How to foreach gridview on button

Try below..
Html below..

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
           <Columns>
               <asp:BoundField DataField="RowNumber" HeaderText="Row Number" />
               <asp:TemplateField HeaderText="From">
                   <ItemTemplate>
                       <asp:Label ID="Label1" runat="server" />
                   </ItemTemplate>
               </asp:TemplateField>
               <asp:TemplateField HeaderText="To">
                   <ItemTemplate>
                       <asp:Label ID="Label2" runat="server" />
                   </ItemTemplate>
               </asp:TemplateField>
           </Columns>
       </asp:GridView>
       <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><asp:TextBox ID="TextBox2"
           runat="server"></asp:TextBox><asp:Button ID="Button1" runat="server" Text="Button"
               OnClick="Button1_Click" />

Code behind..

protected void Page_Load(object sender, EventArgs e)
    {
        if (!this.IsPostBack)
        {
            SetInitialRow();
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        AddNewRowToGrid();
    }
    private void SetInitialRow()
    {
        DataTable dt = new DataTable();
        DataRow dr = null;
        dt.Columns.Add(new DataColumn("RowNumber", typeof(string)));
        dt.Columns.Add(new DataColumn("From", typeof(string)));
        dt.Columns.Add(new DataColumn("To", typeof(string)));
        dr = dt.NewRow();
        dr["RowNumber"] = 1;
        dr["From"] = string.Empty;
        dr["To"] = 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 rowIndex = 0;
        if (ViewState["CurrentTable"] != null)
        {
            DataTable dtCurrentTable = (DataTable)ViewState["CurrentTable"];
            DataRow drCurrentRow = null;
            if (dtCurrentTable.Rows.Count > 0)
            {
                int row = dtCurrentTable.Rows.Count;
                //extract the TextBox values
                Label box1 = (Label)GridView1.Rows[rowIndex].Cells[1].FindControl("Label1");
                Label box2 = (Label)GridView1.Rows[rowIndex].Cells[2].FindControl("Label2");
                drCurrentRow = dtCurrentTable.NewRow();
                drCurrentRow["RowNumber"] = row + 1;
                dtCurrentTable.Rows[row - 1]["From"] = TextBox1.Text;
                dtCurrentTable.Rows[row - 1]["To"] = TextBox2.Text;
                rowIndex++;

                dtCurrentTable.Rows.Add(drCurrentRow);
                ViewState["CurrentTable"] = dtCurrentTable;
                GridView1.DataSource = dtCurrentTable;
                GridView1.DataBind();
            }
        }
        else
        {
            Response.Write("ViewState is null");
        }
        //Set Previous Data on Postbacks
        SetPreviousData();
    }
    private void SetPreviousData()
    {
        int rowIndex = 0;
        if (ViewState["CurrentTable"] != null)
        {
            DataTable dt = (DataTable)ViewState["CurrentTable"];
            if (dt.Rows.Count > 0)
            {
                for (int i = 0; i < dt.Rows.Count; i++)
                {
                    Label box1 = (Label)GridView1.Rows[rowIndex].Cells[1].FindControl("Label1");
                    Label box2 = (Label)GridView1.Rows[rowIndex].Cells[2].FindControl("Label2");
                    box1.Text = dt.Rows[i]["From"].ToString();
                    box2.Text = dt.Rows[i]["To"].ToString();
                    rowIndex++;
                }
            }
        }
    }

Modify the code previous i posted.
using session or viewstate add more records to gridview.

aspx.cs:
 protected void btnclick_Click(object sender, EventArgs e)
    {
        DataTable dt = new DataTable();
        if (Session["DataTable"] != null)
        {
            dt = (DataTable)Session["DataTable"];
        }
        else
        {
            DataColumn cl = new DataColumn("From");
            dt.Columns.Add(cl);
            cl = new DataColumn("To");
            dt.Columns.Add(cl);
        }

        //-- add first row in datatable
        DataRow dr = dt.NewRow();
        dr[0] = txt1.Text;
        dr[1] = txt2.Text;
        dt.Rows.Add(dr);
        grdview.DataSource = dt;
        grdview.DataBind();
        Session["DataTable"] = dt;
    }

hi Kurac1,
Try this code. hope it helpful...

<asp:TextBox ID="txt1" runat="server"></asp:TextBox>
   <asp:TextBox ID="txt2" runat="server"></asp:TextBox>
   <asp:Button ID="btnclick" Text="click" runat="server" OnClick="btnclick_Click" />
   <asp:GridView ID="grdview" runat="server" EnableModelValidation="True" AutoGenerateColumns="False">
       <Columns>
           <asp:TemplateField HeaderText="From">
               <ItemTemplate>
                   <asp:Label ID="Label1" runat="server" Text='<%# DataBinder.Eval(Container.DataItem,"From")%>' />
               </ItemTemplate>
           </asp:TemplateField>
           <asp:TemplateField HeaderText="To">
               <ItemTemplate>
                   <asp:Label ID="Label2" runat="server" Text='<%# DataBinder.Eval(Container.DataItem,"To")%>' />
               </ItemTemplate>
           </asp:TemplateField>
       </Columns>
   </asp:GridView>

aspx.cs:
protected void btnclick_Click(object sender, EventArgs e)
{
DataTable dt = new DataTable();
DataColumn cl = new DataColumn("From");
dt.Columns.Add(cl);
cl = new DataColumn("To");
dt.Columns.Add(cl);

//-- add first row in datatable
DataRow dr = dt.NewRow();
dr[0] = txt1.Text;
dr[1] = txt2.Text;
dt.Rows.Add(dr);
grdview.DataSource = dt;
grdview.DataBind();
}