Difference Between a Field and a Property

What is the difference between a field and a property?

Properties expose fields. Fields should (almost always) be kept private to a class and accessed via get and set properties. Properties provide a level of abstraction allowing you to change the fields while not affecting the external way they are accessed by the things that use your class.

public class MyClass
{
    // this is a field.  It is private to your class and stores the actual data.
    private string _myField;

    // this is a property. When accessed it uses the underlying field,
    // but only exposes the contract, which will not be affected by the underlying field
    public string MyProperty
    {
        get
        {
            return _myField;
        }
        set
        {
            _myField = value;
        }
    }

    // This is an AutoProperty (C# 3.0 and higher) - which is a shorthand syntax
    // used to generate a private field for you
    public int AnotherProperty { get; set; } 
}

@Kent points out that Properties are not required to encapsulate fields, they could do a calculation on other fields, or serve other purposes.

@GSS points out that you can also do other logic, such as validation, when a property is accessed, another useful feature.

What is the difference between properties and fields?

This one is just a field, which is what you call a variable that is declared directly on a class or struct:

public string Name;

This one is a property:

public string Name { get; set; } 

They are functionally similar. The getter and setter allow you to customize what happens when the variable is accessed or set, respectively. Traditionally, a property is used to expose a private field that is used inside the class to do work. You do this to expose the variable without allowing people to reach in and change or break the inner workings of the class.

This is part of Encapsulation.

In this particular syntax you are "auto-implementing" the getter and setter, which creates a hidden backing field that you never see anywhere in your code. The result is that working with this will seem extremely similar to just using a field.

Why not just use a field in this case? Aside from leading to better design, there are important implications that have more to do with convention. In a lot of cases where data binding is involved, for example, only properties are considered for binding. Fields are ignored. So in Entity Framework, a property might represent a column in a database... And it has to be a property. It cannot be a plain field.

The key lesson here is that if you want to expose a field outside you class, you should use a property to promote encapsulation and to signal to others (people and also code in some cases) that you really did intend to expose what you exposed.

More: What is the difference between a field and a property?

What is the difference between field, variable, attribute, and property in Java POJOs?

From here: http://docs.oracle.com/javase/tutorial/information/glossary.html

---

• field

  • A data member of a class. Unless specified otherwise, a field is not static.

---

• property

  • Characteristics of an object that users can set, such as the color of a window.

---

• attribute

  • Not listed in the above glossary

---

• variable

  • An item of data named by an identifier. Each variable has a type, such as int or Object, and a scope. See also class variable, instance variable, local variable.

What is the difference between `Fields` and `Properties` in C#?

The main advantage is that you can attach all sorts of functionality to a property such as validation, synchronization etc. You can't do that for a class field. For example, a field can throw BCL exceptions on assignment but it can't throw an exception that make sense with logic in your problem domain.

Also imagine trying to protect a field for thread synchronization. You have to write extra code in all the places in your code where the field is accessed. To do that with a property you can simply wrap the getter and setter with a lock in one place. (But beware! The ease of using lock in property getters and setters can give you a false sense of security if you're working with mutable types. See the accepted answer in this post.)

Now, you might think that validation and synchronization are not important to you for this particular value, and they may never be for this particular instance. But by using properties instead of direct field access is making your application much more maintainable in the future. (Suppose the value of an integer field suddenly needs to come from a source different from the original implementation and it needs to be converted from a string to an int. If you use properties to wrap the field then you make the change in one place and all the client code that uses that property does not need to change at all!)

Also, for managing information shared across many classes (global) take a look at the singleton pattern. But beware! Even though it looks neat and clean you can still get into trouble with it. Though if you really need global data you should use properties contained in a singleton. If nothing else, it's a good organization strategy.

To avoid issues with singletons or "global" data take a look at dependency injection as a much better alternative.

Difference between Property and Field in C# 3.0+

Encapsulation.

In the second instance you've just defined a variable, in the first, there is a getter / setter around the variable. So if you decide you want to validate the variable at a later date - it will be a lot easier.

Plus they show up differently in Intellisense :)

Edit: Update for OPs updated question - if you want to ignore the other suggestions here, the other reason is that it's simply not good OO design. And if you don't have a very good reason for doing it, always choose a property over a public variable / field.

Why ever use fields instead of properties?

Typically, properties need a backing field unless they are simple getter/setter "automatic properties".

So, if you're just doing...

public string Name { get; set; } // automatic property

...you don't need a field, and I agree, no reason to have one.

However, if you're doing...

public string Name
{
    get { return _name; }
    set 
    {
       if (value = _name) return;
       _name = value;
       OnPropertyChange("Name");
    }
}

...you need that _name backing field.

For private variables that don't require any special get/set logic, it's really a judgment call whether to do a private automatic property or just a field. I usually do a field, then, if I need it to be protected or public, I will change it to an automatic property.

Update

As noted by Yassir, if you use automatic properties, there's still a field lurking behind the scenes, it's just not something you actually have to type out. So, the bottom line is: properties don't store data, they provide access to data. Fields are what actually hold the data. So, you need them even if you can't see them.

Update 2

Regarding your revised question...

is there any time that SomeType someField; is preferable to SomeType SomeProperty { get; set; }?

...one thing that comes to mind: If you have a private field, and (according to convention for private fields) you call it _name, that signals to you and anyone reading your code that you are working directly with private data. If, on the other hand, you make everything a property, and (according to convention for properties) call your private property Name, now you can't just look at the variable and tell that it is private data. So, using only properties strips away some information. I haven't tried working with all properties to gauge whether that is crucial information, but something is definitely lost.

Another thing, more minor, is that public string Name { get; set; } requires more typing (and is a little messier) than private string _name.

What is the difference between a field and a property in Elasticsearch?

Subfields are indexed from the parent property source. While sub-properties need to have a "real" value in the document's source.

If your source contains a real object, you need to create properties. Each property will correspond to a different value from your source.

If you only want to index the same value but with different analyzers then use subfields.

It is often useful to index the same field in different ways for
different purposes. This is the purpose of multi-fields. For instance,
a string field could be mapped as a text field for full-text search,
and as a keyword field for sorting or aggregations:

(sorry I find its hard to explain =| )

C# field vs. property

I found out there is also a disadvantage to properties. They cannot be accessed by reference if they are of a value type. Why is this

Because under the covers, a property is just a method. If you look at the IL, you'll see methods like get_PropertyName and set_PropertyName. The problem with that is in order to support working with references, you would need to be able to return a reference for a method.

public ref T MyProperty
{
    get
    {
         return ref _underlyingField;
    }
}

Update: Starting in C# 7.0, this is possible using the syntax describe above.

Remainder of previous answer:

This of course, is something entirely possible in the CLR; but not exposed by the C# language.

Though it is possible, the CLR needs some tweaks to keep it as verifiable. The syntax for the property would have to support it to.

However, is any of that useful? As you stated, a field can do it. If you need it; use a field. Supporting it would take a lot of work. There are probably a very few cases where it is appropriate; and would create many cases where just using a field might have been better in the first place.

What is the difference between a field and a property?

Properties expose fields. Fields should (almost always) be kept private to a class and accessed via get and set properties. Properties provide a level of abstraction allowing you to change the fields while not affecting the external way they are accessed by the things that use your class.

public class MyClass
{
    // this is a field.  It is private to your class and stores the actual data.
    private string _myField;

    // this is a property. When accessed it uses the underlying field,
    // but only exposes the contract, which will not be affected by the underlying field
    public string MyProperty
    {
        get
        {
            return _myField;
        }
        set
        {
            _myField = value;
        }
    }

    // This is an AutoProperty (C# 3.0 and higher) - which is a shorthand syntax
    // used to generate a private field for you
    public int AnotherProperty { get; set; } 
}

@Kent points out that Properties are not required to encapsulate fields, they could do a calculation on other fields, or serve other purposes.

@GSS points out that you can also do other logic, such as validation, when a property is accessed, another useful feature.

What is the difference between class fields and properties in Javascript

What is the difference between the two besides the syntax?

There isn't any. A public field is an instance property, just one created with a field definition rather than by assignment. Other than how they're created, they're exactly the same thing.

The term "field" was used so it could cover both public and private (since private fields are not properties).

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