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Метод CustomValidationAttribute "<var>Method name</var>" типа "<var>Type Name</var>" должен возвращать System.ComponentModel.DataAnnotations.ValidationResult. Чтобы указать успешное выполнение, используйте System.ComponentModel.DataAnnotations.ValidationResult.Success.
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The CustomValidationAttribute method '<var>Method name</var>' in type '<var>Type Name</var>' must return System.ComponentModel.DataAnnotations.ValidationResult. Use System.ComponentModel.DataAnnotations.ValidationResult.Success to represent success.
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