Random String Generator

I encountered the problem where I needed to build a lot of controls at runtime and of course I needed the to have a name. I decided to use a Random String builder and here is what I cam up with:

 public static string RandomString(this String str, int size, bool lowerCase)

        {

            StringBuilder builder = new StringBuilder();

            Random random = new Random();

            char ch;

            for (int i = 0; i < size; i++)

            {

                ch = Convert.ToChar(Convert.ToInt32(Math.Floor(2 * random.NextDouble() + 28)));

                builder.Append(ch);

            }

            if (lowerCase)

                return builder.ToString().ToLower();

            return builder.ToString();

        }

* One small side note is that the 2 and the 28 in the sample have no correlation to the result, I just wanted a completely random result.