c# - 用于链接查询组合的 Linq 语法

Question

在某些情况下，我有多种选择，需要根据所选内容过滤数据(在本例中使用 EF)。有没有比列出所有组合更简洁的方式来编写以下内容？

我下面的初步尝试似乎过于复杂，尤其是当组合变得更大时。

DateTime today = DateTime.Now.Date;
DateTime yesterday = today.AddDays(-1);
DateTime tomorrow = today.AddDays(1);

var query = db.Products.AsQueryable();
if (userSettings.DisplayYesterday && userSettings.DisplayToday && userSettings.DisplayTomorrow)
{
    query = query.Where(x => DbFunctions.TruncateTime(x.ListedTime) == yesterday || DbFunctions.TruncateTime(x.ListedTime) == today || DbFunctions.TruncateTime(x.ListedTime) == tomorrow);
}
else if (!userSettings.DisplayYesterday && userSettings.DisplayToday && userSettings.DisplayTomorrow)
{
    query = query.Where(x => DbFunctions.TruncateTime(x.ListedTime) == today || DbFunctions.TruncateTime(x.ListedTime) == tomorrow);
}
else if (userSettings.DisplayYesterday && !userSettings.DisplayToday && userSettings.DisplayTomorrow)
{
    query = query.Where(x => DbFunctions.TruncateTime(x.ListedTime) == yesterday || DbFunctions.TruncateTime(x.ListedTime) == tomorrow);
}
else if (userSettings.DisplayYesterday && userSettings.DisplayToday && !userSettings.DisplayTomorrow)
{
    query = query.Where(x => DbFunctions.TruncateTime(x.ListedTime) == yesterday || DbFunctions.TruncateTime(x.ListedTime) == today);
}
else if (!userSettings.DisplayYesterday && !userSettings.DisplayToday && userSettings.DisplayTomorrow)
{
    query = query.Where(x => DbFunctions.TruncateTime(x.ListedTime) == tomorrow);
}
else if (!userSettings.DisplayYesterday && userSettings.DisplayToday && !userSettings.DisplayTomorrow)
{
    query = query.Where(x => DbFunctions.TruncateTime(x.ListedTime) == today);
}
else if (userSettings.DisplayYesterday && !userSettings.DisplayToday && !userSettings.DisplayTomorrow)
{
    query = query.Where(x => DbFunctions.TruncateTime(x.ListedTime) == yesterday);
}
else if (userSettings.DisplayYesterday && !userSettings.DisplayToday && !userSettings.DisplayTomorrow)
{
    query = query.Where(x => DbFunctions.TruncateTime(x.ListedTime) == yesterday);
}
else if (!userSettings.DisplayYesterday && !userSettings.DisplayToday && !userSettings.DisplayTomorrow)
{
    //  If nothing is selected then default to today
    query = query.Where(x => DbFunctions.TruncateTime(x.ListedTime) == today);
}

List<Products> productList = await query.ToListAsync();

Answer 1

是的，您可以安装 LinqKit nuget 包并使用其 PredicateBuilder(如果出于某种原因您不能\不想安装第三方包 - 您可以自己实现类似的功能)。使用 PredicateBuilder，您可以这样做:

// start with false, because building OR expression
// and false OR something is the same as just something
var condition = PredicateBuilder.New<Product>(false);
if (userSettings.DisplayYesterday)
    condition = condition.Or(x => DbFunctions.TruncateTime(x.ListedTime) == yesterday);
if (userSettings.DisplayToday)
    condition = condition.Or(x => DbFunctions.TruncateTime(x.ListedTime) == today);
if (userSettings.DisplayTomorrow)
    condition = condition.Or(x => DbFunctions.TruncateTime(x.ListedTime) == tomorrow);
query = query.Where(condition);

如果您使用 AND(而不是像此处那样的 OR)连接多个条件，则不需要谓词构建器，只需链接 Where 调用:

if (someCondition)
    query = query.Where(x => x.SomeField == someValue);
if (anotherCondition)
    query = query.Where(x => x.SomeAnotherField == someAnotherValue);