如何创建将返回公共接口派生类型的工厂类

Question

I have an Interface and it is implemented by many classes. Each class also has its own set of properties which are not present in interface. Now, if I want to design a factory which returns of type interface, I cannot set some of the derived class properties as they are not member of interface.

How to address this scenario?

Answer 1

If you have an Interface like

public interface MyInterface 
{
    string Name { get; }
}

and implementations like

public class MyClass : MyInterface
{
    string Name { get; set; }
    int Something { get; set; }
}

public class MySecondClass : MyInterface
{
    string Name { get; set; }
    decimal SomethingElse { get; set; }
}

you can create your class in the factory like

public class MyFactory
{
    public MyInterface createMyClass() 
    {
        return new MyClass() { Name = "foo", Something = 42 };
    }

    public MyInterface createMySecondClass() 
    {
        return new MySecondClass() { Name = "bar", SomethingElse = 4.2M };
    }
}

Of course this way you can't access the members you don't have declared in your interface.

var something = myFactory.createMyClass().Something;

This wouldn't work. You can only access the name property:

var name = myFactory.createMyClass().Name;

If you would want to access the special property of your class, you would have to cast your interface to the actual class:

var something = ((MyClass)myFactory.createMyClass()).Something;