From the code above, I suppose you are trying to extract the text portion between ':' and ')'.
I would code like:
string sText = PrintInfoTextBlk.Text;
int nStartIndex = sText.IndexOf(':');
int nEndIndex = sText.IndexOf(')');
string mystring = null;
if ((nStartIndex >= 0) && (nEndIndex > nStartIndex))
mystring = sText.Substring(nStartIndex + 1, nEndIndex - nStartIndex - 1);
else if ((nStartIndex >= 0) && (nEndIndex < 0))
mystring = sText.Substring(nStartIndex + 1);
ValueCntTextBox.Text = nStartIndex;
UInt32.Parse(nStartIndex, out _ValueCntTextBox);