least one sequence has run out, so they are equal

26: // if both are empty

27: return !hasFirst && !hasSecond;

` 28: }`

` 29: }`

Fortunately, you don’t have to do any of this work, LINQ has already done this for you in an extension method called **SequenceEqual()**.

### SequenceEqual() – Compare two sequences for value equality

The **Enumerable.SequenceEqual()** extension method is a very general algorithm that compares any two sequences to make sure they have the same count of items, and the same value in each corresponding position.

It can determine the count much like my second code example above, where it will iterate through the sequences together, and the moment one of them runs out of elements it can simply make sure the other has no elements as well.

Now, on equality of the values, it will use the default comparer unless you specify an alternate comparer. Thus the two forms of **SequenceEqual()** extension method are as follows:

**first.SequenceEqual(second)***Compares the first sequence to the second using the***EqualityComparer<T>.Default**comparer for the their type.

**first.SequenceEqual(second, comparer)***Compares the first sequence to the second using the specified***IEqualityComparer<T>**comparer.

Since both of these are based solely on **IEnumerable<T>**, this means we can compare any two sequences of values as long as the type of value in the sequence is the same. That is, we can use this method to compare an **int[]** to a **List<int> **if we wish.

1: var evens = new int[] { 2, 4, 6, 8, 10 };

2: var odds = new List<int> { 1, 3, 5, 7, 9 };

` 3: `

4: // false, not same values

5: Console.WriteLine("Evens and odds are same: " +

` 6: evens.SequenceEqual(odds));`

` 7: `

` 8: var evensAsList = evens.ToList();`

` 9: `

10: // true, different containers, but same values

11: Console.WriteLine("Evens array and evens list are same: " +

` 12: evens.SequenceEqual(evensAsList));`

` 13: `

` 14: var evensBelowSeven = evens.TakeWhile(e => e < 7);`

` 15: `

16: // false, same starting values but one sequence longer

17: Console.WriteLine("Evens array and evens < 7 are same: " +

` 18: evens.SequenceEqual(evensBelowSeven));`

Keep in mind with these methods that the order of the elements is the order the elements are iterated over, which may not be the original order added, based on whether the container is ordered, un-ordered, sorted, etc.

1: var orderedNumbers = new int[] { 1, 200, 13, 57, 132 };

2: var sortedNumbers = new SortedSet<int> { 1, 200, 13, 57, 132 };

` 3: `

4: // false, sortedNumbers will actually iterate as { 1, 13, 57, 132, 200 }

5: Console.WriteLine("Ordered same as sorted: " +

` 6: orderedNumbers.SequenceEqual(sortedNumbers));`

These methods can come in handy for comparing two sequences to make sure the number and order of the items is correct, which can be handy for a variety of needs.

### Unit Testing - Using SequenceEqual() to test expected results

So what if you are wanting to verify a method returns a particular sequence of values in a unit test? For example, say we have a utility method which is a generator (returns a theoretically infinite sequence) of Fibonacci numbers (yes, this has overflow/safety concerns, but hey! It’s fun!):

1: // A utility class that holds math utility functions.

2: public static class MathUtility

` 3: {`

4: // This method returns the fibonacci sequence: 0, 1, 1, 2, 3, 5, 8, ...

5: public static IEnumerable<int> GetFibonacciSequence()

` 6: {`

7: int first = 0;

8: int second = 1;

` 9: `

10: // first and second result are 0 and 1.

11: yield return first;

12: yield return second;

` 13: `

14: // this enumerable sequence is bounded by the caller.

15: while(true)

` 16: {`

17: // each subsequent number sum of previous two

18: int current = first + second;

19: yield return current;

` 20: `

21: // wind up for next number if we're requesting one

` 22: first = second;`

` 23: second = current;`

` 24: }`

` 25: }`

` 26: }`

Remember that **yield return** creates an iterator which only returns the next value when it’s called for. Thus while the above looks like an infinite loop, in reality it depends how it’s being called. If we use a method to pull it such as **Take(10), **the method above will only yield 10 numbers and then halt. This is part of the magic of iterators and deferred execution!

But I digress, we can now use **SequenceEqual()** to easily check to see whether we get the results we expect for the first 10 values:

` 1: [TestClass]`

2: public class FibonacciTest

` 3: {`

` 4: [TestMethod]`

5: public void TestFirstTen()

` 6: {`

` 7: var results = MathUtility.GetFibonacciSequence();`

` 8: `

` 9: Assert.IsTrue(results.Take(10).SequenceEqual(`

10: new[] { 0, 1, 1, 2, 3, 5, 8, 13, 21, 34 }));

` 11: }`

` 12: }`

And if we compile and run this test class in a unit test project, we will indeed see the Fibonacci method is indeed working as we expected!

So aside from everyday uses, the **SequenceEqual()** method is also very handy for checking expected results in unit tests as well.

### Summary

LINQ adds many wonderful extension methods that can be called off of any implementation of **IEnumerable<T>**, one of these is the **SequenceEqual()** method that checks two sequences to see if they contain the same number of values, and the same value in each position of the sequence.

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Print | posted on Thursday, February 16, 2012 7:14 PM |