How to parse string to byte[] in silverlight4

i have Two textboxes for upload file the uploades file will be stored in Article Table the DataField is; article=varbinary(max); articleimage=varbinary(max) i Used the Following Coding For Store Data... in That i Want to Parse string To byte how can i parse the textbox input to byte[]
 Article magazine = new Article();
       
            magazinearticle.=(txtBook.Text);
            magazine.articleimage = txtCover.Text;