Problem with objectdatasource with method with object for parameter

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hi all,

i have in a my page (asp.net c#) a objectdatasource in binding with my
business object.

I've created a method with parameter follow the example found in
http://msdn.microsoft.com/it-it/library/system.web.ui.webcontrols.parameter.aspx


web page have a listview control vith a DropDownList (ddl) control and a
text box

when i select an item of ddl this change a text value in Text control:

1) if ojectdatasource use method without parameter the page call only
method on ddl and return to webbrowser (text control have the new value)

2) if ojectdatasource use method with parameters the page call the
method of ddl, then call the Select method of object datasource and
therefore clear the page to the original value (text control have no value)

page, objectdatasource , listview, ddl and text control have viewstate=true

any suggestion to resolve the problem?

Thanks many