.net核心中的Func委托如何使用协变 [英] How does contravariance work with Func delegate in .net core

问题描述

我有以下代码段，试图为我的域对象编写通用验证规则.在这样做的时候，我遇到一个问题来解决Func代表支持差异的问题

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) ;
    }
}

使用上面的代码，编译器会显示一条错误消息

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

行上的编译器错误:base.AddValidationRule(empPredicate);参数1:无法从'System.Func<> Employee，bool>'转换为'System.Func<>人员，布尔>

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

我已提到 https://docs.microsoft.com/zh-cn/previous-versions/visualstudio/visual-studio-2010/dd465122%28v%3dvs.100%29 但我仍然无法使得编译器在这里了解这种矛盾，

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，以便我更好地理解

Appreciate your help so I understand this better

推荐答案

不能从'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 需要一个可以在任何 Person 上运行的函数.您的函数只能在限制更严格的 Exployee 上运行.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，但可能是 BaseValidation 实现的 IValidation< Person> .

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

最好的解决方法是确保您从 IValidation< Employee> 继承，这可以通过使 BaseValidation 通用来实现.

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

这项工作可以吗?

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

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

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