应用基于多维数组的LINQ过滤器 [英] Applying LINQ filters based on a multi-dimensional array

问题描述

给出一个实体框架查询，例如

Given an entity framework query, such as

var query = (from property in _dbContext.Properties
        join location in _db.Locations
            on property.Id equals location.PropertyId
select new PropertyDetail
{
    Url = property.Url,
    Type = property.Type,
    Title = property.Title,
    Continent = location.Continent,
    Country = location.Country,
    State = location.State,
});

我应用了以下过滤器:

if (!string.IsNullOrWhitespace(searchFilters.Type))
{
    query = query.Where(model => model.Type == searchFilters.Type);
}
if (!string.IsNullOrWhitespace(searchFilters.Title))
{
    query = query.Where(model => model.Title.Contains(searchFilters.Title));
}

给出以下多维数组

var locations = new[]
{
    new[] {"Africa", "Algeria", ""},
    new[] {"Asia", "Hong Kong", ""},
    new[] {"Asia", "Singapore", ""},
    new[] {"Oceania", "Australia", "New South Wales"},
    new[] {"North America", "United States", "California"}
};

如何进一步限制查询"，使其仅包括与指定位置{Continent，Country，State(可选)}匹配的那些条目?

How can the "query" be further restricted to only include those entries that match the specified locations {Continent, Country, State(optional)} ?

推荐答案

不幸的是，LINQ to Entities当前不支持联接到内存集合，也不支持Contains用于非原始内存集合.我看到的唯一方法(实际上这里还有另一种方法

Unfortunately LINQ to Entities currently does not support joins to inmemory collection, nor Contains for non primitive inmemory collection. The only way I see (actually there is another one described here Entity Framework LINQ Get all items part of another collection, but now I think this is more appropriate) is to construct OR filter using some expression build helper.

例如，使用

For instance, using the PredicateUtils class from Establish a link between two lists in linq to entities where clause, it could be like this:

首先，添加一些辅助方法

First, add a little helper method

static Expression<Func<PropertyDetail, bool>> LocationFilter(string value, int index)
{
    if (!string.IsNullOrEmpty(value))
    {
        if (index == 0) return d => d.Continent == value;
        if (index == 1) return d => d.Country == value;
        if (index == 2) return d => d.State == value;
    }
    return null;
}

然后使用

var locationsFilter = locations.Select(location => location.Select(LocationFilter)
    .Aggregate(PredicateUtils.And)).Aggregate(PredicateUtils.Or);
if (locationsFilter != null)
    query = query.Where(locationsFilter);

为完整起见，以下是使用的帮助程序类:

For completeness, here is the helper class used:

public static class PredicateUtils
{
    sealed class Predicate<T>
    {
        public static readonly Expression<Func<T, bool>> True = item => true;
        public static readonly Expression<Func<T, bool>> False = item => false;
    }
    public static Expression<Func<T, bool>> Null<T>() { return null; }
    public static Expression<Func<T, bool>> True<T>() { return Predicate<T>.True; }
    public static Expression<Func<T, bool>> False<T>() { return Predicate<T>.False; }
    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> left, Expression<Func<T, bool>> right)
    {
        if (Equals(left, right)) return left;
        if (left == null || Equals(left, True<T>())) return right;
        if (right == null || Equals(right, True<T>())) return left;
        if (Equals(left, False<T>()) || Equals(right, False<T>())) return False<T>();
        var body = Expression.AndAlso(left.Body, right.Body.Replace(right.Parameters[0], left.Parameters[0]));
        return Expression.Lambda<Func<T, bool>>(body, left.Parameters);
    }
    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> left, Expression<Func<T, bool>> right)
    {
        if (Equals(left, right)) return left;
        if (left == null || Equals(left, False<T>())) return right;
        if (right == null || Equals(right, False<T>())) return left;
        if (Equals(left, True<T>()) || Equals(right, True<T>())) return True<T>();
        var body = Expression.OrElse(left.Body, right.Body.Replace(right.Parameters[0], left.Parameters[0]));
        return Expression.Lambda<Func<T, bool>>(body, left.Parameters);
    }

    static Expression Replace(this Expression expression, Expression source, Expression target)
    {
        return new ExpressionReplacer { Source = source, Target = target }.Visit(expression);
    }

    class ExpressionReplacer : ExpressionVisitor
    {
        public Expression Source;
        public Expression Target;
        public override Expression Visit(Expression node)
        {
            return node == Source ? Target : base.Visit(node);
        }
    }
}

更新:按照注释中的要求，这是locations是List<Location>的解决方案:

UPDATE: As requested in the comments, here is the solution for locations being List<Location>:

var locationsFilter = locations.Select(location =>
{
    var filter = PredicateUtils.Null<PropertyDetail>();
    if (!string.IsNullOrEmpty(location.Continent))
        filter = filter.And(d => d.Continent == location.Continent);
    if (!string.IsNullOrEmpty(location.Country))
        filter = filter.And(d => d.Country == location.Country);
    if (!string.IsNullOrEmpty(location.State))
        filter = filter.And(d => d.State == location.State);
    return filter;
}).Aggregate(PredicateUtils.Or);

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