c# - 创建用于 Entity Framework 6 的表达式

Question

我有以下方法启动 IQueryable .

public static IQueryable<StorageDetail> QueryStorageDetails(ApplicationDbContext dbContext, DateRange dateRange)
{
    return from detail in dbContext.StorageDetails
           where (detail.ArrivalDate != null && detail.ArrivalDate <= dateRange.EndDate &&
                (detail.ReleaseDate == null || detail.ReleaseDate >= dateRange.StartDate)) ||
                (detail.TakeOrPayStartDate != null && detail.TakeOrPayStartDate <= dateRange.EndDate &&
                (detail.TakeOrPayEndDate == null || detail.TakeOrPayEndDate >= dateRange.StartDate))
           select detail;
}

我尝试如下重构它。

public static bool FilterByDate(StorageDetail detail, DateRange dateRange)
{
    return (detail.ArrivalDate != null && detail.ArrivalDate <= dateRange.EndDate &&
        (detail.ReleaseDate == null || detail.ReleaseDate >= dateRange.StartDate)) ||
        (detail.TakeOrPayStartDate != null && detail.TakeOrPayStartDate <= dateRange.EndDate &&
        (detail.TakeOrPayEndDate == null || detail.TakeOrPayEndDate >= dateRange.StartDate));
}

public static IQueryable<StorageDetail> QueryStorageDetails(ApplicationDbContext dbContext, DateRange dateRange)
{
    return dbContext.StorageDetails
        .Where(d => FilterByDate(d, dateRange));
}

但这会产生错误。

The LINQ expression 'DbSet<StorageDetail>
    .Where(s => QueryHelper.FilterByDate(
        detail: s, 
        dateRange: __dateRange_0))' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

我很确定有一种方法可以做到这一点，可能是通过创建一个 Expression但我似乎无法正确使用语法。有人知道怎么做吗？

Answer 1

FilterByDate应该返回 Expression<Func<StorageDetail, bool>>因此 EF 将能够将您的代码转换为查询。尝试这样的事情:

public static Expression<Func<StorageDetail, bool>> FilterByDate(DateRange dateRange)
{
    return detail => (detail.ArrivalDate != null && detail.ArrivalDate <= dateRange.EndDate &&
        (detail.ReleaseDate == null || detail.ReleaseDate >= dateRange.StartDate)) ||
        (detail.TakeOrPayStartDate != null && detail.TakeOrPayStartDate <= dateRange.EndDate &&
        (detail.TakeOrPayEndDate == null || detail.TakeOrPayEndDate >= dateRange.StartDate));
}

和

public static IQueryable<StorageDetail> QueryStorageDetails(ApplicationDbContext dbContext, DateRange dateRange)
{
    return dbContext.StorageDetails
        .Where(FilterByDate(dateRange));
}