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Gets or sets the value or expression associated with the conditional format or data validation.
The returned object represents the first part of the validation criteria and can be a constant value, a string value, a cell reference, or a formula.
object Formula1 { get; set; }
Property Formula1 As Object
IValidation validation = worksheet.Range["A1"].Validation;
validation.Add(ValidationType.List, ValidationAlertStyle.Stop, ValidationOperator.Between, "Yes,No,Maybe", null);
object formula1 = validation.Formula1;