Question

Lets say we have a generic list of Class1, typically having ~100 objects for a given session.
I would like to see if the list has a particular object. ASP.NET 2.0 allows me to do this:

Dim objResult as Class1 = objList.Find(objSearch)

How does this approach rate when compared to a traditional For loop, looking at a performance perspective?

How would this vary with increase or decrease in length of the list?

Answer 1

You can easily see how .Net List implements the Find method using Reflector:

Public Function Find(ByVal match As Predicate(Of T)) As T
    If (match Is Nothing) Then
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match)
    End If
    Dim i As Integer
    For i = 0 To Me._size - 1
        If match.Invoke(Me._items(i)) Then
            Return Me._items(i)
        End If
    Next i
    Return CType(Nothing, T)
End Function

The only difference between both implementations is that Find requires a call to match instead of having this logic inlined in the loop.

Interestingly, this simple performance:

var persons = new List<Person>();
for (int i = 0; i < 100; i++)
{
    persons.Add(new Person { ID = i });
}

GC.Collect();
var sw = Stopwatch.StartNew();
for (int i = 0; i < 10000000; i++)
{
    persons.Find(person => person.ID == i % 100);

}
sw.Stop();
Console.WriteLine(sw.Elapsed);

GC.Collect();
sw = Stopwatch.StartNew();
for (int i = 0; i < 10000000; i++)
{
    for (int j = 0; j < 100; j++)
    {
        if (persons[j].ID == i % 100)
        {
            break;
        }
    }
}
sw.Stop();
Console.WriteLine(sw.Elapsed);

Shows that:
Total time required to query the list using Find is 05.7990078 seconds.
Total time required to query the list using loop is 06.3551074 seconds.

This results seems consistent in several executions.

Edit – Found explanation for the advantage of Find:
Find works faster since it accesses directly the underlying array in each iteration. The loop access it through the List indexer, which requires for every access index verification:

Public Default Property Item(ByVal index As Integer) As T
    Get
        If (index >= Me._size) Then
            ThrowHelper.ThrowArgumentOutOfRangeException
        End If
        Return Me._items(index) // _items is the underlying array.
    End Get