C# Code Optimization -- Finding nearest key in a SortedList

Just wondering if anyone can see a way to optimize this piece of code. It is a key piece of my program and needs to run as quickly as possible. The part of the code I am unsure of is the while loop for finding the nearest key -- but any assistance with optimizing the code would be appreciated.

// TODO: Move to snippets lib or create a new collection type that supports this feature
private string _getTrait(SortedList thisList, decimal thisValue)
{
    // Check to see if we need to search the list.
    if (thisList == null || thisList.Count  upper) { searching = false; }
    }

    // Check to see if we are under or over the max values.
    if (index >= thisList.Count - 1) { return thisList.Values[thisList.Count - 1]; }
    if (index < 0) { return thisList.Values[0]; }

    // Check to see if we should have rounded up instead
    if (thisList.Keys[index + 1] - thisValue < thisValue - (thisList.Keys[index])) { index++; }

    // Return the correct/closest string
    return thisList.Values[index];
}

I am using C#, .net4.0 -- I need to use a Generic SortedList ( http://msdn.microsoft.com/en-US/library/ms132319(v=vs.100).aspx )

Your loop is pretty well written. The only thing that sticks out to me is your use of a constant in the while loop and manually breaking. I also prefer to use a slightly different version of CompareTo, but it doesn't make much difference:

while ( (lower<=upper))
{
    int comparisonResult = thisValue.CompareTo( thisList.Keys [index]);
    if (comparisonResult == 0) { return thisList.Values[index]; }

    if (comparisonResult < 0)
    {
        upper = index - 1;
    }
    else
    {
        lower = index + 1;
    } 
    index = (lower + upper) / 2; 
}

You might also be looking for a solution that's just easier to read or shorter. Are you familiar with LINQ? This is one possibility for how your function might look.

private static string GetTraitRefactor (SortedList thisList, decimal thisValue)
    {
        var keys = thisList.Keys;
        var nearest = thisValue -
            keys.Where(k => k  thisValue - k);
        return thisList[nearest];
    }

StackExchange Code Review Q#18155, answer score: 3