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class Program
{
    static void Main(string[] args)
    {
        string test = "Hallo Welt";

        string[] splitted = test.SplitByIndex(2, 4, 6).ToArray();
        string[] splittedFromEnd = test.SplitByIndex(test.Length - 2).ToArray();

    }
}

public static class Helper
{
    public static IEnumerable<string> SplitByIndex(this string source, params int[] indexes)
    {
        int lastIndex = 0;
        foreach (var index in indexes)
        {
            if (index >= source.Length)
                break;

            yield return source.Substring(lastIndex, index - lastIndex);
            lastIndex = index;
        }
        yield return source.Substring(lastIndex);
    }
}					

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    static void Main()
    {
        string str = "Hallo Welt";        
        var l = str.Length;

        char[] array = new char[] {str[l-2], str[l-1]};
    }