問題描述

我之前使用過基于 lamdas 的 C# 表達式，但我沒有手工編寫它們的經(jīng)驗.給定一個 Expression>originalPredicate，我想創(chuàng)建一個Expression>翻譯謂詞.

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.

在這種情況下 SomeType 和 OtherType 具有相同的字段，但它們不相關(guān)(沒有繼承，也不是基于公共接口).

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:我有一個基于 LINQ to SQL 的存儲庫實現(xiàn).我將 LINQ to SQL 實體投影到我的模型實體，以將我的模型保存在 POCO 中.我想將表達式傳遞給存儲庫(作為規(guī)范的一種形式)，但它們應該基于模型實體.但我無法將這些表達式傳遞給數(shù)據(jù)上下文，因為它需要基于 LINQ to SQL 實體的表達式.

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.

推薦答案

用Expression，最簡單的方法就是用一個轉(zhuǎn)換表達式:

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 });
    }
}

但是請注意，不同的提供商會對此提供不同的支持.例如，EF 可能不喜歡它，即使 LINQ-to-SQL 喜歡.

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

另一種選擇是完全重建表達式樹，使用反射找到對應的成員.復雜得多.

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

這篇關(guān)于C#如何轉(zhuǎn)換一個表達式<Func<SomeType>>到表達式<Func<OtherType>>的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網(wǎng)！