委托和查询谓词 [英] Delegates and Predicate enquiry

问题描述

我了解lambda表达式，我试图了解为什么，虽然具有EmployeeFilterAge类，像这样：

I'm learning about lambdas and I'm trying to understand why, while having EmployeeFilterAge class like so:

public class EmployeeFilterAge
{
    int _age;
    public EmployeeFilterAge(int age)
    {
        _age = age;
    }
    public bool OlderThan(Employee employee)
    {
        return employee._age > _age;
    }
}

当我试图做到这一点：

var filterByAge = new EmployeeFilterAge(29);
var del = new AgeExclusion(filterByAge.OlderThan);

我得到一个错误说不重载方法匹配... ...委托，而代表看起来像：

I get an error saying "no overload method ... matches delegate ...", while delegate looks like that:

public delegate bool AgeExclusion(object person);

这是因为它的谓词？

Is this because its a predicate?

推荐答案

委托说，参数可以是任何类型的。当有人调用方法，他们可以在一个狮子，老虎，熊（噢，我的！），或者其他任何他们想要的，你的方法需要能够处理它传递。

The delegate says that the parameter can be of any type. When someone invokes the method they can pass in a Lion, a Tiger, a Bear (Oh My!!!), or whatever else they want, and your method needs to be able to handle it.

您正试图分配一个永远只能接受一个方法的员工，那是不允许接受任何其他类型作为参数，所以很明显它不是分配给委托类型的变量的一种有效方法。

You're trying to assign a method that can only ever accept an Employee, and that isn't allowed to accept any other type as an argument, so obviously it's not a valid method to assign to a variable of that delegate type.

现在如果类型互换，这将是罚款。如果代表说，它可以接受，需要一个方法只有一个员工作为参数，那么你可以把它认为是能够利用的什么方法作为参数，因为毕竟，所有的员工，它会通过，这将是一个方法有效参数，可以接受的任何的。这种情况的技术术语是逆变。

Now if the types were reversed, it would be fine. If the delegate said that it could accept a method that takes only an Employee as an argument, then you could give it a method that is able to take anything as an argument, because after all, all of the employees that it will pass it would be valid arguments for a method that can accept anything. The technical term for this is contravariance.

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