C#.NET - Inheritance - Number of objects created in memory

Question

Purpose of question is to understand how inheritance works under the hood; I am aware about what it is and when to use it.

Following is use case -

class A {}

class B : A {}

class C
{
B b = new B();
}

Now, how many objects (EXCLUDING the one for C class as it will be entry point and any default DotNet/CLR objects) are created in memory? Is it two (one for A and other for B)? Or is it only one for B which also contains members of A? Some explanation will help.

Answer 1

Since it seems that the purpose of your question is to basically to see the "physical" difference between using composition and inheritance, I'm going to focus on that.

When you use new, a single instance of the type is created, and the appropriate constructor of the type (and all of its "parents") is executed once.

In C#, the default inheritance approach (the B : A kind) is subclassing. In this approach, a derived class is basically a copy of its parent, plus the instance fields of the derived class, plus the metadata associated with e.g. any new virtual methods.

This means that in your case, calling new B() only creates a single object instance, and that's it. The instance of B contains within itself the fields and metadata of A, but not a reference to an instance of A.

If you define B2 like this:

class A2
{
  int myInt;
}

class B2
{
  A2 aInstance = new A2();
}

Then the B2 constructor also creates an instance of A2, so you have two object instances, one of type A2, another of type B2. B2 only contains a reference to the A2 instance, rather than a copy of it.

How does this translate to runtime costs?

• The second approach means a layer of indirection. This can impact data locality, though not in the usual case, due to the way .NET allocation works - in practice, A2 will tend to be allocated right behind B2.
• The second approach means you'll need a bit of extra metadata, since you have two instances instead of one. This basically means a pointer to a type handle and a syncblock index. This is a fixed cost per instance - 4 bytes for the syncblock, 4 bytes for the type handle. I'm not sure if this changes on 64-bit or not. Unless your instances have very little instance data, this is not a huge cost. I'm pretty sure this is not contractual, though, and in fact, the actual minimum size is 12 bytes, rather than 8 (or at least it used to be with early GCs).
• The second approach means an extra instance for the GC to worry about. I'm not sure how much of an impact this can have in practice - the GC still has to go through the same amount of memory, and I think that's more important for GC performance in practice than the amount of objects. But that's just my ballpark estimate :)
• The allocation cost of both should be pretty much the same, affording for the few extra bytes of instance metadata. .NET heap allocations are really more like stack allocations - you just shift a pointer. This is unlikely to make a difference, especially compared to the cost of collecting and compacting the memory :)

The result? Well, I don't think it's something you need to care about much in advance. There is a cost to having the extra instances, but unless you're allocating millions of instances, it probably isn't going to make much of an observable difference. If your application allows it, you might even have a net gain, since the composition model can allow you to reuse the same instance in multiple places which simply isn't possible with subclassing. Sometimes this makes sense, sometimes it doesn't :)

And of course, note that you don't always have to use classes. For example, A2 can easily be a struct, eliminating the extra instance - again, impossible with subclassing, since structs can't be inherited from. In that case, the two approaches become equivalent.

As is usually the case with performance, you really need to do practical profiling to get your answer. And the result will probably be something like "99.9% of the code performs fine, but this one class could save us a lot of CPU/RAM if we changed it to a struct and moved the polymorphism to a higher layer".

Finally, I'm pretty sure none of this is part of the contract. If Microsoft decides to change the way inheritance works in a future version of the .NET framework, and makes it create a new instance instead of "inlining" the parent, I don't think it's going to violate the specification in any way. Unless you absolutely need to depend on this information, don't.

Answer 2

For the sake of argument lets say you make an instance of C

var c = new C();

at this point you have two object instances, because during construction C makes an instance of B.

To answer your question, You have an instance of C and an instance of B. You don't have an instance of A, even though B is derived from A. (Update: Ignoring C and any reflection, you have one object instance of B. )

You can prove this with some code:

class A { }

class B : A { }

class C
{
    public B B = new B();
}

class Program
{
    static void Main(string[] args)
    {
        var c = new C();

        var b = c.B;

        var BasA = (A)b;

        bool BisA = BasA.GetType() == typeof (A);

        Console.WriteLine($"Assert That B is not A: {!BisA}");
    }
}

Plus, You can see all your memory via the debugger:

Also be careful with terminology. A, B and C are classes. Objects are instances of classes. In C# the information that describes a class can be encapsulated in an instance of a System.Type class.

So lets go down the rabbit hole a little; How many objects do you have in memory, in a executing assembly?

Limiting our scope to those classes only, in addition to the two objects you get from instanciating C, you will also have three instance of System.RuntimeType one for A, B and C:

  var assemblyTypes = Assembly.GetExecutingAssembly().GetTypes();

  foreach (var classType in assemblyTypes)
       Console.WriteLine("Type instance: " + classType);

Again, this is showing how you have three instance of System.RuntimeType, that describe the classes A, B and C.

Being pedantic, you will also get an instance of RuntimeAssembly (for your executable) and RuntimeType (for your console Program class), as well as others

Answer 3

If you ignore C (as program entry) there will be one B object and two System.RuntimeType objects that describe the A and B classes.

Refer to CLR via C#, Fourth edition, P100-110

Answer 4

New objects are created when you use keyword new.

The source code is more like a blueprint where for simplicity you specify that one object extends behavior and members of another object (inherits it). This information is needed for describing the type itself. The actual object is constructed based on the description of the type by using the new keyword.

In your case only one object will be created.