Hello Please Dont used this type you Must used Sql Parameter
Like...
SqlCommand cmd = new SqlCommand("insert into table (name,address,comment) values(@name,@address,@comment)", cnn);
cmd.Parameters.AddWithValue(@name, txtname.text);
cmd.Parameters.AddWithValue(@address, txtaddress.text);
cmd.Parameters.AddWithValue(@comment, txtcomment.text);
then execute query ..
i Hope ypur problem will solve if not please give me your comment