What type does the 'this' keyword actually refer to?

Question

I have the following code

public class Base {

    public Base() {}

    public virtual void IdentifyYourself() {
         Debug.Log("I am a base");
    }

    public void Identify() { this.IdentifyYourself(); }
}

public class Derived : Base {

    public Derived() {}

    public override void IdentifyYourself() {
        Debug.Log("I am a derived");
    }
}

I run the following test code in a different entrypoint:

Base investigateThis = new Derived();
investigateThis.Identify()

and the output is: "I am a derived"

So no matter where the C# 'this' keyword is used; does it always refer to the run-time type no matter what scope 'this' is used in?

Bonus points to anyone who was able to 'Google' better than me and find MSDN documentation on specifically 'this' (pun intended) behavior.

Lastly, does anyone happen to know what is happening under the hood? Is it just a cast?

Update #1: Fixed typo in code; With the current set of answers, I guess I did not fully understand the implications of what the MSDN documentation meant by "..is the current instance..".

Update #2: Apologies, I wasn't sure if I should have made a separate question, but on further investigation, I've confused myself again; given this updated code, why is the output both: "I am a derived" & "It is a base!".

Didn't other people answer that 'this' is indeed the run-time type? Let me know if my updated question still is not clear.

Updated code:

public class Base {

    public Base() {}

    public virtual void IdentifyYourself() {
         Debug.Log("I am a base");
    }

    //Updated the code here...
    public void Identify() { this.IdentifyYourself(); AnotherTake(); }

    public void AnotherTake() { WhatIsItExactly(this); }

    public void WhatIsItExactly(Derived thing) {
         Debug.Log("It is a derived!");
    }

    public void WhatIsItExactly(Base thing) {
         Debug.Log("It is a base!");
    }
}

public class Derived : Base {

    public Derived() {}

    public override void IdentifyYourself() {
        Debug.Log("I am a derived");
    }
}

It's a logical thing: what could `this` refer to than the current object? It's not `that`, it's `this`. Your question isn't really about `this` even, it's about virtual methods. — Sami Kuhmonen, Oct 03 '16 at 14:33
google `c# this` and this (pun intended) was the first result https://msdn.microsoft.com/en-us/library/dk1507sz.aspx — Nkosi, Oct 03 '16 at 14:33
Your derived class does not derive from anything other than object at the moment. — Nkosi, Oct 03 '16 at 14:35
@Nkosi yep *"The this keyword refers to the current instance of the class..."* — Zack, Oct 03 '16 at 14:36
I would ask Update #2 as a separate question since it's now about run-time binding versus polymorphism. — D Stanley, Oct 03 '16 at 19:01

score 1 · Accepted Answer · answered Oct 03 '16 at 14:27

1

Absolutely! investigateThis refers to an instance of Derived.

So the virtual method IdentifyYourself in Derived will be called. This is run-time polymorphism in effect.

The scope does not matter.

Under the hood, an virtual function table is built, and there is a pointer in the object that points to that table.

answered Oct 03 '16 at 14:27

This was generally helpful information, but with my recent code update; I'm confused again on the implications of using 'this' – AMemberofDollars Oct 03 '16 at 18:42

Callum Bradbury · Answer 2 · 2016-10-03T14:47:16.220

'this' always refers to the current instance, whereas 'base' always refers to the inherited type, regardless of whether the code using 'this' is in the base or the child class, it will always refer to the child (unless the base is instantiated itself, of course). It's just a reference to the current instance, like 'self' in python. Useful if a parameter has the same name as a private field, but as far as I'm aware it has no functional purpose other than that, I use it for readability, to clearly show when something belongs to the class I'm working in.

score 0 · Answer 3 · answered Oct 03 '16 at 14:43

if you google: c# this the following link is the first result returned.

https://msdn.microsoft.com/en-us/library/dk1507sz.aspx

The this keyword refers to the current instance of the class and is also used as a modifier of the first parameter of an extension method.

You may also want to take a look at base while you are at it.

https://msdn.microsoft.com/en-us/library/hfw7t1ce.aspx

The base keyword is used to access members of the base class from within a derived class:

Call a method on the base class that has been overridden by another method.

Specify which base-class constructor should be called when creating instances of the derived class.

What type does the 'this' keyword actually refer to?

3 Answers3