What is the difference between a property and a variable

Question

I have a confusion about understanding Property and Variables

public class ABC() {     public int A;     public int B { get; set; } } 

What is the exact difference between in A and B?

Answer 1

As many have pointed out, A is a field, B is a property.

The real question is, why should you care, and what to use?

I refer to a blog post of Jonathan Aneja:

(Its in VB, but it applies to C# as well ;))

So why use properties over fields, 5 reasons:

1. Fields can’t be used in Interfaces

You can’t enforce the existence of a field in an object’s public contract through an interface. For properties though it works fine.

2. Validation

While your application currently may not require any validation logic to set a particular value, changing business requirements may require inserting this logic later. At that point changing a field to a property is a breaking change for consumers of your API. (For example if someone was inspecting your class via reflection).

3. Binary Serialization

Changing a field to a property is a breaking change if you’re using binary serialization. Incidentally, this is one of the reasons VB10’s auto-implemented properties have a “bindable” backing field (i.e. you can express the name of the backing field in code) – that way, if you change an auto-implemented property to an expanded property, you can still maintain serialization compatibility by keeping the backing field name the same (in C# you’re forced to change it because it generates backing fields with unbindable names).

4. A lot of the .NET databinding infrastructure binds to properties but not fields

I’ve heard arguments on both sides as to whether or not that’s a good thing, but the reality is that’s the way it works right now. (Note from me: WPF bindings work on properties)

5. Exposing a public field is an FxCop violation

For many of the reasons listed above :)

There might be more reasons.

I would also like to point to a blog post of Jeff Atwood and conclude with a quote from it:

The really important thing to take away here is to avoid writing code that doesn't matter. And property wrappers around public variables are the very essence of meaningless code.

Answer 2

A is a field, B is a property. A property is basically syntactic sugar for getters and setters. The class you have defined will be compiled into something like this:

public class ABC() {     public int A;      private int backing_B;      public void set_B(int value)     {         backing_B = value;     }      public int get_B()     {         return backing_B;     } } 

Note that this kind of conversion is true for all C# properties -- accesses to ABC.B will be converted to method calls. Properties basically provide the illusion of a "variable" while actually just being a cleverly disguised pair of methods.

This is important, because it allows you to declare your own get and set method body, which can validate values or do other interesting things:

private int b;  public int B {     get { return b; }     set {         if (value < 0) throw new ArgumentOutOfRangeException("value");         b = value;     } } 

Note that most properties will use a field to store their value. Properties seldom exist on their own, apart from fields.