LevenshteinDistance.cs

public static class Distance
{
    public static double Levenshtein(string source, string target)
    {
        if (source.Length == 0)
            return target.Length;

        if (target.Length == 0)
            return source.Length;

        if (source == target)
            return 0;

        var memo = new int[target.Length + 1];

        for (int k = 0; k <= target.Length; k++)
            memo[k] = k;

        for (int i = 1; i <= source.Length; i++)
        {
            var currentRowMemo = new int[target.Length + 1];
            currentRowMemo[0] = i;

            for (int k = 1; k <= target.Length; k++)
            {
                currentRowMemo[k] = Math.Min(
                    Math.Min(currentRowMemo[k - 1] + 1, memo[k] + 1),
                    memo[k - 1] + (source[i - 1] != target[k - 1] ? 1 : 0));
            }

            memo = currentRowMemo;
        }

        return memo[target.Length];
    }
}