Insert Data from GridView bound using ViewState to Database Table in ASP.Net

This Way:

HTML:

<form id="form1" runat="server">
    <div>
        Enter Name
        <asp:TextBox ID="txtName" runat="server" />
        <br />
        Enter City
        <asp:TextBox ID="txtCity" runat="server" />
        <br />
        <asp:Button ID="btnAdd" Text="Add" OnClick="Add" runat="server" />
        <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
            <Columns>
                <asp:BoundField DataField="Id" HeaderText="Id" />
                <asp:BoundField DataField="Name" HeaderText="Name" />
                <asp:BoundField DataField="City" HeaderText="City" />
            </Columns>
        </asp:GridView>
        <asp:Button runat="server" OnClick="Save" Text="Save" />
    </div>
    </form>

C#:

 protected void Page_Load(object sender, EventArgs e)
    {
        if (!this.IsPostBack)
        {
            this.GetMaxId();
            this.GetPersons();
        }
    }

    private void GetPersons()
    {
        string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand("SELECT * FROM Person", con))
            {
                using (SqlDataAdapter da = new SqlDataAdapter(cmd))
                {
                    DataTable dt = new DataTable();
                    da.Fill(dt);
                    if (dt.Rows.Count > 0)
                    {
                        this.GridView1.DataSource = dt;
                        this.GridView1.DataBind();
                        ViewState["CurrentTable"] = dt;
                    }


                }
            }
        }
    }

    private void GetMaxId()
    {
        string constr = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand("SELECT MAX(Id) AS ID FROM Person", con))
            {
                using (SqlDataAdapter da = new SqlDataAdapter(cmd))
                {
                    DataTable dt = new DataTable();
                    da.Fill(dt);
                    if (dt.Rows.Count > 0)
                    {
                        string str = string.Empty;
                        if (str != dt.Rows[0]["ID"].ToString())
                        {
                            int id = Convert.ToInt32(dt.Rows[0]["ID"].ToString());
                            ViewState["Id"] = id;
                        }
                    }
                }
            }
        }
    }

    protected void Add(object sender, EventArgs e)
    {
        if (ViewState["CurrentTable"] == null)
        {
            int id = Convert.ToInt32(ViewState["Id"]) + 1;
            ViewState["Id"] = id;
            DataTable dt = new DataTable();
            dt.Columns.Add("Id", typeof(int));
            dt.Columns.Add("Name", typeof(string));
            dt.Columns.Add("City", typeof(string));
            dt.Rows.Add(id, this.txtName.Text, this.txtCity.Text);
            ViewState["CurrentTable"] = dt;
            this.GridView1.DataSource = dt;
            this.GridView1.DataBind();
        }
        else
        {
            int id = Convert.ToInt32(ViewState["Id"]) + 1;
            ViewState["Id"] = id;
            DataTable dt2 = (DataTable)ViewState["CurrentTable"];
            dt2.Rows.Add(this.txtName.Text, id, this.txtCity.Text);
            this.GridView1.DataSource = dt2;
            this.GridView1.DataBind();
            ViewState["CurrentTable"] = dt2;
        }
    }

    protected void Save(object sender, EventArgs e)
    {
        DataTable dt = (DataTable)ViewState["CurrentTable"];
        string name, city;
        int id;
        foreach (DataRow row in dt.Rows)
        {
            name = row["Name"].ToString();
            city = row["City"].ToString();
            id = int.Parse(row["Id"].ToString());
            this.InsertRows(name, city, id);
        }
    }

    private void InsertRows(string name, string city, int id)
    {
        string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand("Insert_Person", con))
            {
                con.Open();
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@Id", id);
                cmd.Parameters.AddWithValue("@Name", name);
                cmd.Parameters.AddWithValue("@City", city);
                cmd.ExecuteNonQuery();
                con.Close();
            }
        }
    }

SQL: this stored procedure will add new data only

CREATE PROCEDURE Insert_Person
	@Id INT,
	@Name VARCHAR(50),
	@City VARCHAR(50)
AS
BEGIN	
	SET NOCOUNT ON;
	IF NOT EXISTS(SELECT * FROM Person WHERE Id = @Id)
	BEGIN
		INSERT INTO Person (Id,Name,City) VALUES(@Id,@Name,@City)
	END    
END
GO

Table:

CREATE TABLE [dbo].[Person](
	[name] [varchar](50) NULL,
	[Id] [int] NULL,
	[City] [varchar](50) NULL
) ON [PRIMARY]

GO

Thank You.