creating new row for a gridview dynamically

if (!IsPostBack)
{
getdata();
}
----------------------------
void getdata()
{
dalLogin dal = new dalLogin();
DataSet ds = new DataSet();
ds= dal.getdata();
ViewState["table"] = ds.Tables[0];
bindgrid();
}
void bindgrid()
{
GridView1.DataSource = (DataTable)ViewState["table"];
GridView1.DataBind();
}
-------------------------------
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.Header)
{
e.Row.Cells[5].Attributes.Add("style", "display:none");
}
if (e.Row.RowType == DataControlRowType.DataRow)
{
for (int i = 0; i < GridView1.Rows.Count; i++)
GridView1.Rows[i].Cells[5].Attributes.Add("style", "display:none");
}
}
-----------------------------------
protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
{
if (e.CommandName == "newrow")
{
DataTable dt = (DataTable)ViewState["table"];
dt.Rows.Add(dt.NewRow());
ViewState["table"] = dt;
GridView1.EditIndex = GridView1.Rows.Count;
bindgrid();
Button btn = (Button)GridView1.Rows[GridView1.Rows.Count - 1].FindControl("Button1");
btn.Text = "save";
btn.CommandName = "sav";
}
else if (e.CommandName == "sav")
{
TextBox t1 = (TextBox)GridView1.Rows[GridView1.Rows.Count - 1].Cells[0].Controls[0];
TextBox t2 = (TextBox)GridView1.Rows[GridView1.Rows.Count - 1].Cells[1].Controls[0];
TextBox t3 = (TextBox)GridView1.Rows[GridView1.Rows.Count - 1].Cells[2].Controls[0];
TextBox t4 = (TextBox)GridView1.Rows[GridView1.Rows.Count - 1].Cells[3].Controls[0];
TextBox t5 = (TextBox)GridView1.Rows[GridView1.Rows.Count - 1].Cells[4].Controls[0];
DataTable dt = (DataTable)ViewState["table"];
dt.Rows.RemoveAt(GridView1.Rows.Count - 1);
DataRow dr = dt.NewRow();
dr[0] = t1.Text; dr[1] = t2.Text; dr[2] = t3.Text; dr[3] = t4.Text; dr[4] = t5.Text;
dt.Rows.Add(dr);
ViewState["table"] = dt;
GridView1.EditIndex = -1;
bindgrid();
Button btn = (Button)GridView1.Rows[GridView1.Rows.Count - 1].FindControl("Button1");
btn.Text = "new";
btn.CommandName = "newrow";
}
}