how to convert a illegible string to legible string?

Question

hi

i want to convert a illegible string to legible.i try this code: 

 

string encoded_text = richTextBox1.Text;

string decoded_text;

byte[] b = System.Text.Encoding.Default.GetBytes(encoded_text);

decoded_text = System.Text.Encoding.UTF8.GetString(b);

richTextBox2.Text = decoded_text;

 

but when i convert encoded_text just show ? in richtextbox2.

can you help me?

 

 you can download my project by this link: download my project

 

thanks