Char 9: error: unknown type name 'StringBuilder' StringBuilder builder = new StringBuilder();

This error occurs because the compiler does not recognize the type 'StringBuilder'.

To fix this error, you need to include the namespace that contains the 'StringBuilder' class at the top of your file.

For example, if you are using the 'StringBuilder' class from the 'System.Text' namespace, add the following line at the top of your file:

using System.Text;

If you have already included the namespace, make sure that you have spelled the class name correctly and that you have referenced the correct assembly if necessary.