C＃如何转换的表达式来; Func键<&SOMETYPE GT;>一个表达式来; Func键<&OTHERTYPE GT;>

问题描述：

我已经使用基于lamdas之前，C＃则表达式，但我没有经验用手将它们组成。给定一个表达式来; Func键&LT; SOMETYPE，布尔&GT;&GT; originalPredicate ，我想创建一个表达式来; Func键&LT; OTHERTYPE，布尔&GT;&GT; translatedPredicate 。

I have used C# expressions before based on lamdas, but I have no experience composing them by hand. Given an Expression<Func<SomeType, bool>> originalPredicate, I want to create an Expression<Func<OtherType, bool>> translatedPredicate.

在这种情况下，SOMETYPE和OTHERTYPE有相同的领域，但他们基于通用接口上不相关（不继承，不）。

In this case SomeType and OtherType have the same fields, but they are not related (no inheritance and not based on a common interface).

背景：我有一个基于LINQ到SQL存储库的实现。我投射的LINQ to SQL实体我的模型实体，以保持我的POCO模型。欲表达传递给库（作为规范的形式），但它们应该基于模型实体。但我不能通过这些表达式的数据上下文，因为它预期基础上的LINQ to SQL实体表达式。

Background: I have a repository implementation based on LINQ to SQL. I project the LINQ to SQL entities to my Model entities, to keep my model in POCO. I want to pass expressions to the repository (as a form of specifications) but they should be based on the model entities. But I can't pass those expressions to the data context, since it expects expressions based on the LINQ to SQL entities.

答

随着表达式，最简单的方法是用一个转换的表达式

With Expression, the simplest way is with a conversion expression:

class Foo {
    public int Value { get; set; }
}
class Bar {
    public int Value { get; set; }
}
static class Program {
    static void Main() {
        Expression<Func<Foo, bool>> predicate =
            x => x.Value % 2 == 0;
        Expression<Func<Bar, Foo>> convert =
            bar => new Foo { Value = bar.Value };

        var param = Expression.Parameter(typeof(Bar), "bar");
        var body = Expression.Invoke(predicate,
              Expression.Invoke(convert, param));
        var lambda = Expression.Lambda<Func<Bar, bool>>(body, param);

        // test with LINQ-to-Objects for simplicity
        var func = lambda.Compile();
        bool withOdd = func(new Bar { Value = 7 }),
             withEven = func(new Bar { Value = 12 });
    }
}

请注意，这将是由不同的供应商不同的支持。 EF可能不喜欢它，例如，即使LINQ到SQL一样。

Note however that this will be supported differently by different providers. EF might not like it, for example, even if LINQ-to-SQL does.

另外一种选择是重建表达式树的完全，使用反射来找到相应的成员。复杂得多。

The other option is to rebuild the expression tree completely, using reflection to find the corresponding members. Much more complex.

C＃如何转换的表达式来; Func键<&SOMETYPE GT;>一个表达式来; Func键<&OTHERTYPE GT;>

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