如何为日期范围创建条件 lambda 表达式？

Question

在我的 ASP.net MVC 项目中，我有一个包含 7 个搜索变量的搜索表单。它们都不是强制性的，但是，应该至少选择一个来执行搜索操作，如图所示。

在这种情况下使用 lambda 表达式执行搜索的最佳方法是什么，因为用户可以选择任何搜索参数组合？目前，我正在使用一长串“if-else if”结构并为每个结构编写单独的 lambda 表达式。有没有更好/更有效的方法？

更新 1： 这只是条件的一半，现在我被告知，即使是三个日期标准中的每一个的“日期”都是可选的，这将进一步增加检查列表。这是标准的做法吗？

IEnumerable<MyRecord> Request = null;
        if (!status.Equals(""))
        {
            if (deliveryDateFrom == null && caseDateFrom == null && pickupDateFrom == null)
            {
                //only request status
                Request = GetRecord("EquipRequest")
                    .Where(r => r.Status == status);                            
            }
            else if (deliveryDateFrom != null && caseDateFrom == null && pickupDateFrom == null)
            {
                // request status and delivery date
                Request = GetRecord("EquipRequest")
                    .Where(r => r.Status == status && (r.DeliveryDate >= deliveryDateFrom && r.DeliveryDate <= deliveryDateTo));
            }
            else if (deliveryDateFrom == null && caseDateFrom != null && pickupDateFrom == null)
            {
                // request status and case date 
                Request = GetRecord("EquipRequest")
                    .Where(r => r.Status == status && (r.CaseDate >= caseDateFrom && r.CaseDate <= caseDateTo));
            }
            else if (deliveryDateFrom == null && caseDateFrom == null && pickupDateFrom != null)
            {
                // request status and pickup date
                Request = GetRecord("EquipRequest")
                    .Where(r => r.Status == status && (r.PickupDate >= pickupDateFrom && r.PickupDate <= pickupDateTo));  
            }
            else if (deliveryDateFrom != null && caseDateFrom != null && pickupDateFrom == null)
            {
                // request status, delivery date and case date
                Request = GetRecord("EquipRequest")
                    .Where(r => r.Status == status && (r.DeliveryDate >= deliveryDateFrom && r.DeliveryDate <= deliveryDateTo)
                    && (r.CaseDate >= caseDateFrom && r.CaseDate <= caseDateTo));  
            }
            else if (deliveryDateFrom != null && caseDateFrom == null && pickupDateFrom != null)
            {
                // request status, delivery date and pickup date
                Request = GetRecord("EquipRequest")
                    .Where(r => r.Status == status && (r.DeliveryDate >= deliveryDateFrom && r.DeliveryDate <= deliveryDateTo)
                    && (r.PickupDate >= pickupDateFrom && r.PickupDate <= pickupDateTo)); 
            }
            else if (deliveryDateFrom == null && caseDateFrom != null && pickupDateFrom != null)
            {
                // request status, case date and pickup date
                Request = GetRecord("EquipRequest")
                    .Where(r => r.Status == status && (r.CaseDate >= caseDateFrom && r.CaseDate <= caseDateTo)
                    && (r.PickupDate >= pickupDateFrom && r.PickupDate <= pickupDateTo)); 
            }
            else if (deliveryDateFrom != null && caseDateFrom != null && pickupDateFrom != null)
            {
                // request status, delivery date, case date and pickup date
                Request = GetRecord("EquipRequest")
                    .Where(r => r.Status == status && (r.DeliveryDate >= deliveryDateFrom && r.DeliveryDate <= deliveryDateTo)
                    && (r.CaseDate >= caseDateFrom && r.CaseDate <= caseDateTo)
                    && (r.PickupDate >= pickupDateFrom && r.PickupDate <= pickupDateTo));
            }                
        }

Answer 1

你可以做的是使用如下函数：

public static IEnumerable<TSource> WhereIf<TSource>(this IEnumerable<TSource> source, bool condition, Func<TSource, bool> predicate)
{
    return condition ? source.Where(predicate) : source;
}

如果 bool 为真，它将通过您的 lambda 表达式缩小范围，如果为假，它将继续。

并像这样使用它：

var whatever = GetRecord("EquipRequest")
    .Where(r.Status == status)
    .WhereIf(deliveryDateFrom != null, r => r.DeliveryDate >= deliveryDateFrom)
    .WhereIf(deliveryDateTo != null, r => r.DeliveryDate <= deliveryDateTo)
    .WhereIf(caseDateFrom != null, r => r.CaseDate >= caseDateFrom)
    .WhereIf(caseDateTo != null, r => r.CaseDate <= caseDateTo)
    .WhereIf(pickupDateFrom != null, r => r.PickupDate >= pickupDateFrom)
    .WhereIf(pickupDateTo != null, r => r.PickupDate <= pickupDateTo)

以此类推

Answer 2

首先我建议你获取LinqPad，学习它，如果你想尝试这样的东西。

然后编写类似这样的 LinqPad 脚本并运行它以使您的 Linq 语句正确：

public class SearchViewModel {
    public DateTime DeliveryDateFrom {get; set;}
    public DateTime DeliveryDateTo {get; set;}
    public DateTime CaseDateFrom {get; set;}
    public DateTime CaseDateTo {get; set;}
    public DateTime PickupDateFrom {get; set;}
    public DateTime PickupDateTo {get; set;}
};



public class Data {
    public DateTime DeliveryDate {get; set;}
    public DateTime CaseDate {get; set;}
    public DateTime PickupDate {get; set;}
};

void Main()
{
    var model = new SearchViewModel();  // insert your model stuff here
    // you can use LinqPad to hook up directly to your project DBContext dll here
    var dataBase = new List<Data>() { 
        new Data() { DeliveryDate = DateTime.Now, CaseDate = DateTime.Now, PickupDate = DateTime.Now }
    };  

    // experiment with Linq statement here
    var result = dataBase.Where(x =>
        (model.DeliveryDateFrom >=  x.DeliveryDate && model.DeliveryDateTo <=  x.DeliveryDate) &&
        (model.CaseDateFrom >=  x.CaseDate && model.CaseDateTo <=  x.CaseDate) &&
        (model.PickupDateFrom >=  x.PickupDate && model.PickupDateTo <=  x.PickupDate)
    );

    result.Dump();
}

Answer 3

您可以链接 where 子句。

IEnumerable<MyRecord> query = GetRecord("EquipRequest");
if (!status.Equals(""))
{
    if (deliveryDateFrom.HasValue) 
        query = query.Where(x => x.DeliveryDate >= deliveryDateFrom);
    if (deliveryDateTo.HasValue) 
        query = query.Where(x => x.DeliveryDate <= deliveryDateTo);
}
IEnumerable<MyRecord> result = query.ToList();