在另一个lambda表达式中使用lambda表达式 [英] Use lambda expression in another lambda expression

问题描述

我需要组合两个lambda表达式，方式是第二个表达式包含第一个。
我搜索了很多但没有找到任何明确的答案...

I need to combine two lambda expressions, in a way that the second expression "contains" the first. I searched a lot but did not find any clear answer...

我想要做的是如下：
第一个表达式expression1作为一个参数传递给一个方法，只用于定义第二个lambda必须在哪个字段或属性上运行。

What i am trying to do is the following : A first expression "expression1" is passed as a parameter to a method, and is simply used to define on which field or property the second lambda must operate.

原来，我试图请执行以下操作：

Schematically, I am trying to do the following :

// simple field selector :
Expression<Func<T, string>> expression1 = obj => obj.field; 

// trying to use the field selector in 2nd expression :
Expression<Func<T, bool>> e2 = obj => [[Result of expression1]].Equals("myValue");

换句话说，我想得到：

Expression<Func<T, bool>> e2 = obj => obj.field.Equals("myValue"); 

我需要这样做，因为它并不总是 Equals ）方法将被调用，但有许多不同的方法。

I need to do it this way because it is not always the Equals() method that will be called, but many different methods.

我尝试将expression1编译成一个 Func< T ，string> 为了在expression2中调用此Func，但是正如我在LinQ中使用此函数一样，我收到一个异常，因为LinQ不支持Invoke节点类型。

I tried to compile expression1 to a Func<T, string> in order to invoke this Func in expression2, but as I am using this with LinQ, I get an exception because LinQ does not support Invoke node types.

所以我的问题是：是否有一种方法来组合两个表达式的正文，以及如何？

So my question is : is there a way to just combine the bodies of the two expressions, and how please ?

感谢提前！ p>

Thanks by advance !

推荐答案

嗯，这应该是诀窍：

void Main()
{
    var execute = Create<TestClass>(
            first => first.MyStringField, // field selector
            second => second.Equals("1234") // method selector
        );

    Console.WriteLine(execute(new TestClass("1234"))); // true
    Console.WriteLine(execute(new TestClass("4321"))); // false
}

class TestClass
{
    public string MyStringField;

    public TestClass(string val){
        MyStringField = val;
    }

}

static Func<TSource, bool> Create<TSource>(
                    Expression<Func<TSource, string>> fieldSelector,
                    Expression<Func<string, bool>> methodSelector
                )
{
    // todo: classical validation of fieldSelector, if necessary.
    // todo: classical validation of methodSelector, if necessary.

    var compiledFieldSelector = fieldSelector.Compile();
    var compiledMethodSelector = methodSelector.Compile();

    return T => compiledMethodSelector(compiledFieldSelector(T));
}

注意，由于lambda表达式的本质，您需要验证字段选择器方法选择器，否则可以做一些非常奇怪的事情。

Note, due the nature of lambda expressions, you need to validate the field selector and method selector, otherwise it's possible to do some very weird things.

或者，下一个方法创建不会构成事物的lambda，在这个意义上，这是更好，因为LinqToEntities不应该解释查询问题。

Alternatively, the next approach creates lambda that does not "compose" things, in the sense, this is better since LinqToEntities shouldn't have problems interpreting the query.

 static Func<TSource, bool> Create<TSource>(
                    Expression<Func<TSource, string>> memberSelector,
                    Expression<Func<string, bool>> methodSelector
                )
{
    // todo: classical validation of memberSelector, if necessary.
    // todo: classical validation of methodSelector, if necessary.

    var memberExpression = (MemberExpression)(memberSelector.Body);
    var methodCallExpression = (MethodCallExpression)(methodSelector.Body);

    // input TSource => ..
    var input = Expression.Parameter(typeof(TSource));

    var call = Expression.Call(
                Expression.MakeMemberAccess(
                                input, 
                                memberExpression.Member), 

                methodCallExpression.Method, 
                methodCallExpression.Arguments);

    return Expression.Lambda<Func<TSource, bool>>(call, input)
                .Compile();
}

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