What is the fastest way to get all the keys between 2 keys in a SortedList?

Question

Given a populated SortedList. I would like to get all the keys (or their index range, what should be a closed int interval (mis

Answer 1

I wondered why SortedList doesn't provide a BinarySearch when it's already sorted by the keys. It also uses the method itself(f.e. in IndexOf), but the array used is a private field.

So i've tried to create an extension method for this, have a look:

public static class SortedListExtensions
{
    public static int BinarySearch(this SortedList sortedList, TKey keyToFind, IComparer comparer = null)
    {
        // need to create an array because SortedList.keys is a private array
        var keys = sortedList.Keys;
        TKey[] keyArray = new TKey[keys.Count];
        for (int i = 0; i < keyArray.Length; i++)
            keyArray[i] = keys[i];

        if(comparer == null) comparer = Comparer.Default;
        int index = Array.BinarySearch(keyArray, keyToFind, comparer);
        return index;
    }

    public static IEnumerable GetKeyRangeBetween(this SortedList sortedList, TKey low, TKey high, IComparer comparer = null)
    {
        int lowIndex = sortedList.BinarySearch(low, comparer);
        if (lowIndex < 0)
        {
            // list doesn't contain the key, find nearest behind
            // If not found, BinarySearch returns the complement of the index
            lowIndex = ~lowIndex;
        }

        int highIndex = sortedList.BinarySearch(high, comparer);
        if (highIndex < 0)
        {
            // list doesn't contain the key, find nearest before
            // If not found, BinarySearch returns the complement of the index
            highIndex = ~highIndex - 1;
        }

        var keys = sortedList.Keys;
        for (int i = lowIndex; i < highIndex; i++)
        {
            yield return keys[i];
        }
    }
}

Create a sample SortedList:

DateTime start = DateTime.Today.AddDays(-50);
var sortedList = new SortedList();
for(int i = 0; i < 50; i+=2)
{
    var dt = start.AddDays(i);
    sortedList.Add(dt, string.Format("Date #{0}: {1}", i, dt.ToShortDateString()));
}

DateTime low = start.AddDays(1);   // is not in the SortedList which contains only every second day
DateTime high = start.AddDays(10);

Now you can use the extension method above to get the range of keys between a low- and high-key:

IEnumerable dateRange = sortedList.GetKeyRangeBetween(low, high).ToList();

Result:

04/04/2014
04/06/2014
04/08/2014
04/10/2014

Note that this is built from scratch and not really tested, but it could give you an idea anyway.