How to validate data/parameters before the data gets transfered?

I am using the createuserwizard to create users, and I have added some extra fields that collects user data and stores it in a custom table. I want to validate the data/parameters before the data gets transfered to the database. Why does not this code work?
Thanks for any help!

protected void CreateUserWizard_FinishButtonClick(object sender, WizardNavigationEventArgs e)
{
SqlDataSource DataSource = (SqlDataSource)CreateUserWizardStep3.ContentTemplateContainer.FindControl("personsSqlDataSource");
Parameter Morradi = new Parameter("ReceiveMail", TypeCode.Boolean, "true");
DataSource.InsertParameters.Add(Morradi);
}   ...