string split by index / params?

All other answers just seemed too complicated, so I took a stab.

using System.Linq;

public static class StringExtensions
{
    /// <summary>
    ///     Returns a string array that contains the substrings in this instance that are delimited by specified indexes.
    /// </summary>
    /// <param name="source">The original string.</param>
    /// <param name="index">An index that delimits the substrings in this string.</param>
    /// <returns>An array whose elements contain the substrings in this instance that are delimited by one or more indexes.</returns>
    /// <exception cref="ArgumentNullException"><paramref name="index" /> is null.</exception>
    /// <exception cref="ArgumentOutOfRangeException">An <paramref name="index" /> is less than zero or greater than the length of this instance.</exception>
    public static string[] SplitAt(this string source, params int[] index)
    {
        index = index.Distinct().OrderBy(x => x).ToArray();
        string[] output = new string[index.Length + 1];
        int pos = 0;

        for (int i = 0; i < index.Length; pos = index[i++])
            output[i] = source.Substring(pos, index[i] - pos);

        output[index.Length] = source.Substring(pos);
        return output;
    }
}

You could use the String instance method Substring.

string a = input.Substring(0, 10);
string b = input.Substring(10, 5);
string c = input.Substring(15, 3);