c# - LINQ to SQL 复杂查询

Question

我有以下对象

public Cell {
    public string CategoryId {get; set;}
    public DateTime FromTime {get; set}
    public DateTime ToTime {get; set}
}

我有一个名为 Item 的数据库表，如下所示：

Item

-------------------------
CategoryId    LastUpdated

现在在代码中，我有一个List<Cell> ToBeFetchedFromDB包含多个 Cell 的 Cell 列表，假设列表包含Fooand Bar，我想动态构建这样的查询，BY INTERATING THROUGH THE COLLECTION ToBeFetchedFromDB WITHIN MY LINQ TO SQL QUERY而不是静态构建查询：

from x in Item
where x.CategoryId == Foo.CategoryId && Foo.FromTime < x.LastUpdated < Foo.ToTime
      || x.CategoryId == Bar.CategoryId && Bar.FromTime < x.LastUpdated < Bar.ToTime
select x

我一直在尝试，但无法弄清楚:(

多谢你们！

Answer 1

使用PredicateBuilder，您可以使用 a 浏览列表中的每个项目，foreach并为您的过滤器添加一个新选项：

var filter = PredicateBuilder.False<Item>();

foreach (var cell in ToBeFetchedFromDB)
{
    filter = PredicateBuilder.Or(filter, item => 
        item.CategoryId == cell.CategoryId &&
        cell.FromTime < item.LastUpdated &&
        item.LastUpdated < cell.ToTime);
}

var query = Item.Where(filter);

PredicateBuilder来自链接的副本：

public static class PredicateBuilder
{
    public static Expression<Func<T, bool>> True<T>() { return f => true; }
    public static Expression<Func<T, bool>> False<T>() { return f => false; }

    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1,
                                                        Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, bool>>
                (Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
    }

    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,
                                                            Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, bool>>
                (Expression.AndAlso(expr1.Body, invokedExpr), expr1.Parameters);
    }
}