Slice()

/// <summary>
/// Extracts a section of the string given a start and end index within the string.
/// </summary>
/// <param name="value">The given string</param>
/// <param name="start">The start index to slice from.</param>
/// <param name="end">The end index to slice to.</param>
/// <returns>The section of string sliced.</returns>
public static string Slice(this string value, int start, int end)
{
    if (string.IsNullOrEmpty(value))
    {
        return value;
    }

    int upperBound = value.Length - 1;

    //
    // Check the arguments
    //
    if (start < 0)
    {
        throw new ArgumentOutOfRangeException("start", "start cannot be less than zero");
    }

    if (end > upperBound)
    {
        throw new ArgumentOutOfRangeException("end", string.Format("end cannot be greater than {0}", upperBound));
    }

    if (start > end)
    {
        throw new ArgumentOutOfRangeException("start", "start may not be greater than end");
    }

    return value.Substring(start, (end - start +  1));
}

string result = "0123456789".Slice(4,8); // Returns "45678"
string result = result.Slice(2,3); // Returns "67"