What's the Difference Between Encapsulating a Private Member as a Property and Defining a Property Without a Private Member

What's the difference between encapsulating a private member as a property and defining a property without a private member?

The code that the C# compiler generates for auto-implemented properties is almost identical to your first example (it uses a private, backing field), so I wouldn't worry about it too much.

The only real difference is that it decorates the property getter and setter with the [CompilerGenerated] attribute. This shouldn't have any impact on the performance of getting and setting the property. (As a minor nitpick, that should increase the size of the assembly's binary ever so slightly).

What I like about auto-implemented properties, other than brevity of course, is that it prevents even the declaring type from accessing the backing-field instead of the property (the backing-field is anonymous). This brings clarity to the code, and generally makes refactoring / changing the property implementation easier too.

What is difference in defining properties in these two methods in c#?

Both methods use the same number of variables, in the first version the extra variable is just hidden by the compiler, if you decompiled the code you would see it too uses a variable.

Defining both Private Field and Property vs just Property

There really is no difference. If no private variable is created by the user then the code will be generated automatically for the private field. However, if the user wishes to do additional logic in the getter or setter of the property then declaring a private field is necessary.

C# , Difference between property with variable and without variable

The later is called an Automatic Property and is the same.They were introduced in C#3, you can read more about them here: http://trashvin.blogspot.com/2008/05/automatic-properties-and-object.html

Simply put, Automatic Properties are syntactic sugar so the developer has to type less code and the compiler will generate the private field and the public setter and getter for you.

Property and Encapsulation

Encapsulation helps by insulating calling classes from changes.

Let's imagine you have a simple class that models a car engine (cause all OO examples should involve a car analogy :) ). You may have a simple field like this:

private bool engineRunning;

Simply making this field public or providing an IsEngineRunning() getter doesn't appear to be any different.

Now suppose you make your class more sophisticated, you want to remove that field and replace it with:

private bool ignitionOn;
private bool starterWasActivated;

Now if you have lots of classes accessing the old engineRunning field you have to go and change them all (bad times).

If instead you had started with:

public bool IsEngineRunning()
{
    return this.engineRunning;
}

you could now change it to :

public bool IsEngineRunning()
{
    return ignitionOn && starterWasActivated;
}

and the class's interface remains the same (good times).

Private vs. Public members in practice (how important is encapsulation?)

It depends. This is one of those issues that must be decided pragmatically.

Suppose I had a class for representing a point. I could have getters and setters for the X and Y coordinates, or I could just make them both public and allow free read/write access to the data. In my opinion, this is OK because the class is acting like a glorified struct - a data collection with maybe some useful functions attached.

However, there are plenty of circumstances where you do not want to provide full access to your internal data and rely on the methods provided by the class to interact with the object. An example would be an HTTP request and response. In this case it's a bad idea to allow anybody to send anything over the wire - it must be processed and formatted by the class methods. In this case, the class is conceived of as an actual object and not a simple data store.

It really comes down to whether or not verbs (methods) drive the structure or if the data does.

Property with and without { get; set; }

The first is a public field, the second an automatically implemented public property.

They are not the same. With the auto implemented property the compiler will generate a private backing field.

Though both can work as a way to expose data from your class, you should be using properties following the principle of information hiding - fields should be private and only accessed through properties. This allows you to make changes to the implementation without breaking the callers.

What are the differences between the private keyword and private fields in TypeScript?

Private keyword

The private keyword in TypeScript is a compile time annotation. It tells the compiler that a property should only be accessible inside that class:

class PrivateKeywordClass {
    private value = 1;
}

const obj = new PrivateKeywordClass();
obj.value // compiler error: Property 'value' is private and only accessible within class 'PrivateKeywordClass'.

However compile time checking can be easily bypassed, for example by casting away the type information:

const obj = new PrivateKeywordClass();
(obj as any).value // no compile error

The private keyword is also not enforced at runtime

Emitted JavaScript

When compiling TypeScript to JavaScript, the private keyword is simply removed:

class PrivateKeywordClass {
    private value = 1;
}

Becomes:

class PrivateKeywordClass {
    constructor() {
        this.value = 1;
    }
}

From this, you can see why the private keyword does not offer any runtime protection: in the generated JavaScript it's just a normal JavaScript property.

Private fields

Private fields ensure that properties are kept private at runtime:

class PrivateFieldClass {
    #value = 1;

    getValue() { return this.#value; }
}

const obj = new PrivateFieldClass();

// You can't access '#value' outside of class like this
obj.value === undefined // This is not the field you are looking for.
obj.getValue() === 1 // But the class itself can access the private field!

// Meanwhile, using a private field outside a class is a runtime syntax error:
obj.#value

// While trying to access the private fields of another class is 
// a runtime type error:
class Other {
    #value;

    getValue(obj) {
        return obj.#value // TypeError: Read of private field #value from an object which did not contain the field
    }
}

new Other().getValue(new PrivateKeywordClass());

TypeScript will also output a compile time error if you try using a private field outside of a class:

Error on accessing a private field

Private fields originates from a TC-39 ECMAScript proposal and are part of the 2021 ECMAScript specification, which means that they can be used in both normal JavaScript and TypeScript.

Emitted JavaScript

If you use private fields in TypeScript and are targeting ES2021 or older versions of JavaScript for your output, TypeScript will generate code that emulates the runtime behavior of private fields using WeakMap (source)

class PrivateFieldClass {
    constructor() {
        _x.set(this, 1);
    }
}
_x = new WeakMap();

If you are targeting anything later than ES2021, TypeScript will emit the private field:

class PrivateFieldClass {
    constructor() {
        this.#x = 1;
    }
    #x;
}

Which one should I use?

It depends on what you are trying to achieve.

The private keyword is a fine default. It accomplishes what it was designed to accomplish and has been used successfully by TypeScript developers for years. And if you have an existing codebase, you do not need to switch all of your code to use private fields. This is especially true if you are not targeting esnext, as the JS that TS emits for private fields may have a performance impact. Also keep in mind that private fields have other subtle but important differences from the private keyword

However if you need to enforce runtime privateness or are outputting esnext JavaScript, than you should use private fields.

Also keep in mind that organization/community conventions on using one or the other will also evolve as private fields become more widespread within the JavaScript/TypeScript ecosystems

Other differences of note

Private fields are not returned by Object.getOwnPropertyNames and similar methods
Private fields are not serialized by JSON.stringify
There are importance edge cases around inheritance.
TypeScript for example forbids declaring a private property in a subclass with the same name as a private property in the superclass.
```
class Base {
    private value = 1;
}

class Sub extends Base {
    private value = 2; // Compile error:
}
```
This is not true with private fields:
```
class Base {
    #value = 1;
}

class Sub extends Base {
    #value = 2; // Not an error
}
```

A private keyword private property without an initializer will not generate a property declaration in the emitted JavaScript:

class PrivateKeywordClass {
    private value?: string;
    getValue() { return this.value; }
}

Compiles to:

class PrivateKeywordClass {
    getValue() { return this.value; }
}

Whereas private fields always generate a property declaration:

class PrivateKeywordClass {
    #value?: string;
    getValue() { return this.#value; }
}

Compiles to (when targetting esnext):

class PrivateKeywordClass {
    #value;
    getValue() { return this.#value; }
}

What is the use of encapsulation when I'm able to change the property values with setter methods?

Assume you have an age property.

The user can enter a value of -10, which although is a valid number, is an invalid age. A setter method could have logic which would allow you to catch such things.

Another scenario, would be to have the age field, but hide it. You could also have a Date of Birth field, and in it's setter you would have something like so:

...
private int age
private Date dob

...
public void setDateOfBirth(Date dob)
{
    this.dob = dob;
    age = ... //some logic to calculate the age from the Date of Birth.
}

What's the Difference Between Encapsulating a Private Member as a Property and Defining a Property Without a Private Member