Rubberduck.Parsing.Preprocessing.BinaryMinusExpression.Evaluate C# (CSharp) 메소드

        public override IValue Evaluate()
        {
            var left = _left.Evaluate();
            var right = _right.Evaluate();
            if (left == null || right == null)
            {
                return null;
            }
            else if (left.ValueType == ValueType.Date && right.ValueType == ValueType.Date)
            {
                // 5.6.9.3.3 - Effective value type exception.
                // If left + right are both Date then effective value type is double.
                decimal leftValue = left.AsDecimal;
                decimal rightValue = right.AsDecimal;
                decimal difference = leftValue - rightValue;
                return new DecimalValue(difference);
            }
            else if (left.ValueType == ValueType.Date || right.ValueType == ValueType.Date)
            {
                decimal leftValue = left.AsDecimal;
                decimal rightValue = right.AsDecimal;
                decimal difference = leftValue - rightValue;
                try
                {
                    return new DateValue(new DecimalValue(difference).AsDate);
                }
                catch
                {
                    return new DecimalValue(difference);
                }
            }
            else
            {
                decimal leftValue = left.AsDecimal;
                decimal rightValue = right.AsDecimal;
                return new DecimalValue(leftValue - rightValue);
            }
        }
    }