运算符之间的LINQ

Question

以下适用于IEnumerable类型,但有没有办法得到这样的东西使用IQueryable类型对sql数据库？

class Program
{
    static void Main(string[] args)
    {
        var items = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, };

        foreach (var item in items.Where(i => i.Between(2, 6)))
            Console.WriteLine(item);
    }
}

static class Ext
{
   public static bool Between<T>(this T source, T low, T high) where T : IComparable
   {
       return source.CompareTo(low) >= 0 && source.CompareTo(high) <= 0;
   }
}

Answer 1

如果你把它表达为一个where子句,它可以只使用LINQ to SQL开箱即用,如果你可以构造一个合适的表达式.

在表达树方面可能有更好的方法 - 马克格拉维尔可能能够改善它 - 但值得一试.

static class Ext
{
   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 = Expression.Invoke(keySelector, 
            keySelector.Parameters.ToArray());
       Expression lowerBound = Expression.GreaterThanOrEqual
           (key, Expression.Constant(low));
       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);
   }
}

它可能取决于所涉及的类型 - 特别是,我使用了比较运算符而不是IComparable<T>.我怀疑这更有可能被正确地翻译成SQL,但CompareTo如果你愿意,你可以改变它以使用该方法.

像这样调用它:

var query = db.People.Between(person => person.Age, 18, 21);