How can I know the Client have 'Adobe Acrobat Reader' by C# in ASP.NET?If someone want to download a book in my Web,I'd like to check whether he have a Reader and advise him to download a Reader.

There is no direct method in .NET. You have a few options:

1. Some third party controls will fetch you certain data (BrowserHawk is one that comes to mind). However, this might not be a reliable method.

2. Sometimes you can sniff into the browser DOM. I've seen cases where people look in some collection of plugins and could potentially find something that contains "acrobat" to indicate the presence of the reader or regular acrobat. But keep in mind that you'll need to be able to communicate this back to the server with a auto-posting form or something similar.

-Peter