Hi,
There are three ways to do this kind issues.
One Way.
By using CrossPostBack functionality of ASP.NET,you can pass the one aspx page data to another page.
Go through the CrossPostBack function in ASP.NET
http://msdn.microsoft.com/en-us/library/ms178139.aspx[
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http://www.c-sharpcorner.com/UploadFile/DipalChoksi/xpgpostbk_asp2_dc08102006235543PM/xpgpostbk_asp2_dc.aspx[
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Second Way.
Instead of displaying gridview with checkbox in another page,display it in same page.By clicking add button for selected items in gridview,you can directly add those items to listbox in the same page itself.
Third Way.
By using Session concept.
you can put the selected check box item in session,then use that session variable in listbox page.
http://www.codeproject.com/KB/aspnet/state_management_intro.aspx[
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The above technies will solve your issue.
Still if you are not happy with this,let me know.
Regards,
Kiran.