逆变如何与 .net 核心中的 Func 委托一起使用 [英] How does contravariance work with Func delegate in .net core

问题描述

我有以下一段代码，我正在尝试为我的域对象编写通用验证规则.这样做时，我有一个问题来处理 Func 委托支持差异

I have the following piece of code where I am trying to write a generic validation rule for my domain objects. while doing so I have an issue to deal Func delegate supporting variance

public class Person { }
public class Employee : Person { }

internal interface IValidation<T> where T: Person  
{
     void AddValidationRule(Func<T,bool> predicateFunction);
}

internal class BaseValidation : IValidation<Person>
{
    void IValidation<Person>.RegisterValidationRules(Person person)
    {

    }
}

internal class EmployeeValidation : BaseValidation
{
    void RegisterValidation()
    {
        Func<Employee,bool> empPredicate = CheckJoiningDate;
        base.AddValidationRule(empPredicate);
    }

    bool CheckJoiningDate(Employee employee)
    {
        return employee.JoiningDate > DateTime.Now.AddDays(-1) ;
    }
}

有了上面的代码，编译器给出一个错误信息说

With the above code in place, compiler gives an error message saying

在线编译器错误:base.AddValidationRule(empPredicate);参数 1:无法从 'System.Func<>Employee, bool>' 转换为'System.Func<>Person, bool>

Compiler Error on line : base.AddValidationRule(empPredicate); Argument 1: cannot convert from 'System.Func<>Employee, bool>' to 'System.Func<>Person, bool>

我提到了这个 https://docs.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2010/dd465122%28v%3dvs.100%29 但我还是做不到't 让编译器理解这里的逆变，

I had referred to this https://docs.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2010/dd465122%28v%3dvs.100%29 but I still couldn't make the compiler to understand about the contravariance here,

感谢您的帮助，让我更好地理解这一点

Appreciate your help so I understand this better

推荐答案

无法从 'System.Func<>Employee, bool>' 转换为 'System.Func<>Person, bool>

cannot convert from 'System.Func<>Employee, bool>' to 'System.Func<>Person, bool>

base.AddValidationRule 需要一个可以对任何 Person 进行操作的函数.您的函数只能对限制性更强的 Exployee 进行操作.这是错误的方差形式.

base.AddValidationRule requires a function that can operate on any Person. Your function can only operate on Exployee which is more restrictive. This is the wrong form of variance.

此处未显示，但可能 BaseValidation 已实现 IValidation.

It is not shown here but likely BaseValidation implemented IValidation<Person>.

可能，最好的解决方法是确保您从 IValidation 继承，可能是通过使 BaseValidation 通用.

Likely, the best fix is to ensure that you inherit from IValidation<Employee>, possibly by making BaseValidation generic.

这行得通吗?

internal class BaseValidation<T> : IValidation<T>
{
    void IValidation<T>.RegisterValidationRules(T entity)
    {
    }
}

internal class EmployeeValidation : BaseValidation<Employee>
{
 //...
}

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