How to get checkbox inside a gridview in webmethod in asp.net

public partial class _Default : System.Web.UI.Page  
{    
    SqlConnection con = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\Database.mdf;Integrated Security=True;User Instance=True");  
    protected void Page_Load(object sender, EventArgs e)  
    {  
        if (!Page.IsPostBack)  
        {  
            refreshdata();  
        }  
    }  
      public void refreshdata()  
    {          
        SqlCommand cmd = new SqlCommand("select * from tbl_data", con);  
        SqlDataAdapter sda = new SqlDataAdapter(cmd);  
        DataTable dt = new DataTable();  
        sda.Fill(dt);  
        GridView1.DataSource = dt;  
        GridView1.DataBind();  
       }  
    protected void Button1_Click(object sender, EventArgs e)  
    {  
        foreach (GridViewRow gvrow in GridView1.Rows)  
        {  
            var checkbox = gvrow.FindControl("CheckBox1") as CheckBox;  
            if (checkbox.Checked)  
            {  
                var lblID = gvrow.FindControl("Label1") as Label;  
                var lblName = gvrow.FindControl("Label2") as Label;  
                var lblCity = gvrow.FindControl("Label3") as Label;  
                SqlCommand cmd = new SqlCommand("insert into tbl_save (id,name,city) values (@id,@name,@city)", con);  
                cmd.Parameters.AddWithValue("id", lblID.Text);  
                cmd.Parameters.AddWithValue("name", lblName.Text);  
                cmd.Parameters.AddWithValue("city", lblCity.Text);  
                con.Open();  
                int i = cmd.ExecuteNonQuery();  
                con.Close();  
                refreshdata();  
              }  
              }  
  }  
}