我的项目有许多带有日期字段的对象,我经常需要选择其中一个字段在日期范围内的所有对象。
例如:
public class Contract
{
public DateTime SignDate { get; set; }
public DateTime ReleaseDate { get; set; }
}
public class PersonalCheck
{
public DateTime SignDate { get; set; }
public DateTime ProcessDate { get; set; }
public DateTime VoidDate { get; set; }
}
如果我只关心SignDate,事情就简单多了。我会声明一个接口...
public interface IObjectWithSignDate
{
DateTime SignDate { get; set; }
}
...change我的其他对象以继承它,然后创建一个方法,如下所示:
public static IQueryable<T> SignedWithin<T>(this IQueryable<T> items, DateTime start, DateTime end) where T : IObjectWithSignDate
{
return items.Where(q => q.SignDate >= start && q.SignDate <= end);
}
如何避免为ReleaseDate、ProcessDate、VoidDate等重写此函数?我能让这个方法接受任何对象的IQueryable和一个变量,告诉它对哪个日期字段运行这个选择器吗?
注意:这必须能够a)在LinqToEntities中执行才能在数据库上运行,b)不会增加很多开销(我担心反射可能会这样做)
发布于 2018-06-02 15:29:03
简单但具体的
您可以像这样添加扩展方法:
public static class DateTimeExtensions
{
public static bool IsBetween(this DateTime thisDateTime, DateTime start, DateTime end)
{
return thisDateTime >= start && thisDateTime <= end;
}
}
您可以隔离地对其进行单元测试。
然后,您可以在任何想要检查的DateTime字段上使用它。例如:
var start = new DateTime(2017, 1, 1);
var end = new DateTime(2017, 12, 31, 23, 59, 59);
IList<Contract> contracts = new List<Contract>(); // or anything enumerable
var contractsSignedBetween = contracts.Where(x => x.SignDate.IsBetween(start, end));
var contractsReleasedBetween = contracts.Where(x => x.ReleaseDate.IsBetween(start, end));
(请注意,我如何将开始日期时间设置为00:00:00时间,将结束日期时间设置为23:59:59时间,也可以随意包括毫秒,以便包括前一天内的时间。)
创建可重用的
如果你发现自己经常需要这样做,你可以做一个扩展
public static class EnumerableContractsExtensions
{
public static IEnumerable<Contract> SignedBetween(this IEnumerable<Contract> contracts, DateTime start, DateTime end)
{
return contracts.Where(x => x.SignDate.IsBetween(start, end));
}
}
并像这样使用它
var contractsSignedBetween = contracts.SignedBetween(start, end);
这也可以在隔离中进行单元测试。
更灵活但更具体的
使用表达式表示您想要的日期...
public static class EnumerableContractsExtensions
{
public static IEnumerable<Contract> Between(this IEnumerable<Contract> contracts, Func<Contract, DateTime> selector, DateTime start, DateTime end)
{
return contracts.Where(x => selector(x).IsBetween(start, end));
}
}
然后执行以下操作:
var contractsSignedBetween = contracts.Between(x => x.SignDate, start, end);
var contractsReleasedBetween = contracts.Between(x => x.ReleaseDate, start, end);
灵活而通用的
全力以赴去做泛型(尽管你不能把它变成一个扩展方法,因为它是泛型的):
public static class EnumerableExtensions
{
public static IEnumerable<T> Between<T>(IEnumerable<T> items, Func<T, DateTime> selector, DateTime start, DateTime end)
{
return items.Where(x => selector(x).IsBetween(start, end));
}
}
同样,这本身是可测试的,并且可以像这样使用:
IList<Contract> contracts = new List<Contract>();
IList<PersonalCheck> personalChecks = new List<PersonalCheck>();
var contractsSignedBetween = EnumerableExtensions.Between(contracts, x => x.SignDate, start, end);
var checksSignedBetween = EnumerableExtensions.Between(personalChecks, x => x.SignDate, start, end);
Making it IQueryable
因为LINQ To Entities不知道如何将方法转换为IQueryable
,所以为了使其作为SQL语言工作,该方法需要转移到expression tree。
public static IQueryable<TSource> Between<TSource, TKey>(
this IQueryable<TSource> source,
Expression<Func<TSource, TKey>> keySelector,
TKey low,
TKey high)
where TKey : IComparable<TKey>
{
Expression key = keySelector.Body;
Expression lowerBound = Expression.LessThanOrEqual(Expression.Constant(low), key);
Expression upperBound = Expression.LessThanOrEqual(key, Expression.Constant(high));
Expression and = Expression.AndAlso(lowerBound, upperBound);
Expression<Func<TSource, bool>> lambda =
Expression.Lambda<Func<TSource, bool>>(and, keySelector.Parameters);
return source.Where(lambda);
}
它仍然会像这样使用:
var contractsSignedBetween = contracts.Between(x => x.SignDate, start, end);
这也适用于DateTime
之外的其他东西。希望这能有所帮助。
https://stackoverflow.com/questions/50648265
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