如何构建这个复杂的表达式Func？ [英] How to build this complex Expression Func?
问题描述
我目前正在使用一个 EF 网站，我有这个数据库对象：
public partial class Association
{
public Association（）
{
this.Users = new HashSet< User>（）;
}
public Guid Id {get;组; }
public Guid ItemId {get;组; }
[ForeignKey（ItemId）]
public virtual Item Item {get;组; }
[InverseProperty（Association）]
public virtual ICollection< User>用户{get;组;
}
项目类：
public partial class Item
{
public Item（）
{
this.Associations = new HashSet< Association>（） ;
}
public Guid Id {get;组; }
public ItemTypeEnum Type {get;组; }
[InverseProperty（Item）]
public virtual ICollection< Association>协会{get;组; }
}
此字典：
public static Dictionary< ItemTypeEnum，int> MaxOccupationRange = new Dictionary< ItemTypeEnum，int>（）
{
{ItemTypeEnum.Type1,1}，
{ItemTypeEnum.Type2,2}，
{ItemTypeEnum.Type3,5 }，
{ItemTypeEnum.Type4,10}，
};
我需要一个 Expression&FunC< Association，bool>> / code>将返回 true if association.Users.Count< MaxOccupationRange [association.Item.Type] ，否则 false 。
我认为这应该大致如下但不幸的是，我对于表达式的概念不够熟悉，可以实现这种做法。
调试时以下结果，似乎它计算（一个未优化但是）正确的表达式。
public static Expression< Func< TSource，布尔>> MyExprMethod< TSource，TKey，TKey2，TValue>（表达<功能>源，TKey>源，表达< Func< TSource，TKey2>> source2，IReadOnlyDictionary< TKey，TValue> dict）
{
var body = dict
.Aggregate（（Expression）null，（next，dicEntry）=> next == null
？Expression.Condition
（
Expression.LessThan （source2.Body，Expression.Constant（dicEntry.Value）），
Expression.Constant（true），
Expression.Constant（false）
）
：Expression.Condition
（
Expression.Equal（source.Body，Expression.Constant（dicEntry.Key）），
Expression.Condition
（
Expression.LessThan（source2.Body， Expression.Constant（dicEntry.Value）），
Expression.Constant（true），
next
），
next
）
）;
return Expression.Lambda
我尝试这样调用：
var expr = MyHelper.MyExprMethod（（Association x）=> x.Item.Type，（Association b）=> b.Users.Count，Item .MaxOccupationRange）;
但显然，它抱怨 b 参数由于它没有出现在结果lambda中，因此不被分配。
我尝试了许多东西（匿名类型而不是两个来源等），但是似乎我对这个概念太陌生了，我不能找到一种方式来产生这样一个表达方式。
有没有人可以给我看？
非常感谢。 / p>
编辑
这是呼叫上下文： / p>
var filter = PredicateUtils.True< Association>（）;
var expr = MyHelper.MyExprMethod（（Association x）=> x.Item.Type，（Association b）=> b.Users.Count，Item.MaxOccupationRange）;
filter = filter.And（expr）;
var associations = entities.Associations.Where（filter）;
返回关联.OrderBy（a => a.Id）.ToPagedList（pageNb，2）;
这里是最后一行显示的消息（ToPagedList）：参数'b'未绑定到指定的LINQ to Entities查询表达式
我想你需要重新参数化第二个表达式
public static Expression< Func< TSource，bool>> MyExprMethod< TSource，TKey，TKey2，TValue>（表达<功能>源，TKey>源，表达< Func< TSource，TKey2>> source2，IReadOnlyDictionary< TKey，TValue> dict）
{
source2 = PredicateRewriter.Rewrite（source2，source.Parameters [0]）;
var body = dict
通过助手
public class PredicateRewriter
{
public static Expression< Func< T，U>>重写< T，U>（表达式< Func< T，U> exp，// string newParamName
ParameterExpression param）
{
// var param = Expression.Parameter参数[0] .Type，newParamName）;
var newExpression = new PredicateRewriterVisitor（param）.Visit（exp）;
return（Expression< Func< T，U>）newExpression;
}
与访客模式
私人类PredicateRewriterVisitor：ExpressionVisitor
{
private readonly ParameterExpression _parameterExpression;
public PredicateRewriterVisitor（ParameterExpression parameterExpression）
{
_parameterExpression = parameterExpression;
}
protected override表达式访问参数（ParameterExpression node）
{
return _parameterExpression;
}
}
}
I'm currently working on an EF website where I have this database object:
public partial class Association
{
public Association()
{
this.Users = new HashSet<User>();
}
public Guid Id { get; set; }
public Guid ItemId { get; set; }
[ForeignKey("ItemId")]
public virtual Item Item { get; set; }
[InverseProperty("Association")]
public virtual ICollection<User> Users { get; set; }
}
An Item class:
public partial class Item
{
public Item()
{
this.Associations = new HashSet<Association>();
}
public Guid Id { get; set; }
public ItemTypeEnum Type { get; set; }
[InverseProperty("Item")]
public virtual ICollection<Association> Associations { get; set; }
}
And this dictionary:
public static Dictionary<ItemTypeEnum, int> MaxOccupationRange = new Dictionary<ItemTypeEnum, int>()
{
{ ItemTypeEnum.Type1, 1 },
{ ItemTypeEnum.Type2, 2 },
{ ItemTypeEnum.Type3, 5 },
{ ItemTypeEnum.Type4, 10 },
};
I would need an Expression<Func<Association, bool>> that would return true if association.Users.Count < MaxOccupationRange[association.Item.Type], and else false.
I think this should roughly look like the following method but unfortunately, I'm not familiar enough with the Expression concept to achieve this kind of thing.
When debugging the following result, it seems like it computes (an not optimized but) correct expression.
public static Expression<Func<TSource, bool>> MyExprMethod<TSource, TKey, TKey2, TValue>(Expression<Func<TSource, TKey>> source, Expression<Func<TSource, TKey2>> source2, IReadOnlyDictionary<TKey, TValue> dict)
{
var body = dict
.Aggregate((Expression)null, (next, dicEntry) => next == null
? Expression.Condition
(
Expression.LessThan(source2.Body, Expression.Constant(dicEntry.Value)),
Expression.Constant(true),
Expression.Constant(false)
)
: Expression.Condition
(
Expression.Equal(source.Body, Expression.Constant(dicEntry.Key)),
Expression.Condition
(
Expression.LessThan(source2.Body, Expression.Constant(dicEntry.Value)),
Expression.Constant(true),
next
),
next
)
);
return Expression.Lambda<Func<TSource, bool>>(body, source.Parameters[0]);
}
I try to call it this way:
var expr = MyHelper.MyExprMethod((Association x) => x.Item.Type, (Association b) => b.Users.Count, Item.MaxOccupationRange);
But obviously it complains about the b parameter to be unassigned since it does not appear in the result lambda.
I tried many things (anonymous type instead of two sources etc.) but it seems that I'm too unfamiliar with the concept, I could not find a way to generate such an expression.
Would anybody please show me ?
Thanks a lot.
EDIT
Here's the call context:
var filter = PredicateUtils.True<Association>();
var expr = MyHelper.MyExprMethod((Association x) => x.Item.Type, (Association b) => b.Users.Count, Item.MaxOccupationRange);
filter = filter.And(expr);
var associations = entities.Associations.Where(filter);
return associations .OrderBy(a => a.Id).ToPagedList(pageNb, 2);
And here's the message, shown on the last line (ToPagedList): The parameter 'b' was not bound in the specified LINQ to Entities query expression
I guess you need to re-parametrize the second expression
public static Expression<Func<TSource, bool>> MyExprMethod<TSource, TKey, TKey2, TValue>(Expression<Func<TSource, TKey>> source, Expression<Func<TSource, TKey2>> source2, IReadOnlyDictionary<TKey, TValue> dict)
{
source2 = PredicateRewriter.Rewrite(source2, source.Parameters[0]);
var body = dict
through a helper
public class PredicateRewriter
{
public static Expression<Func<T, U>> Rewrite<T,U>(Expression<Func<T, U>> exp, //string newParamName
ParameterExpression param)
{
//var param = Expression.Parameter(exp.Parameters[0].Type, newParamName);
var newExpression = new PredicateRewriterVisitor(param).Visit(exp);
return (Expression<Func<T, U>>)newExpression;
}
with a Visitor pattern
private class PredicateRewriterVisitor : ExpressionVisitor
{
private readonly ParameterExpression _parameterExpression;
public PredicateRewriterVisitor(ParameterExpression parameterExpression)
{
_parameterExpression = parameterExpression;
}
protected override Expression VisitParameter(ParameterExpression node)
{
return _parameterExpression;
}
}
}
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