How to create auto generated Product Id in textbox and store in database ?

Unless your auto generated id must follow a given pattern, it is a good idea (as already suggested) to make the database itself generate it for you.

If you really need to do that in code then you have to save the last generated id (or the whole set of generated ones, if you aren't generating a sequence) to prevent creating duplicates.

Did you mean coding for these answer?
This is my advice for auto generate in C#

con.Open();
string str = @"select count(*) from product";

SqlCommand cmd = new SqlCommand(str, con);
int i = (int)cmd.ExecuteScalar();
i++;
txtNo.Text = i.ToString();

con.Close();

do you want random product id or auto increment product id....
auto increment product id:
1.your table have column name id datatype maybe int
2.select max(id) table_name
3.display in textbox+1