RequireIfAttribute.cs

   public class RequireIfAttribute : ValidationAttribute
    {
        public RequireIfMode Mode { get; }

        public Type TargetType { get; }

        public string Target { get; }

        public bool TargetEmptyAsExists { get; set; }

        private Func<object, object> _Accessor;

        public RequireIfAttribute(RequireIfMode mode, Type targetType, string target)
        {
            Mode = mode;
            TargetType = targetType;
            Target = target;
            _Accessor = MemberAccessor(targetType, target);
        }
        
        public static Func<object, object> MemberAccessor(Type type, string member)
        {
            var p = Expression.Variable(typeof(object));
            var convert = Expression.Convert(Expression.PropertyOrField(Expression.Convert(p, type), member), typeof(object));
            return Expression.Lambda<Func<object, object>>(convert, p).Compile();
        }
        
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            if (validationContext.ObjectInstance == null)
            {
                return ValidationResult.Success;
            }
            var o = _Accessor(validationContext.ObjectInstance);
            var check = false;
            switch (Mode)
            {
                case RequireIfMode.IfExists:
                    check = o != null && (TargetEmptyAsExists || o is string && !string.IsNullOrWhiteSpace(o as string));
                    break;
                case RequireIfMode.IfNotExists:
                    check = o == null || (!TargetEmptyAsExists && (o is string && string.IsNullOrWhiteSpace(o as string)));
                    break;
                default:
                    throw new ArgumentOutOfRangeException();
            }
            if (check)
            {
                if (value == null)
                {
                    return new ValidationResult($"{validationContext.DisplayName}不能为空");
                }
                if (value is string s && s.IsNullOrWhiteSpace())
                {
                    return new ValidationResult($"{validationContext.DisplayName}不能为空");
                }
            }
            return ValidationResult.Success;
        }

        public string GetDescription()
        {
            string s;
            switch (Mode)
            {
                case RequireIfMode.IfExists:
                    s = "不";
                    break;
                case RequireIfMode.IfNotExists:
                    s = "";
                    break;
                default:
                    throw new ArgumentOutOfRangeException();
            }
            return $"当{Target}{s}为空时必填";
        }