validate a number

hey, im using asp.net/c#
i build a page with a 5 textbox's
i want to validate that in all 5 of the textboxs the data is representing a number (DOUBLE)
unless all the data will be number the user wont be able to move to the next page.
 
i tried to use a custom validator to validate all the textboxs but with no luck..
 
thanks in advance :)

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