Page.IsValid não pode ser chamado antes da validação. Ele deve ser consultado no manipulador de eventos para um controle com CausesValidation=True ou após chamar Page.Validate.

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Page.IsValid cannot be called before validation has taken place. It should be queried in the event handler for a control that has CausesValidation=True and initiated the postback, or after a call to Page.Validate.

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