What are Automatic Properties in C# and what is their purpose?

Question

Could someone provide a very simple explanation of Automatic Properties in C#, their purpose, and maybe some examples? Try to keep things in layman's terms, please!

Solution 1

Automatic Properties are used when no additional logic is required in the property accessors.
The declaration would look something like this:

public int SomeProperty { get; set; }

They are just syntactic sugar so you won't need to write the following more lengthy code:

 private int _someField;
 public int SomeProperty 
 {
    get { return _someField;}
    set { _someField = value;}
 }

Solution 2

Edit: Expanding a little, these are used to make it easier to have private variables in the class, but allow them to be visible to outside the class (without being able to modify them)

Oh, and another advantage with automatic properties is you can use them in interfaces! (Which don't allow member variables of any kind)

With normal properties, you can do something like:

private string example;
public string Example 
{
    get { return example; }
    set { example = value; }
}

Automatic properties allows you to create something really concise:

public string Example { get; set; }

So if you wanted to create a field where it was only settable inside the class, you could do:

public string Example { get; private set; }

This would be equivalent to:

private string example;
public string Example 
{
    get { return example; }
    private set { example = value; }
}

Or in Java:

private String example;

public String getExample() {
    return example;
}

private void setExample(String value) {
    example = value;
}

Edit: @Paya also alerted me to:

• http://msdn.microsoft.com/en-us/library/bb384054.aspx
• http://weblogs.asp.net/dwahlin/archive/2007/12/04/c-3-0-features-automatic-properties.aspx

Solution 3

If you are asking why you would use Properties or Automatic Properties, this is the design philosophy behind it.

One important design principle is that you never expose fields as public, but rather always access everything via properties. This is because you can never tell when a field is accessed and more importantly when it is set. Now, a lot of the time, there is never any processing needed while setting or getting the value (for example, range checking). This is why Automatic Properties were created. They are a simple, one-line way of creating a property. The backing store for it is created by the compiler.

While this is what I do even for my internal programs, it is probably more important for ones designed for public use (for sale, open source, etc). If you use an Automatic Property and later decide that you need to do something else in the set or get, you can easily change your code without breaking the public interface.

Update

As a point of clarification to a comment below, if all of the code is your own, then no, it may not make much of a difference between a property and a field to you. But, if you are designing a library that will be consumed by others then switching back and forth between public fields and properties will cause exceptions unless the code using the library is recompiled first.

As a test, I created a library project and declared a property called TestData. I created a whole new project just to consume this library. All worked as expected. I then changed the property to a public field (the name stayed the same) and copied over the new library DLL without recompiling the consuming project. The outcome was an exception thrown as the code was expecting to find the methods property methods get_TestData and set_TestData, but fields are not accessed via methods.

Unhandled Exception: System.MissingMethodException: Method not found: 'Void TestLibrary.TesterClass.set_TestData(System.String)'.
   at TestLibraryConsumer.Program.Main(String[] args)

Solution 4

Many people have already stated that auto properties are syntactic sugar - a shorthand way of writing simple properties. I will deal with the differences between a public variable and a public property and why, when switching between the two, you would need to recompile. Take the following:

public class MyClass
{
    public int MyPublicVariable = 0;

    public int MyPublicProperty
    {
        get;
        set;
    }
}

Once compiled, conceptually, it actually ends up being similar to the following:

public class MyClass
{
    public int MyPublicVariable = 0;

    private int MyPublicProperty = 0;

    public int get_MyPublicProperty()
    {
        return MyPublicProperty;
    }

    public void set_MyPublicProperty( int value )
    {
        MyPublicProperty = value;
    }
}

A long time ago, properties were invented to be a quick and easy way to define pairs of get and set methods. It made code more readable and easier to understand as they conveyed intent and ensured consistency.

MyClass myClass = new MyClass();

myClass.MyPublicVariable = 2;

myClass.MyPublicProperty = 2;

Once compiled, again conceptually, it ends up being similar to the following:

MyClass myClass = new MyClass();

myClass.MyPublicVariable = 2;

myClass.set_MyPublicProperty( 2 );

So, one of the reasons for preferring public properties over public variables is if you need to use different logic as your code evolves then consumers of your code don't necessarily need to re-compile. This is why it is often considered best practice. It is also the reason auto properties were invented - to speed along code writing whilst maintaining this best practice.

There has also been some comments about interfaces. Interfaces are essentially a contract that guarantees the presence of certain methods within any class implementing them. As we know from the above, properties represent one or two methods so they work just fine in interfaces.

Hope this helps a little.

Here is another example that may be of interest:

public class MyClass
{
    private int[] _myArray = new int[ 5 ];

    public int MyArray[ int index ]
    {
        get
        {
            return _myArray[ index ];
        }
        set
        {
            _myArray[ index ] = value;
        }
     }
}

public class MyClass
{
    private int[] _myArray = new int[ 5 ];

    public int get_MyArray( int index )
    {
        return _myArray[ index ];
    }

    public void set_MyArray( int index, int value )
    {
        _myArray[ index ] = value;
    }
}

Please note: I cannot exactly remember the method signatures used when decompiling properties. I think it is 'get_XXX' and 'set_XXX' but it could be something else that is very similar. As long as the understanding is there, it probably doesn't matter too much. In the end, they all become memory addresses anyway :-)

Solution 5

They are just a coding shortcut to save the programmer a few keystrokes. Instead of typing all this:

private string _lastName;
public string LastName {
    get {
        return _lastName;
    }
    set {
        _lastName = value;
    }
}  

you can just type:

public string LastName {
    get; set;
} 

and let the compiler generate the rest automatically.

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Comments

[nawfal]

possible duplicate of C# 3.0 auto-properties - useful or not?

[Paya]

I would also add these two links: MSDN and ASP.NET Blog.

[Chris Fulstow]

The compiler also creates a private, anonymous backing field that can only be accessed through the property's get and set accessors.

[Cody Gray]

It's C# 3.0 and later. The documentation you link to says it at the top.

[user2048204]

@Cody Gray @Stecya Why can't i just use public int SomeProperty; Will this not be the same? I mean if there is no private variable at all here, then why do we even need Properties?

[Slai]

@user2048204 because property is different from variable blog.codinghorror.com/properties-vs-public-variables

[Salem]

The equivalent in Java would need setExample to be private or no setter at all.

[Sepster]

@user2048204 There is a private variable; It is an auto generated anonymous private field. But you write your code to access it from within the class, via the public property name (which would be functionally identical to accessing it directly, in this case where there was no additional logic on the accessor).

[Ammar Shaukat]

when you explore an auto-implemented property with ildasm tool, you'll come to know that behind each auto-implemented property there are following field created automatically a private data member and two methods that work as getter and setter for that private field.

[ruffin]

Interestingly, you can still set a value for public int SomeProperty { get; } (no private set) in your constructor, as if your implicit backing field were marked readonly.

[Gregory Fenn]

There's something missing from Stecya/AustinWBryan's answer. if an autoproperty creates a private field behind the scenes, why is is that an interface can define an autoproperty but not fields? It can't be the case that autoproproperties are "just" syntactic sugar, because it it was there would be compiler errors when I use them in interfaces.

[DragonSpit]

This suggestion works for the case when you want to have a Property that you need to be "gettable only" by other classes, and to be able to update it by other methods within this class.

[mins]

"If you use an Automatic Property and later decide that you need to do something else in the set or get, you can easily change your code without breaking the public interface". it's equally easy to replace a field with a property of the same name if you later need some processing before value assignment or reading. But until you need this processing, there is no visible benefit to declare a property over declaring a field. So the OP is still valid (this answer describes the actual benefit of having empty get/set methods).

[mins]

This should be the selected answer, as the current one provides no actual reason to use empty get/get over using a simple field (said abruptly, it just states there is a simple way to declare useless methods).

[Jim]

@mins. If the code is all yours, you are correct. I have updated my answer to further clarify why flipping between properties and fields can still go wrong though.

[Rob]

I'm not sure that this answer adds anything as it doesn't "provide a very simple explanation of Automatic Properties in C#, their purpose, and maybe some examples", it also doesn't compile.

[leoap]

@GregoryFenn Not if the fields are autogenerated for the class that implements the interface, instead for the interface

[Szesan]

It's astonishing that out of this 5 replies in this thread nobody answered the question, which is "why do we even need Properties". He wasn't asking how it works "under the hood", but for it's practical purposes. I think when you don't have an actual reason to use a property (such as implementing an interface, or using reflection) you indeed better of making the field public and be done with it. You don't need the getter-setter if do nothing with them.