Fastest way to compare two lists

Question

I have a List (Foo) and I want to see if it's equal to another List (foo). What is the fastest way ?

Answer 1

From 3.5 onwards you may use a LINQ function for this:

List<string> l1 = new List<string> {"Hello", "World","How","Are","You"};
List<string> l2 = new List<string> {"Hello","World","How","Are","You"};
Console.WriteLine(l1.SequenceEqual(l2));

It also knows an overload to provide your own comparer

Answer 2

Here are the steps I would do:

1. Do an object.ReferenceEquals() if true, then return true.
2. Check the count, if not the same, return false.
3. Compare the elements one by one.

Here are some suggestions for the method:

1. Base the implementation on ICollection. This gives you the count, but doesn't restrict to specific collection type or contained type.
2. You can implement the method as an extension method to ICollection.
3. You will need to use the .Equals() for comparing the elements of the list.

Answer 3

Something like this:

public static bool CompareLists(List<int> l1, List<int> l2)
{
    if (l1 == l2) return true;
    if (l1.Count != l2.Count) return false;
    for (int i=0; i<l1.Count; i++)
        if (l1[i] != l2[i]) return false;
    return true;
}

Some additional error checking (e.g. null-checks) might be required.