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This property can be used in addition to the ValidationSummary.ShowSummary property to control where the validation summary is displayed. If this property and ValidationSummary.EnableClientScript are both set to true, the validation summary is displayed in a message box. If ValidationSummary.EnableClientScript is set to false, this property has no effect.
If both the ValidationSummary.ShowMessageBox and ValidationSummary.ShowSummary properties are set to true, the validation summary is displayed in both a message box and on the Web page.