How to get value from HiddenField within Gridview in ASP.Net using C#

This post will guide you how to get value from hiddenfield within gridview in asp.net. In this example, I have a button and a gridview with a checkbox, a hiddenfield  and a boundfield. I want to get value row that i checked on checkbox when I click on a button. Please look at code below:
Code Design :

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">

  <Columns>

    <asp:TemplateField HeaderText="Select">

       <ItemTemplate>
         <asp:CheckBox ID="chkBxSelect" runat="server" AutoPostBack="true"/>

       </ItemTemplate>

    </asp:TemplateField>

    <asp:TemplateField>

       <ItemTemplate>

          <asp:HiddenField ID="Invoice_ID" runat="server" Value='<%#Eval("Invoice_ID")%>'/>

       </ItemTemplate>

    </asp:TemplateField>

   <asp:BoundField DataField="Invoice" HeaderText="Invoice" SortExpression="Invoice" />

 </Columns>

</asp:GridView>

<asp:Button ID="Button1" runat="server" Text="Search" Width="80px" Height="25px" onclick="Button1_Click"/>

 Code Behind : 

protected void Button1_Click(object sender, EventArgs e)

{

   string invoiceID="";
   string invoice="";
   string message="";

for (int k = 0; k < GridView1.Rows.Count; k++)

 {

   //declare checkbox 

   CheckBox chk = (CheckBox)GridView1.Rows[k].FindControl("chkBxSelect");

   if (chk.Checked == true)

   {

    //The first or only a checkbox is checked

     if (k==0)

     {

        //get value from hiddenfield

         invoiceID = ((HiddenField)GridView1.Rows[k].Cells[1].FindControl("Invoice_ID")).Value;

        //get value from boundfied                      

        invoice= GridView1.Rows[k].Cells[2].Text;

      }     
       //the next checkboxs are checked

      else

      {

        invoiceID += "," +

((HiddenField)GridView1.Rows[k].Cells[1].FindControl("Invoice_ID")).Value;

        invoice += "," + GridView1.Rows[k].Cells[2].Text;

      }

    }    
    //if no checkboxs is checked

    else

    {

       //display message from page

       message = "Please check on checkbox to get value";

       Page.ClientScript.RegisterStartupScript(Page.GetType(), "shwomessage", "alert('" + message + "');", true);

     } 

    }

}

Done .....Hope this post will help you.