如何使用LinqKit的PredicateBuilder构建带Or条件的C# LINQ查询？

问题背景 #

原本有一段可正常运行的LINQ查询，通过多次Where逐步过滤结果，但因业务需求需要添加0至多个OR条件，标准LINQ无法满足，遂尝试使用LinqKit的PredicateBuilder，却在结合Include关联查询时出现编译错误。

原标准LINQ代码 #

IQueryable<Booking> booking;
booking = (
    from b in _context.Bookings
        .Include(v => v.Vendor)
        .Include(v => v.PurchaseOrder)
        .ThenInclude(v => v.PurchaseOrderDestinations)
        .Include(l => l.LineLevelBookings)
        .Include(d => d.BookingDangerousGoods)
    select b
);

if (searchModel.BookingStatusId != null)
{
    booking = booking.Where(b => b.BookingStatusId == searchModel.BookingStatusId);
}

PredicateBuilder条件构建代码 #

var predicate = PredicateBuilder.New<Booking>(true);

if (searchModel.PoNumber != null)
{
    predicate = predicate.And(b => b.PoNumber.ToString().StartsWith(searchModel.PoNumber.ToString()));
}

错误的查询代码 #

booking = _context.Bookings
    .Include(v => v.Vendor)
    .Include(v => v.PurchaseOrder)
    .ThenInclude(v => v.PurchaseOrderDestinations)
    .Include(l => l.LineLevelBookings)
    .Include(d => d.BookingDangerousGoods)
    .AsExpandable()
    .Where(t => predicate)
    .ToList();

错误提示 #

Cannot convert Lambda Expression to intended delegate type because some of the return trypes in the block are not implicitly convertible to the delegate return type

解决方案 #

编译错误的核心原因是Where方法的参数使用错误：你将predicate直接嵌套在Lambda表达式中，而正确用法是直接传入predicate本身，或调用其Invoke方法。

修正写法1：直接传入predicate #

这是最简洁的用法，直接将PredicateBuilder构建的表达式传入Where：

booking = _context.Bookings
    .Include(v => v.Vendor)
    .Include(v => v.PurchaseOrder)
    .ThenInclude(v => v.PurchaseOrderDestinations)
    .Include(l => l.LineLevelBookings)
    .Include(d => d.BookingDangerousGoods)
    .AsExpandable()
    .Where(predicate)
    .ToList();

修正写法2：Lambda中调用Invoke（适用于需额外组合逻辑的场景） #

如果需要在Where中结合其他条件，可以通过Invoke方法调用predicate：

booking = _context.Bookings
    .Include(v => v.Vendor)
    .Include(v => v.PurchaseOrder)
    .ThenInclude(v => v.PurchaseOrderDestinations)
    .Include(l => l.LineLevelBookings)
    .Include(d => d.BookingDangerousGoods)
    .AsExpandable()
    .Where(t => predicate.Invoke(t))
    .ToList();

添加OR条件示例 #

若需实现多条件OR逻辑，直接使用PredicateBuilder的Or方法即可：

if (searchModel.CustomerName != null)
{
    predicate = predicate.Or(b => b.CustomerName.Contains(searchModel.CustomerName));
}

内容的提问来源于stack exchange，提问作者bilpor