InnerTruncate

public static string InnerTruncate(this string value, 
     int maxLength)
{
    // If there is no need to truncate then
    // return what we were given.
    if (string.IsNullOrEmpty(value) 
            || value.Length <= maxLength)
    {
        return value;
    } // end if

     // Figure out how many characters would be in 
     // each  half if we were to have
     // exactly the same length string on either side 
     // of the elipsis.
     int charsInEachHalf = (maxLength - 3) / 2;

     // Get the string to the right of the elipsis 
     // and then trim the beginning.  There is no
     // need to have a space immediately following 
     // the elipsis.
     string right = value.Substring(
         value.Length - charsInEachHalf, charsInEachHalf)
         .TrimStart();

      // Get the string to the left of the elipsis.
      // We don't use "charsInEachHalf " here
      // because we may be able to take more characters
      // than that if "right" was trimmed.
      string left = value.Substring(
          0, (maxLength - 3) - right.Length)
          .TrimEnd();

       // Concatenate and return the result.
       return string.Format("{0}...{1}", left, right);
} // end InnerTruncate

string sentence = "Truncate the really long string.";

var truncated = sentence.InnerTruncate(10);

Assert.AreEqual("Truncate...string.", truncated);