如何在C＃中链接方法时强制转换

Question

I'm trying to code something, and I always return the object itself so I can keep chaining. Like this

object.SetThis().SetThat().SetThisThirdThing().setThisFourthThing();

I return this in all my methods so I can keep doing this. But one of my methods is something I made in my base class, which of course then returns the base class.

So instead of public MyClass SetThat() it returns public SuperClass SetThat() . And because it returns SuperClass and not MyClass I can't call SetThisThirdThing() because the base class doesn't know about it.

So how do I cast it so I can keep the chain? What is the syntax? I tried

object.SetThis().(MyClass)SetThat().SetThisThirdThing().setThisFourthThing();

Or is there a way to make a superClass method return the subclass when called from the subclass without having to override it in all the subclasses?

This is one of the things that all the subclasses have in common, so it would be really nice if I would be able to circumvent this somehow without having to override it in all my subclasses.

Answer 1

is there a way to make a superClass method return the subclass when called from the subclass without having to override it in all the subclasses?

Can you make the superclass generic?

public class SuperClass<T> where T: SuperClass<T>
{
    public T SetThis()
    {
        ....
        return (T)this;
    }
}

public class SubClass : SuperClass<SubClass>
{
}

Note that it's not 100% guaranteed since you could also do this:

public class EvilSubClass : SuperClass<SubClass>
{
}

which fits the generic contraints, but now the return type of SetThis() is SubClass and not EvilSubClass

Answer 2

var obj = ((MyType)myObject.SetThis()).SetThat();

PS你应该避免这样做，因为阅读可能会让人感到困惑。

Answer 3

试试这样：

((MyClass)object.SetThis().SetThat()).SetThisThirdThing()

Answer 4

To cast a (sub)expression, you need to put the class name in front (and use parentheses).

((MyClass)obj.SetThis().SetThat()).SetThisThirdThing().setThisFourthThing();

(note that object is a reserved word, so you can't use it as a variable name)

An alternative is to use the as keyword:

(obj.SetThis().setThat() as MyClass).SetThisThirdThing().setThisFourthThing();

Note that this will behave differently if setThat() doesn't return a MyClass ; the first version will throw an InvalidCastException , the second version a NullReferenceException because as returns null if the cast is not possible.

Answer 5

Just use parenthesis:

(MyClass)(object.SetThis().SetThat()).SetThisThirdThing().setThisFourthThing();

I don't know why the object is there, but you can safely remove it.

(MyClass)(SetThis().SetThat()).SetThisThirdThing().setThisFourthThing();

Answer 6

Without modifying your classes, you can also write an extension method As like this

public static T As<T>(this MyBaseClass obj) where T : MyBaseClass
{
   return (T)obj;
}

--

Usage would be similar to this

new MyChildClass().SetBase().As<MyChildClass>().SetThis();

Answer 7

Why not do it like this? A simple Cast method to help you cast it back to the correct type...

class Program
{
    static void Main(string[] args)
    {
        var obj = new MyClass();
        //obj.SetThis().SetThat().SetThisThirdThing().setThisFourthThing(); // Compile error
        obj.SetThis().SetThat().Cast<MyClass>().SetThisThirdThing().setThisFourthThing();
    }
}

class SuperClass
{
    public SuperClass SetThat()
    {
        return this;
    }

    public T Cast<T>() where T : SuperClass
    {
        return (T)this;
    }
}

class MyClass : SuperClass
{
    public MyClass SetThis()
    {
        return this;
    }

    public MyClass SetThisThirdThing()
    {
        return this;
    }

    public MyClass setThisFourthThing()
    {
        return this;
    }
}