Comments by grgran (Top 3 by date)

Thanks for sharing. You might want to consider some changes. For example the StringBuilder class now has a Clear method so you should be able to remove that for 4.0 code. You might also consider some optimization for anything that does a ToString() like SubString(). Consider this (untested, but the ideas should be clear):

///

/// Returns a substrings the specified StringBuilder, like String.Substring().
///

/// <param name="sb">The StringBuilder source.</param>
/// <param name="startIndex">The start index.</param>
/// <param name="length">The length.</param>
/// <returns>
public static string Substring(this StringBuilder sb, int startIndex, int length) {
if (sb == null) {
sb = new StringBuilder();
// or throw ArgumentNullException
}
if (length == 0) { return String.Empty; }
if (length > (sb.Length / 2)) { // just a guess, prob'ly needs a more dynamic algo
return sb.ToString(startIndex, length);
}
if (startIndex < 0) { throw new ArgumentException(); }
if (length < 0) { throw new ArgumentException(); }
var end = (startIndex + length) - 1; // end is never < 0
if (end > sb.Length) {
end = sb.Length - 1;
length = end - startIndex;
// or throw ArgumentException
}
var chars = new char[length];
int k = 0;
for (int i = startIndex; i <= end; i++) {
chars[k++] = sb[i];
}
return new String(chars);
}

This adds some safety checks and would provide better performance when short substrings are required from large stringbuilders.

Cheers