how to add new row and new values in gridview in asp.net and c#

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ASP.NET

here i am using code but gridview adding new rocords overriding old records Please help me out...
thank you.. :)

private void SetInitialRow()
       {

           DataTable datatable = new DataTable();

           datatable.Columns.Add("SlNo", typeof(string));
           datatable.Columns.Add("ReceiptNo", typeof(string));
           datatable.Columns.Add("ItemName", typeof(string));
           datatable.Columns.Add("UOM", typeof(string));
           datatable.Columns.Add("Description", typeof(string));
           datatable.Columns.Add("Rate", typeof(string));
           datatable.Columns.Add("Quatity", typeof(string));
           datatable.Columns.Add("Amount", typeof(string));
           DataRow dr = null;

           dr = datatable.NewRow();

           dr["SlNo"] = "1";
           dr["ReceiptNo"] = string.Empty;// txtRcptHdNo.Text;
           dr["ItemName"] = string.Empty;// ddlRcptDtlsItem.SelectedItem.Text;
           dr["UOM"] = string.Empty;// txtRcptDtlsUOM.Text;
           dr["Description"] = string.Empty;// txtRcptDtlsDescr.Text;
           dr["Rate"] = string.Empty;// txtRcptDtlsRt.Text;
           dr["Quatity"] = string.Empty;// txtRcptDtlsQty.Text;
           dr["Amount"] = string.Empty;// txtRcptDtlsAmount.Text;

           datatable.Rows.Add(dr);
           //dr = dt.NewRow();

           //Store the DataTable in ViewState
           ViewState["CurrentTable"] = datatable;

           dgvRcptDtls.DataSource = datatable;
           dgvRcptDtls.DataBind();
       }

private void AddNewRowToGrid()
{
    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++)
            {

                drCurrentRow = dtCurrentTable.NewRow();
                drCurrentRow["SlNo"] = i + 1;

                dtCurrentTable.Rows[i - 1]["ReceiptNo"] = txtRcptHdNo.Text;
                dtCurrentTable.Rows[i - 1]["ItemName"] = ddlRcptDtlsItem.SelectedItem.Text;
                dtCurrentTable.Rows[i - 1]["UOM"] = txtRcptDtlsUOM.Text;
                dtCurrentTable.Rows[i - 1]["Description"] = txtRcptDtlsDescr.Text;
                dtCurrentTable.Rows[i - 1]["Rate"] = txtRcptDtlsRt.Text;
                dtCurrentTable.Rows[i - 1]["Quatity"] = txtRcptDtlsQty.Text;
                dtCurrentTable.Rows[i - 1]["Amount"] = txtRcptDtlsAmount.Text;

                rowIndex++;
            }
            dtCurrentTable.Rows.Add(drCurrentRow);
            ViewState["CurrentTable"] = dtCurrentTable;

            dgvRcptDtls.DataSource = dtCurrentTable;
            dgvRcptDtls.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++)
            {

                dt.Rows[i]["ReceiptNo"].ToString();// = txtRcptHdNo.Text;
                dt.Rows[i]["ItemName"].ToString();// = ddlRcptDtlsItem.SelectedItem.Text;
                dt.Rows[i]["UOM"].ToString();// = txtRcptDtlsUOM.Text;
                dt.Rows[i]["Description"].ToString();// = txtRcptDtlsDescr.Text;
                dt.Rows[i]["Rate"].ToString();// = txtRcptDtlsRt.Text;
                dt.Rows[i]["Quatity"].ToString();// = txtRcptDtlsQty.Text;
                dt.Rows[i]["Amount"].ToString();// = txtRcptDtlsAmount.Text;

                rowIndex++;
            }
        }
    }
}

protected void btnAdd_Click(object sender, EventArgs e)
        {
            AddNewRowToGrid();
           
        }

page_load()// event
{
SetInitialRow()
}