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針對 Func 與 Action 泛型委派使用變異數 (C#)

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下列範例示範如何在 Func 和 Action 泛型委派中使用共變數和反變數，以便在您的程式碼中重複使用方法並提供更多彈性。

如需共變數與反變數的詳細資訊，請參閱委派中的變異數 (C#)。

使用具有 Covariant 型別參數的委派

下列範例說明在泛型 Func 委派中支援共變數的好處。 FindByTitle 方法使用 String 類型的參數，並傳回 Employee 類型的物件。 不過，您可以將此方法指派給 Func<String, Person> 委派，因為 Employee 會繼承 Person。

// Simple hierarchy of classes.  
public class Person { }  
public class Employee : Person { }  
class Program  
{  
    static Employee FindByTitle(String title)  
    {  
        // This is a stub for a method that returns  
        // an employee that has the specified title.  
        return new Employee();  
    }  
  
    static void Test()  
    {  
        // Create an instance of the delegate without using variance.  
        Func<String, Employee> findEmployee = FindByTitle;  
  
        // The delegate expects a method to return Person,  
        // but you can assign it a method that returns Employee.  
        Func<String, Person> findPerson = FindByTitle;  
  
        // You can also assign a delegate
        // that returns a more derived type
        // to a delegate that returns a less derived type.  
        findPerson = findEmployee;  
  
    }  
}

使用具有 Contravariant 型別參數的委派

下列範例說明在泛型 Action 委派中支援反變數的好處。 AddToContacts 方法使用 Person 類型的參數。 不過，您可以將此方法指派給 Action<Employee> 委派，因為 Employee 會繼承 Person。

public class Person { }  
public class Employee : Person { }  
class Program  
{  
    static void AddToContacts(Person person)  
    {  
        // This method adds a Person object  
        // to a contact list.  
    }  
  
    static void Test()  
    {  
        // Create an instance of the delegate without using variance.  
        Action<Person> addPersonToContacts = AddToContacts;  
  
        // The Action delegate expects
        // a method that has an Employee parameter,  
        // but you can assign it a method that has a Person parameter  
        // because Employee derives from Person.  
        Action<Employee> addEmployeeToContacts = AddToContacts;  
  
        // You can also assign a delegate
        // that accepts a less derived parameter to a delegate
        // that accepts a more derived parameter.  
        addEmployeeToContacts = addPersonToContacts;  
    }  
}

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