用Expression< Func< T,bool>>组成调用.与Func< bool>相同. [英] Composing invocations with Expression<Func<T,bool>> the same way as Func<T,bool>
问题描述
考虑一个可用作多个其他类的成员的类:
Consider a class that can be used as a member of multiple other classes:
class Customer {
public string FirstName {get;set;}
public string LastName {get;set;}
}
// Both "Order" and "Profile" have a "Customer" property
class Order {
public Customer Customer {get;set;}
}
class Profile {
public Customer Customer {get;set;}
}
我想定义一个方法来检查与Customer
关联的对象.如果我想要一个内存中检查器,可以这样:
I want to define a method that makes a checker for an object associated with a Customer
. If I want an in-memory checker, I do it like this:
static Func<T,bool> Check<T>(Func<T,Customer> conv, string first, string last) {
return obj => conv(obj).FirstName == first && conv(obj).LastName == last;
}
我可以将检查器用于内存中序列,如下所示:
I can use my checker for in-memory sequences as follows:
var matchingOrders = orders
.Where(Check<Order>(x => x.Customer, "Foo", "Bar"))
.ToList();
var matchingProfiles = profiles
.Where(Check<Profile>(x => x.Customer, "Foo", "Bar"))
.ToList();
现在我想对Expression<Func<T,bool>>
做同样的事情:
Now I want to do the same thing with Expression<Func<T,bool>>
:
static Expression<Func<T,bool>> Check<T>(Expression<Func<T,Customer>> conv, string first, string last)
不幸的是,相同的技巧不起作用:
Unfortunately, the same trick does not work:
return obj => conv(obj).FirstName == first && conv(obj).LastName == last;
并像这样使用它:
var matchingOrders = dbContext.Orders
.Where(Check<Order>(x => x.Customer, "Foo", "Bar"))
.ToList();
var matchingProfiles = dbContext.Profiles
.Where(Check<Profile>(x => x.Customer, "Foo", "Bar"))
.ToList();
这会触发错误:
CS0119:表达式表示预期存在
variable', where a
方法组
我可以像编写委托一样来编写表达式吗?
Can I compose expressions the same way that I compose delegates?
推荐答案
不幸的是,C#当前不提供从Expression<Func<...>>
对象组成表达式的方法.您必须使用更长的表达式树:
Unfortunately, C# does not currently provide a way to compose expressions from Expression<Func<...>>
objects. You have to use expression trees, which is quite a bit longer:
static Expression<Func<T,bool>> CheckExpr<T>(Expression<Func<T,Customer>> conv, string first, string last) {
var arg = Expression.Parameter(typeof(T));
var get = Expression.Invoke(conv, arg);
return Expression.Lambda<Func<T,bool>>(
Expression.MakeBinary(
ExpressionType.AndAlso
, Expression.MakeBinary(
ExpressionType.Equal
, Expression.Property(get, nameof(Customer.FirstName))
, Expression.Constant(first)
)
, Expression.MakeBinary(
ExpressionType.Equal
, Expression.Property(get, nameof(Customer.LastName))
, Expression.Constant(last)
)
)
, arg
);
}
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