C#

public int Register_User(String Firstname, String LastName, String Username, String Password, String Email)
       {
           int Res=0;

           con = new OleDbConnection(strcon);

           cmdinsert = new OleDbCommand();

           cmdinsert.CommandText = "insert into Users Values(?,?,?,?,?)";

           cmdinsert.CommandTimeout = 0;

           cmdinsert.CommandType = CommandType.Text;

           cmdinsert.Connection = con;

           cmdinsert.Parameters.Add("Username", OleDbType.VarChar,30).Value = Convert.ToString(Username);

           cmdinsert.Parameters.Add("Password", OleDbType.VarChar,30).Value = Convert.ToString(Password);

           cmdinsert.Parameters.Add("Firstname", OleDbType.VarChar,30).Value = Convert.ToString(Firstname);

           cmdinsert.Parameters.Add("LastName", OleDbType.VarChar,30).Value = Convert.ToString(LastName);

           cmdinsert.Parameters.Add("Email", OleDbType.VarChar, 30).Value = Convert.ToString(Email);

           try
           {
               con.Open();

               Res =(int) cmdinsert.ExecuteScalar();


           }
           catch (OleDbException)
           {
               throw;
           }
           finally
           {
               con.Close();
           }
                return Res;
       }

Number of query values and destination fields are not the same.

private void EnumerateTables(ComboBox cmb, String databaseName, bool isAccess)
        {
            //cmb.Items.Clear();
            if (!isAccess)
            {
                var srv = new Server(Properties.Settings.Default.ServerInstance);
                srv.Databases.Refresh();
                foreach (Database db in srv.Databases)
                {
                    if (db.Name == cmbboxSqlDb.Text)
                        
                    {
                        BindingSource bs = new BindingSource();
                        bs.DataSource = db.Tables;
                        bs.DataMember = "Name";
                        cmb.DataSource = bs;
                        //var tableEnumerator = db.Tables.GetEnumerator();

                        //while (tableEnumerator.MoveNext())
                        //{
                        //    cmb.Items.Add(((Table)tableEnumerator.Current).Name);
                        //}
                    }
                }
            }
            else
            {
                
                if (AccessConnString != null)
                {
                    var conn = new OleDbConnection(AccessConnString);

                    // open the connection to the database 
                    conn.Open();
                    // this.Dispose();

                    // Get the Tables
                    var schemaTable = conn.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, new Object[] { null, null, null, "TABLE" });

                    // Store the table names in the class scoped array list of table names
                    for (int i = 0; i < schemaTable.Rows.Count; i++)
                    {
                      cmb.Items.Add(schemaTable.Rows[i].ItemArray[2].ToString());
                    }

                }
            }
        }