User want to select the date from calendar and then display the excel record into Gridview

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C#5.0

i want the user to select the date from calendar and then display the excel record into database.


In run mode as follows


Fromdate calendar1 Todate calendar2 Import(Button)

I want the user to select the Fromdate and Todate
And then click the Import(Button).
Then excel record is displayed in the Gridview.

Suppose when user did not select the from date and todate,and click the Import button.

Then i want to display the message "Please select the From date and To Date"

for that how can i validate in asp.net using c#.