writing code in button's click event in gridview

You can use the PostbackUrl property of the button and set it to the page you want to post. Now at your current page expose the selected row as a public property. Declare the previous page type in in your other page like <%@ PreviousPageType VirtualPath="~/Default.aspx" %>. All set. Now you can access the public property in your page 1 to page 2 by use the object PreviousPage which is actually an instance of your page1 and accessible in page2.

Good luck

For example at the grid view mark up or in the code behind define handlers for RowBound and RowCommand events

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
    DataKeyNames="ProductId" DataSourceID="SqlDataSource1"
        OnRowCommand="GV_RowCommand"   OnRowDataBound="GV_RowBound"
        >

Have template field with button, set its CommandName and PostBackUrl

<asp:TemplateField>
     <ItemTemplate>
         <asp:Button CommandName="Redirect"  ID="Redirect" runat="server" PostBackUrl="~/Default2.aspx" Text="Redirect" />
     </ItemTemplate>
 </asp:TemplateField>

At the event handlers in code behind..

//GridView Row Exposed through a public property
 public GridViewRow SelectedRow { get; set; }
 protected void GV_RowBound(object sender, GridViewRowEventArgs e)
 {
     //Assign the row index as command argument for the row command event
     if (e.Row.RowType == DataControlRowType.DataRow)
     {
         Button btn = e.Row.FindControl("Redirect") as Button;
         btn.CommandArgument = e.Row.RowIndex.ToString();
     }
 }
 protected void GV_RowCommand(object sender, GridViewCommandEventArgs e)
 {
     GridViewRow row = GridView1.Rows[Convert.ToInt32(e.CommandArgument)];
     //set the selected row.
     SelectedRow = row;
 }

At page 2 define the previous page type

<%@ PreviousPageType VirtualPath="~/Default.aspx" %>

Then from the code behind of the page2...

protected void Page_Load(object sender, EventArgs e)
{
    GridViewRow row = this.PreviousPage.SelectedRow;
    
}