如何表达All with Expression Trees

Question

对于给定的搜索标签过滤器,预期结果是表示在给定标签ID列表中具有所有标签的实体的表达式.

Lambda可能将此表达为:

class Tag 
{
   public long TagId { get; set; }
}

class Taggable 
{
   ICollection<Tag> Tags { get; set; }
}

...

IEnumerable<long> searchTags = new List<long>() { 1, 2, 3 };
Func<Taggable, bool> filter = taggable => searchTags.All(qtag => taggable.Tags.Any(tag => tag.TagId == qtag));

尝试将此表示为表达式树失败:

var tagParam = Expression.Parameter(typeof(Tag), "tag");    
var taggableParam = Expression.Parameter(typeof(Taggable), "taggable");
MemberExpression tagsProperty = Expression.Property(taggableParam, "Tags");
ConstantExpression searchTagsConstant = Expression.Constant(searchTags);

var containsCall = Expression.Call(
      typeof(Enumerable), "Contains",
      new[] { typeof(long) },
      searchTagsConstant,
      Expression.Property(tagParam, "TagID")
);

var anyCall = Expression.Call(
     typeof(Enumerable), "Any",
     new[] { typeof(Tag) },
     tagsProperty,
     Expression.Lambda(containsCall, tagParam)
);

// FAILS HERE
var allCall = Expression.Call(
    typeof(Enumerable), "All",
    new[] { typeof(long) },
    searchTagsConstant,
    anyCall
);

类型'System.Linq.Enumerable'上的通用方法'All'与提供的类型参数和参数兼容.如果方法是非泛型的,则不应提供类型参数.

人们认为它会起作用,因为Enumerable.All<TSource, Func<TSource, bool>>应该满足searchTagsConstant并且anyCall？

Answer 1

我认为它会起作用,因为Enumerable.All<TSource, Func<TSource, bool>>应该满足searchTagsConstant和anyCall

不.anyCall不是lambda表达式(Func<TSource, bool>),只是这种表达式的潜在体.

让我们从你的目标开始:

IEnumerable<long> searchTags = new List<long>() { 1, 2, 3 };

Expression<Func<Taggable, bool>> lambda = 
    taggable => searchTags.All(searchTag => taggable.Tags.Any(tag => tag.TagId == searchTag));

学习如何构建表达式树的最简单方法是在编译时创建一个示例表达式,并通过调试器在运行时通过某个反编译器或表达式树检查生成的代码.

无论如何,请注意上面的表达式有3个lambda参数,而在你的尝试中你只有2个.也没有Contains电话,不知道你为什么放在那里.

构建上面的表达式可以是这样的:

var taggable = Expression.Parameter(typeof(Taggable), "taggable");
var tag = Expression.Parameter(typeof(Tag), "tag");
var searchTag = Expression.Parameter(typeof(long), "searchTag");
// tag.TagId == searchTag
var anyCondition = Expression.Equal(
    Expression.Property(tag, "TagId"),
    searchTag);
// taggable.Tags.Any(tag => tag.TagId == searchTag)
var anyCall = Expression.Call(
    typeof(Enumerable), nameof(Enumerable.Any), new[] { typeof(Tag) },
    Expression.Property(taggable, "Tags"), Expression.Lambda(anyCondition, tag));
// searchTags.All(searchTag => taggable.Tags.Any(tag => tag.TagId == searchTag))
var allCall = Expression.Call(
    typeof(Enumerable), nameof(Enumerable.All), new[] { typeof(long) },
    Expression.Constant(searchTags), Expression.Lambda(anyCall, searchTag));
// taggable => searchTags.All(searchTag => taggable.Tags.Any(tag => tag.TagId == searchTag))
var lambda = Expression.Lambda(allCall, taggable);