C# When To Use "This" Keyword

Question

Possible Duplicate:
When do you use the “this” keyword?

Hello, I understand that the This keyword is used to refer to an instance of the class, however, suppose I have a class called Life, which defines two fields, the person (their name) and their partner(their name):

class Life
{
    //Fields
    private string _person;
    private string _partner;

    //Properties
    public string Person
    {
        get { return _person; }
        set { _person = value; }
    }

    public string Partner
    {
        get { return _partner; }
        set { _partner = value; }
    }

    //Constructor 1
    public Life()
    {
        _person = "Dave";
        _partner = "Sarah";

        MessageBox.Show("Life Constructor Called");
    }

    //Constructor 2
    public Life()
    {
        this._person = "Dave";
        this._partner = "Sarah";

        MessageBox.Show("Life Constructor Called");
    }
}

Is there a difference between constructor 1 and constructor 2!? Or is it just better coding practice to use the "This" keyword?

Regards

Solution 1

The constructors are the same. The reason I would prefer the second is that it will allow you to remove the underscores from your private variable names and retain the context (improving understandability). I make it a practice to always use this when referring to instance variables and properties.

I no longer use the this keyword in this way after moving to a different company with different standards. I've gotten used to it and now rarely use it at all when referring to instance members. I do still recommend using properties (obviously).

My version of your class:

class Life
{
    //Fields
    private string person;
    private string partner;

    //Properties
    public string Person
    {
        get { return this.person; }
        set { this.person = value; }
    }

    public string Partner
    {
        get { return this.partner; }
        set { this.partner = value; }
    }


    public Life()
    {
        this.person = "Dave";
        this.partner = "Sarah";

        MessageBox.Show("Life Constructor Called");
    }
}

or, even better, but not as clear about the use of this with fields.

class Life
{

    //Properties
    public string Person { get; set; }
    public string Partner { get; set; }

    public Life()
    {
        this.Person = "Dave";
        this.Partner = "Sarah";

        MessageBox.Show("Life Constructor Called");
    }
}

Solution 2

"this" is also used in .Net 3.5 with extension methods:

public static class MyExtensions
{    
    public static string Extend(this string text)
    {
       return text + " world";
    }
}

would extend the string class

var text = "Hello";
text.Extend();

To answer your question: no, there is no difference in your two constructors. Imo, the "this" clutters the code and should only be used when necessary, e.g. when parameters and field variables have the same names.

There is also a case when the class explicitly implements an interface. If you need to call the interface methods from within your class you would have to cast this to the interface:

class Impl : IFace
{

    public void DoStuff()
    {
        ((IFace)this).SomeMethod();
    }

    void IFace.SomeMethod()
    {
    }
}

Solution 3

There is no difference in the two statements...

//These are exactly the same.

this._person 

//and 

_person 

The reference to "this" is implied in the case of _person. I wouldn't say that it is necessarily "better" coding practice, I would say that it is just preference.

Solution 4

Already discussed

When do you use the "this" keyword?

Solution 5

Since you are using underscores, there is no conflict between the names; so the "this." is redundant and can be omitted. The IL will be unaffected.

As long as there is no ambiguity between a field and variable/parareter, there is only one scenario in which the this keyword (in the context of meaning the current instance - not ctor-chaining) is strictly necessary - invoking an extension method that is defined separately:

this.SomeExtensionMethod();  // works
SomeExtensionMethod();  // fails

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