How to show message Save or Not?

Nothing is required to do in code behind.

In code behind you already have Page.IsValid , so nothing more is required , it's already double confirmed by client.

Just paste the code in aspx page

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how to use this in code .cs page ?

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This is pretty simple and multiple ways are there to achieve the requirement , the easiest one is as shown below

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You can take advantage of Google for the basic question.

Refer

https://www.aspsnippets.com/Articles/Server-Side-Code-Behind-Yes-No-Confirmation-Message-Box-in-ASPNet.aspx

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