Comparing two List<string> for equality

Question

Other than stepping through the elements one by one, how do I compare two lists of strings for equality (in .NET 3.0):

This fails:

        // Expected result.
        List<string> expected = new List<string>();
        expected.Add( "a" );
        expected.Add( "b" );
        expected.Add( "c" );

        // Actual result
        actual = new List<string>();
        actual.Add( "a" );
        actual.Add( "b" );
        actual.Add( "c" );

        // Verdict
        Assert.IsTrue( actual == expected );

Thanks in advance

Answer 1

Try the following

var equal = expected.SequenceEquals(actual);

Test Version

Assert.IsTrue( actual.SequenceEquals(expected) );

The SequenceEquals extension method will compare the elements of the collection in order for equality.

Answer 2

Many test frameworks offer a CollectionAssert class:

CollectionAssert.AreEqual(expected, actual);

Answer 3

You can always write the needed function themselves:

public static bool ListEquals<T>(IList<T> list1, IList<T> list2) {
    if (list1.Count != list2.Count)
        return false;
    for (int i = 0; i < list1.Count; i++)
        if (!list1[i].Equals(list2[i]))
            return false;
    return true;
}

and use it:

// Expected result.
List<string> expected = new List<string>();
expected.Add( "a" );
expected.Add( "b" );
expected.Add( "c" );

// Actual result
actual = new List<string>();
actual.Add( "a" );
actual.Add( "b" );
actual.Add( "c" );

// Verdict
Assert.IsTrue( ListEquals(actual, expected) );

Answer 4

I noticed no one actually told you why your original code didn't work. This is because the == operator in general tests reference equality (i.e. if the two instances are pointing to the same object in memory) unless the operator has been overloaded. List<T> does not define an == operator so the base reference equals implementation is used.

As other posters have demonstrated, you will generally have to step through elements to test "collection equality." Of course, you should use the optimization suggested by user DreamWalker which first tests the Count of the collections before stepping through them.

Answer 5

You could write an extension method like so:

public static class ListExtensions
    {
        public static bool IsEqual<T>(this IList<T> list,IList<T> target, IComparer<T> comparer) where T:IComparable<T>
        {
            if (list.Count != target.Count)
            {
                return false;
            }
            int index = 0;
            while (index < list.Count && 
                   comparer.Compare(list[index],target[index]) == 0)
            {
                index++;
            }
            if (index != list.Count)
            {
                return false;
            }
            return true;
        }
    }

And call it like so:

List<int> intList = new List<int> { 1, 234, 2, 324, 324, 2 };
List<int> targetList = new List<int> { 1, 234, 2, 324, 324 };
bool isEqual = intList.IsEqual(targetList, Comparer<int>.Default);

EDIT: Updated the code to use a static method instead since OP is using .NET 3.0

public static bool IsEqual<T>(IList<T> sourceList, IList<T> targetList, IComparer<T> comparer) where T : IComparable<T>
        {
            if (sourceList.Count != targetList.Count)
            {
                return false;
            }
            int index = 0;
            while (index < sourceList.Count &&
                   comparer.Compare(sourceList[index], targetList[index]) == 0)
            {
                index++;
            }
            if (index != sourceList.Count)
            {
                return false;
            }
            return true;
        }

Client:

        bool isEqual = IsEqual(intList,targetList, Comparer<int>.Default);