Hi akhter,

Refer below code.

HTML

<asp:Button ID="btn_add" runat="server" Text="Add" OnClick="btn_add_Click" />
<br />
<asp:GridView ID="gvtrans" runat="server" AllowSorting="True" AutoGenerateColumns="False" BackColor="White" BorderColor="#E7E7FF"
    BorderStyle="None" BorderWidth="1px" CellPadding="3" CssClass="active right" GridLines="Horizontal" itemstyle-wrap="True"
    ShowFooter="True" ShowHeaderWhenEmpty="True" Style="margin-top: 0px" Width="667px" Height="86px"
    OnRowDeleting="gvtrans_RowDeleting" DataKeyNames="RowNumber">
    <AlternatingRowStyle BackColor="#F7F7F7" CssClass="altrowstyle" />
    <FooterStyle BackColor="#B5C7DE" ForeColor="#4A3C8C" />
    <HeaderStyle BackColor="#4A3C8C" CssClass="headerstyle" Font-Bold="True" ForeColor="#F7F7F7" />
    <PagerStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" HorizontalAlign="Right" />
    <RowStyle BackColor="#E7E7FF" CssClass="rowstyle" ForeColor="#4A3C8C" />
    <Columns>
        <asp:BoundField DataField="RowNumber" HeaderText="S.NO" />
        <asp:TemplateField HeaderText="Code">
            <ItemTemplate>
                <asp:TextBox ID="txtcode" runat="server" Width="40px"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Bank">
            <ItemTemplate>
                <asp:Label ID="lbbank" runat="server" />
                <asp:DropDownList ID="ddlbank" runat="server" CssClass="form-control js-example-placeholder-single" ToolTip="Select" Width="150px">
                    <asp:ListItem Text="Select" Value="0"></asp:ListItem>
                    <asp:ListItem Text="SBI" Value="1"></asp:ListItem>
                    <asp:ListItem Text="HDFC" Value="2"></asp:ListItem>
                    <asp:ListItem Text="Axis" Value="3"></asp:ListItem>
                </asp:DropDownList>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Loc_ID">
            <ItemTemplate>
                <asp:TextBox ID="txtbank_id" runat="server" Width="50px"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Location">
            <ItemTemplate>
                <asp:Label ID="lbloc" runat="server" Visible="false" />
                <asp:DropDownList ID="ddloc" runat="server" CssClass="form-control js-example-placeholder-single" ToolTip="Select" Width="150px">
                    <asp:ListItem Text="Select" Value="0"></asp:ListItem>
                    <asp:ListItem Text="Malad" Value="1"></asp:ListItem>
                    <asp:ListItem Text="Goregaon" Value="2"></asp:ListItem>
                    <asp:ListItem Text="Andheri" Value="3"></asp:ListItem>
                </asp:DropDownList>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Cheque_No">
            <ItemTemplate>
                <asp:TextBox ID="txtChqno" runat="server" Width="130px"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Date">
            <ItemTemplate>
                <asp:TextBox ID="txtDate" runat="server" TextMode="Date" Width="130px"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Amount">
            <ItemTemplate>
                <asp:TextBox ID="txtamount" runat="server" Text="0" Width="130px"></asp:TextBox>
            </ItemTemplate>
            <FooterTemplate>
                <asp:Label ID="lblTotal" runat="server"></asp:Label>
            </FooterTemplate>
        </asp:TemplateField>
        <asp:CommandField ShowDeleteButton="True" ButtonType="Button" />
    </Columns>
    <SelectedRowStyle BackColor="#738A9C" Font-Bold="True" ForeColor="#F7F7F7" />
    <SortedAscendingCellStyle BackColor="#F4F4FD" />
    <SortedAscendingHeaderStyle BackColor="#5A4C9D" />
    <SortedDescendingCellStyle BackColor="#D8D8F0" />
    <SortedDescendingHeaderStyle BackColor="#3E3277" />
</asp:GridView>

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        SetInitialRow();
    }
}
protected void gvtrans_RowDeleting(object sender, GridViewDeleteEventArgs e)
{
    string index = gvtrans.DataKeys[e.RowIndex].Values[0].ToString();
    DataTable dt = ViewState["CurrentTable"] as DataTable;
    for (int i = 0; i < dt.Rows.Count; i++)
    {
        if (dt.Rows[i]["RowNumber"].ToString() == index)
        {
            dt.Rows.Remove(dt.Rows[i]);
            ViewState["CurrentTable"] = dt;
        }
    }
    gvtrans.DataSource = dt;
    gvtrans.DataBind();
    SetPreviousData();
}
protected void btn_add_Click(object sender, EventArgs e)
{
    addrow();
}
private void SetInitialRow()
{
    DataTable dt = new DataTable();
    DataRow dr = null;
    dt.Columns.Add(new DataColumn("RowNumber", typeof(string)));
    dt.Columns.Add(new DataColumn("txtcode", typeof(string)));
    dt.Columns.Add(new DataColumn("ddlbank", typeof(string)));
    dt.Columns.Add(new DataColumn("ddloc", typeof(string)));
    dt.Columns.Add(new DataColumn("txtbank_id", typeof(string)));
    dt.Columns.Add(new DataColumn("txtChqno", typeof(string)));
    dt.Columns.Add(new DataColumn("txtamount", typeof(string)));
    dt.Columns.Add(new DataColumn("txtdate", typeof(string)));
    dr = dt.NewRow();
    dr["RowNumber"] = 1;
    dr["txtcode"] = String.Empty;
    dr["txtbank_id"] = String.Empty;
    dr["txtChqno"] = String.Empty;
    dr["txtamount"] = String.Empty;
    dr["txtdate"] = String.Empty;
    dt.Rows.Add(dr);
    ViewState["CurrentTable"] = dt;
    gvtrans.DataSource = dt;
    gvtrans.DataBind();
}
private void addrow()
{
    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 box1 = (TextBox)gvtrans.Rows[rowIndex].Cells[1].FindControl("txtcode");
                DropDownList box2 = (DropDownList)gvtrans.Rows[rowIndex].Cells[2].FindControl("ddlbank");

                TextBox box3 = (TextBox)gvtrans.Rows[rowIndex].Cells[5].FindControl("txtChqno");
                TextBox box4 = (TextBox)gvtrans.Rows[rowIndex].Cells[7].FindControl("txtamount");
                TextBox box5 = (TextBox)gvtrans.Rows[rowIndex].Cells[3].FindControl("txtbank_id");
                DropDownList box6 = (DropDownList)gvtrans.Rows[rowIndex].Cells[4].FindControl("ddloc");
                TextBox box7 = (TextBox)gvtrans.Rows[rowIndex].Cells[6].FindControl("txtdate");
                //DropDownList box5 = (DropDownList)gvtrans.Rows[rowIndex].Cells[5].FindControl("ddlType");
                if (box3.Text != "" || box4.Text != "")
                {
                    drCurrentRow = dtCurrentTable.NewRow();
                    drCurrentRow["RowNumber"] = i + 1;
                    dtCurrentTable.Rows[i - 1]["txtcode"] = box1.Text;
                    dtCurrentTable.Rows[i - 1]["ddlbank"] = box2.Text;
                    dtCurrentTable.Rows[i - 1]["txtChqno"] = box3.Text;
                    dtCurrentTable.Rows[i - 1]["txtamount"] = box4.Text;
                    dtCurrentTable.Rows[i - 1]["txtbank_id"] = box5.Text;
                    dtCurrentTable.Rows[i - 1]["ddloc"] = box6.Text;
                    dtCurrentTable.Rows[i - 1]["txtdate"] = box7.Text;
                    //dtCurrentTable.Rows[i - 1]["ddlType"] = box5.Text;
                    rowIndex++;
                }
            }
            if (drCurrentRow != null)
            {
                dtCurrentTable.Rows.Add(drCurrentRow);
            }
            ViewState["CurrentTable"] = dtCurrentTable;
            gvtrans.DataSource = dtCurrentTable;
            gvtrans.DataBind();

            foreach (GridViewRow row in gvtrans.Rows)
            {
                TextBox txtChqno = (TextBox)row.FindControl("txtChqno");
                TextBox txtamount = (TextBox)row.FindControl("txtamount");

                DataTable dt = (DataTable)ViewState["CurrentTable"];

                if (!string.IsNullOrEmpty(dt.Rows[row.RowIndex]["txtChqno"].ToString())
                    || !string.IsNullOrEmpty(dt.Rows[row.RowIndex]["txtamount"].ToString()))
                {
                    //txtChqno.Enabled = false;
                    //txtamount.Enabled = false;
                }
            }
        }
    }
    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++)
            {
                TextBox box1 = (TextBox)gvtrans.Rows[rowIndex].Cells[1].FindControl("txtcode");
                DropDownList box2 = (DropDownList)gvtrans.Rows[rowIndex].Cells[2].FindControl("ddlbank");
                TextBox box3 = (TextBox)gvtrans.Rows[rowIndex].Cells[5].FindControl("txtChqno");
                TextBox box4 = (TextBox)gvtrans.Rows[rowIndex].Cells[7].FindControl("txtamount");
                TextBox box5 = (TextBox)gvtrans.Rows[rowIndex].Cells[3].FindControl("txtbank_id");
                DropDownList box6 = (DropDownList)gvtrans.Rows[rowIndex].Cells[4].FindControl("ddloc");
                TextBox box7 = (TextBox)gvtrans.Rows[rowIndex].Cells[6].FindControl("txtdate");
                box1.Text = dt.Rows[i]["txtcode"].ToString();
                box2.Text = dt.Rows[i]["ddlbank"].ToString();
                box3.Text = dt.Rows[i]["txtChqno"].ToString();
                box4.Text = dt.Rows[i]["txtamount"].ToString();
                box5.Text = dt.Rows[i]["txtbank_id"].ToString();
                box6.Text = dt.Rows[i]["ddloc"].ToString();
                box7.Text = dt.Rows[i]["txtdate"].ToString();
                //      box5.Text = dt.Rows[i]["ddlType"].ToString();
                rowIndex++;
            }
        }
    }
}