binding checkboxes to gridview with respect to date

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        System.Data.DataTable dt = GetTable();
        ViewState["dt"] = dt;
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }

    AddCheckBox(); 
}

void AddCheckBox()
{
    System.Data.DataTable dt = (System.Data.DataTable)ViewState["dt"];
    for (int i = 0; i < dt.Rows.Count; i++)
    {
        if (GridView1.Rows[i].Cells[0].Controls.Count == 0)
        {
            CheckBox cb = new CheckBox();
            GridView1.Rows[i].Cells[0].Controls.Add(cb);
        }
    }
}

void GetCheckBoxValue()
{
    foreach (GridViewRow gr in GridView1.Rows)
    {
        bool cbValue = ((CheckBox)gr.Cells[0].Controls[0]).Checked;

        // do something
    }
}