conversion of bytearray to string

Byte[] bytes = (Byte[])dt.Rows[0]["ColumnName"];
string x = Convert.ToBase64String(bytes);

or

con.Open ();
SqlCommand cmd = new SqlCommand ("select file from Table1", con);
byte[] buffer = (byte[]) cmd.ExecuteScalar ();
con.Close();
using(FileStream fs = new FileStream(@"C:\1.txt", FileMode.Create))
{
  fs.Write(buffer, 0, buffer.Length);
}