将派生类的实例分配给基类的变量

Question

Hello I am newbie to C# and I'm trying to figure out the "implicit conversion" operation. I have a question about it:

    class Animal { }
    class Monkey : Animal { }

    Monkey m = new Monkey();
    Animal a = m; 
    m.GetType()
    [Submission#165+Monkey]
    a.GetType()
    [Submission#165+Monkey]

Monkey m2 = a;// this calls - Compiler Error CS0266. Cannot implicitly convert type 'Animal' to 'Monkey'. An explicit

conversion exists (are you missing a cast?)

I don't understand - if last code line throws an error CS0266, why does GetType method returns that "a" variable has a type "Monkey". If "a" variable is Animal how to find it out? By what method?

Answer 1

GetType() is evaluated at runtime. The error you are getting is at compile time, which is before the program ever runs.

The compiler needs to ensure type safety with the information it has at compile time. Ensuring means making sure nothing can go wrong . Assigning an Animal to a Monkey is, in general, not safe, because a Tiger is also an animal, so you could theoretically end up assigning a tiger to a monkey typed variable.

You can tell the compiler that although the assignment isn't safe, you know what you are doing. You do this with an explicit cast:

Monkey m2 = (Monkey)a;

Here you are telling the compiler; "Hey, I know this isn't generally safe, but trust me, I know a is a Monkey ".

The compiler will accept your promise but won't trust you fully, so a type check will be performed at runtime just to make sure, and if your promise is a lie, you'll get a runtime error.

Answer 2

The type of the variable is Animal , the compiler will not allow you to assign this to a Monkey because an Animal is not necessarily a Monkey . You could assign the other way around because a Monkey is always an Animal . GetType() returns the runtime type of the object the variable points to which the compiler does not, and cannot, know anything about.