C＃：隐含运算符和扩展方法 [英] C#: implicit operator and extension methods

问题描述

我想创建一个 PredicateBuilder< T> 类包装了一个表达式来; Func键< T，BOOL>> 并提供了一​​些方法，可以轻松地建立各种和和或者方法的表达式。我认为这将是冷静，如果我可以用这个 PredicateBuilder< T> 为表达式来; Func键< T，BOOL>> 直接，认为这可以由具有隐含的运营商的方法做事情。

I am trying to create a PredicateBuilder<T> class which wraps an Expression<Func<T, bool>> and provides some methods to easily build up an expression with various And and Or methods. I thought it would be cool if I could use this PredicateBuilder<T> as an Expression<Func<T, bool>> directly, and thought this could be done by having an implicit operator method thing.

简化版的类看起来是这样的：

Stripped down version of the class looks like this:

class PredicateBuilder<T>
{
    public Expression<Func<T, bool>> Predicate { get; protected set; }

    public PredicateBuilder(bool initialPredicate)
    {
        Predicate = initialPredicate 
            ? (Expression<Func<T, bool>>) (x => true) 
            : x => false;
    }

    public static implicit operator Expression<Func<T, bool>>(
        PredicateBuilder<T> expressionBuilder)
    {
        return expressionBuilder.Predicate;
    }
}

然后，就像一个测试，我有这个延伸方法在静态类：

Then, just as a test, I have this extention method in a static class:

public static void PrintExpression<T>(this Expression<Func<T, bool>> expression)
{
    Console.WriteLine(expression);
}

在我的头上，我应该再能够做到这些：

In my head, I should then be able to do these:

var p = new PredicateBuilder<int>(true);

p.PrintExpression();
PredicateExtensions.PrintExpression(p);

但他们没有工作。对于第一个，扩展方法是找不到的。而对于第二个，它说，

However none of them work. For the first one, the extension method is not found. And for the second, it says that

的类型参数的方法ExtravagantExpressions.PredicateHelper.PrintExpression（System.Linq.Expressions.Expression> ）'不能从使用推断。 。尝试显式指定类型参数

The type arguments for method 'ExtravagantExpressions.PredicateHelper.PrintExpression(System.Linq.Expressions.Expression>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

所以我尝试以下，这工作：

So I tried the following, which worked:

PredicateExtensions.PrintExpression<int>(p);

此外，这部作品，当然：

Also, this works, of course:

((Expression<Func<int, bool>>) p).PrintExpression();

但是，是的......为什么不别人打工？我误解了一些关于如何隐含的运营商东西的作品？

推荐答案

这不是特定于扩展方法。 C＃不会隐式转换一个对象为另一种类型，除非有关于目标类型的线索。假设如下：

This is not specific to extension methods. C# won't implicitly cast an object to another type unless there is a clue about the target type. Assume the following:

class A {
    public static implicit operator B(A obj) { ... }
    public static implicit operator C(A obj) { ... }
}

class B {
    public void Foo() { ... }
}

class C {
    public void Foo() { ... }
}

您期望哪种方法在下面的语句？

Which method would you expect to be called in the following statement?

new A().Foo(); // B.Foo? C.Foo? 

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