C＃.NET - 如何使用typeof（）来继承？

Question

I will start by explaining my scenario in code:

public class A { }

public class B : A { }

public class C : B { }

public class D { }

public class Test
{
    private A a = new A ( ) ;
    private B b = new B ( ) ;
    private C c = new C ( ) ;
    private D d = new D ( ) ;

    public Test ( )
    {
        // Evaluates to "false"
        if ( a.GetType == typeof(B) ) { } //TODO: Add Logic

        // Evaluates to "true"
        if ( b.GetType == typeof(B) ) { } //TODO: Add Logic

        // I WANT this to evaluate to "true"
        if ( c.GetType == typeof(B) ) { } //TODO: Add Logic

        // Evaluates to "false"
        if ( d.GetType == typeof(B) ) { } //TODO: Add Logic
    }
}

The important line to take notice of here is:

if ( c.GetType == typeof(B) ) { }

I believe that this will in fact evaluate to "false", since typeof(B) and typeof(C) are not equal to each other in both directions. (C is a B, but B is not necessarily a C.)

But what I need is some kind of condition that will take this into account. How can I tell if an object is a B or anything derived from it?

I don't care if it is an object DERIVED from B, so long as the base B class is there. And I can't anticipate what derived class might show up in my application. I just have to assume that unkown derived classes may exist in the future - and therefore I can only focus on making sure that the base class is what I am expecting.

I need a condition that will perform this check for me. How can this be accomplished?

Answer 1

You can just use is :

if (c is B) // Will be true

if (d is B) // Will be false

Answer 2

Edit: this answers the question in the thread title. cdm9002 has the better answer to the problem as described in the full post.

typeof(B).IsAssignableFrom(c.GetType())

Answer 3

这看起来像是一个多态性的工作，而不是一个带有特定类测试的大型switch语句。

Answer 4

As an alternative to the (c is B) check, you can also do the following:

var maybeB = c as B;
if (maybeB != null) {
   // make use of maybeB
}

This is preferred in some cases since in order to make use of c as a B when using is , you would have to cast anyway.

Answer 5

For VB.NET with Visual Studio 2008, you can check it like:

'MyTextBox control is inherited by Textbox
If Ctl.GetType.Name = "MyTextBox" then    

End If

Answer 6

typeof(B).IsInstanceOfType(c)

Similar to the answer above from sam-harwell, sometimes you may have the type "B" in a variable, so you need to use reflection rather than the "is" operator.

I used Sam's solution, and was pleasantly surprised when Resharper made this suggestion.