Id value is not incremented

Hi

I have created dynamic textboxes inside gridview

While adding new row,already existing records are stored in viewstate

and fetch the data  from viewstate in to gridview.Also  a new row is created.

 ID,Column1,Column2,Column3 are database fields

After adding new row,ID value should be incremented.

but  ID value is not incremented.This ID value is got from database max(ID).I have given code below.

How to make ID value to be incremented .for every new row is added , Id should be incremented 

private void AddNewRowToGrid()
        {
            SqlConnection conn = new SqlConnection(GetConnectionString());
            conn.Open();
            SqlCommand cmd = new SqlCommand("SELECT max(ID) from SampleTable",conn);

            
          
            identity = Convert.ToInt32(cmd.ExecuteScalar());

            int rowIndex = 0;

            if (ViewState["CurrentTable"] != null)
            {
                DataTable dtCurrentTable = (DataTable)ViewState["CurrentTable"];
                DataRow drCurrentRow = null;
                if (dtCurrentTable.Rows.Count > 0)
                {
                    for (int i = 1; i <= dtCurrentTable.Rows.Count; i++)
                    {
                        //extract the TextBox values
                        TextBox box4 = (TextBox)Gridview1.Rows[rowIndex].Cells[1].FindControl("TextBox4");
                        TextBox box1 = (TextBox)Gridview1.Rows[rowIndex].Cells[1].FindControl("TextBox1");
                        TextBox box2 = (TextBox)Gridview1.Rows[rowIndex].Cells[2].FindControl("TextBox2");
                        TextBox box3 = (TextBox)Gridview1.Rows[rowIndex].Cells[3].FindControl("TextBox3");

                        drCurrentRow = dtCurrentTable.NewRow();
                     


                        dtCurrentTable.Rows[i - 1]["ID"] = box4.Text;


                        dtCurrentTable.Rows[i - 1]["Column1"] = box1.Text;
                        dtCurrentTable.Rows[i - 1]["Column2"] = box2.Text;
                        dtCurrentTable.Rows[i - 1]["Column3"] = box3.Text;

                        rowIndex++;
                    }
                    drCurrentRow = dtCurrentTable.NewRow();

                    int identity1 = identity + 1;//This value is not incremented when new row is added
                    drCurrentRow["ID"] = identity1;
                    drCurrentRow["Column1"] = "";
                    drCurrentRow["Column2"] = "sdfsfd";
                    drCurrentRow["Column3"] = "sdfsf";
                    dtCurrentTable.Rows.Add(drCurrentRow);


                    ViewState["CurrentTable"] = dtCurrentTable;

                    Gridview1.DataSource = dtCurrentTable;
                    Gridview1.DataBind();
                   


                }
            }
            else
            {
                Response.Write("ViewState is null");
            }

            //Set Previous Data on Postbacks
            SetPreviousData();
        }

Thanks and Regards

N.Ram