Auto Complete TextBox in C# with DataBase Values | CodeWith C# | Window Based Product

before copy paste code make sure that, you have used using System.Data.SqlClient; in your code

it is also important to connect data base to your application.

below code is for SQL DataBase

Hyper Links contains information from docs.microsoft.com

void autocomplete_txt()
        {
            textBox1.AutoCompleteMode = AutoCompleteMode.Suggest;
            textBox1.AutoCompleteSource = AutoCompleteSource.CustomSource;
            AutoCompleteStringCollection coll_obj = new AutoCompleteStringCollection();

            SqlConnection conn = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\USERS\ANIKET\DOCUMENTS\JERRYINDIA.MDF;Integrated Security=True;Connect Timeout=30");
            conn.Open();
            SqlCommand cmd = new SqlCommand("select * from bazaar", conn);

            SqlDataReader reader = cmd.ExecuteReader();


            //this is for product name
            int itemCodeOrdinal = reader.GetOrdinal("product_name");
            while(reader.Read())
            {
                string prd_nm = reader.GetString(itemCodeOrdinal);
                coll_obj.Add(prd_nm);
            }

            textBox1.AutoCompleteCustomSource = coll_obj;

            
        }