A ValidationSummary control is displayed when the IsValid property of the page is false. It "polls" each of the validation controls on the page and aggregates the text messages exposed by each. It does this through the Page.Validators collection; each validation control is added to this collection, and the validation summary can use this to determine which validators have errors. The following sample illustrates displaying errors with a ValidationSummary.
When the user's input is processed (for example, when the form is submitted), the Web Forms framework passes the user's entry to the associated validation control or controls based on the ValidationGroup property. The validation controls test the user's input and set a property to indicate whether the entry passed the validation test. After all validation controls have been processed, the IsValid property on the page is set; if any of the controls shows that a validation check failed, the entire page is set to invalid.
The ValidationSummary control only displays errors for validators in the same ValidationGroup.