Gridview toujours ajouter de nouvelle ligne lorsque, cliquez sur le bouton?

Je vais écrire le code qui créer une nouvelle ligne dans gridview lors d'un clic de bouton. Le nombre de lignes sont créées par la valeur dans la zone de texte. Ex: Quand j'ai entrer la valeur 2 dans la zone de texte, bien sûr, les deux lignes seront ajoutées, mais quand j'ai cliqué sur le bouton nouveau, troisième rangées encore ajouté. Veuillez vérifier mon code ici:

ASPX

<asp:TextBox ID="txtVisitor" runat="server"></asp:TextBox>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:GridView ID="Gridview1" runat="server" ShowFooter="true" AutoGenerateColumns="false">
            <Columns>
                <asp:BoundField DataField="RowNumber" HeaderText="Row Number" />
                <asp:TemplateField HeaderText="Header 1">
                    <ItemTemplate>
                        <asp:TextBox ID="txtDate" runat="server"></asp:TextBox>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="Header 2">
                    <ItemTemplate>
                        <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="Header 3">
                    <ItemTemplate>
                        <asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
                    </ItemTemplate>
                    <FooterStyle HorizontalAlign="Right" />
                    <FooterTemplate>
                        <asp:Button ID="ButtonAdd" runat="server" Text="Add New Row" OnClick="ButtonAdd_Click" />
                    </FooterTemplate>
                </asp:TemplateField>
            </Columns>
        </asp:GridView>
    </ContentTemplate>
</asp:UpdatePanel>

Code_Behind

protected void ButtonAdd_Click(object sender, EventArgs e)
{
int visitors = Convert.ToInt32(txtVisitor.Text);
AddNewRowToGrid(visitors);
}
private void SetInitialRow()
{
DataTable dt = new DataTable();
DataRow dr = null;
dt.Columns.Add(new DataColumn("RowNumber", typeof(string)));
dt.Columns.Add(new DataColumn("Column1", typeof(string)));
dt.Columns.Add(new DataColumn("Column2", typeof(string)));
dt.Columns.Add(new DataColumn("Column3", typeof(string)));
dr = dt.NewRow();
dr["RowNumber"] = 1;
dr["Column1"] = string.Empty;
dr["Column2"] = string.Empty;
dr["Column3"] = string.Empty;
dt.Rows.Add(dr);
//dr = dt.NewRow();
//Store the DataTable in ViewState
ViewState["CurrentTable"] = dt;
Gridview1.DataSource = dt;
Gridview1.DataBind();
}
private void AddNewRowToGrid(int visitors)
{
if (ViewState["CurrentTable"] != null)
{
DataTable dtCurrentTable = (DataTable)ViewState["CurrentTable"];
DataRow drCurrentRow;
if (dtCurrentTable.Rows.Count > 0)
{
int rowindex = 0;
for (int i = 1; i < visitors; i++)
{
//extract the TextBox values
TextBox box1 = (TextBox)Gridview1.Rows[rowindex].Cells[1].FindControl("txtDate");
TextBox box2 = (TextBox)Gridview1.Rows[rowindex].Cells[2].FindControl("TextBox2");
TextBox box3 = (TextBox)Gridview1.Rows[rowindex].Cells[3].FindControl("TextBox3");
drCurrentRow = dtCurrentTable.NewRow();
drCurrentRow["RowNumber"] = i + 1;
drCurrentRow["Column1"] = box1.Text;
drCurrentRow["Column2"] = box2.Text;
drCurrentRow["Column3"] = box3.Text;
dtCurrentTable.Rows.Add(drCurrentRow);
drCurrentRow = null;
rowindex++;
}
//add new row to DataTable
//Store the current data to ViewState
ViewState["CurrentTable"] = dtCurrentTable;
//Rebind the Grid with the current data
Gridview1.DataSource = dtCurrentTable;
Gridview1.DataBind();
}
}
else
{
Response.Write("ViewState is null");
}
}
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
SetInitialRow();
}
}