为重载方法将对象转换为适当的类型

Question

Say I have a method that is overloaded such as void PrintInfo(Person) and void PrintInfo(Item) , and so on. I try to invoke these methods by passing in an Object.

I'm wondering why it is giving me an error when I do this; aren't all classes inherited from Object? I want to avoid doing an if/switch statement where I check which type the Object is before calling the appropriate method.

What do you guys think is the best approach in this case?

Answer 1

All Person s are object s , but not all object s are Person s. Because of this you can pass a Person to a method that accepts an object but you can't pass an object to a method that requires a Person .

It sounds like you have some common bit of functionality between various objects that you want to use. Given this, it would be best to find either a common ancestor that has all of the functionality that you need, or an interface that they all implement (that again provides everything that you need).

In the case of printing, you may just need the ToString method. In that case, you can just have the method accept an object and call ToString on it. (That's what many print methods do, such as Console.WriteLine .

Answer 2

You need to understand that because C# is a statically typed language (barring dynamic ) the particular overload that is chosen (called overload resolution ) is determined at compile time, not run time. That means that the compiler needs to be able to unequivocally determine what type your argument is. Consider:

Object foo;
foo = "String";
foo = 5;
PrintInfo(foo); // Which overload of printinfo should be called?  The compiler doesn't know!

There are a few ways to solve this- making foo of type dynamic is one- that will cause the correct overload to be chosen at compile time. The problem with that is that you lose type safety- if you don't have an appropriate overload for that type, your application will still compile but will crash when you try to print the unsupported type's info.

An arguably better approach is to ensure that foo is always of the correct type, rather than just Object .

As @Servy suggests, another approach is to attach the behavior to the type itself. You could, for instance, make an interface IHasPrintInfo :

public interface IHasPrintInfo { String PrintInfo { get; } }

and implement that interface on all items whose info you might print. Then your PrintInfo function can just take an IPrintInfo:

public void PrintInfo(IPrintInfo info) { 
    Console.WriteLine(info.PrintInfo);
}

Answer 3

here its ambiguate for compiler; compiler can't figure out which version of method (Person/Item) you are intended to call.