What is the best or most interesting use of Extension Methods you've seen? [closed]

Answer 1

With regular use of StringBuilder, you may see the need to combine AppendFormat() and AppendLine().

public static void AppendFormatLine(this StringBuilder sb, string format, params object[] args)
{
    sb.AppendFormat(format, args);
    sb.AppendLine();
}

Also, since I'm converting an application from VB6 to C#, the following are very useful to me:

public static string Left(this string s, int length)
{
    if (s.Length >= length)
        return s.Substring(0, length);
    throw new ArgumentException("Length must be less than the length of the string.");
}
public static string Right(this string s, int length)
{
    if (s.Length >= length)
        return s.Substring(s.Length - length, length);
    throw new ArgumentException("Length must be less than the length of the string.");
}