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Is there any reason to use field over automatic property? [duplicate]

I just realized that the C# property construct can also be used with a private access modifier:
private string Password { get; set; }
Although this is technically interesting, I can't imagine when I would use it since a private field involves even less ceremony:
private string _password;
and I can't imagine when I would ever need to be able to internally get but not set or set but not get a private field:
private string Password { get; }
or
private string Password { set; }
but perhaps there is a use case with nested / inherited classes or perhaps where a get/set might contain logic instead of just giving back the value of the property, although I would tend to keep properties strictly simple and let explicit methods do any logic, e.g. GetEncodedPassword().
Does anyone use private properties in C# for any reason or is it just one of those technically-possible-yet-rarely-used-in-actual-code constructs?
Addendum
Nice answers, reading through them I culled these uses for private properties:
when private fields need to be lazily loaded
when private fields need extra logic or are calculated values
since private fields can be difficult to debug
in order to "present a contract to yourself"
to internally convert/simplify an exposed property as part of serialization
wrapping global variables to be used inside your class

I use them if I need to cache a value and want to lazy load it.
private string _password;
private string Password
{
get
{
if (_password == null)
{
_password = CallExpensiveOperation();
}
return _password;
}
}

The primary usage of this in my code is lazy initialization, as others have mentioned.
Another reason for private properties over fields is that private properties are much, much easier to debug than private fields. I frequently want to know things like "this field is getting set unexpectedly; who is the first caller that sets this field?" and it is way easier if you can just put a breakpoint on the setter and hit go. You can put logging in there. You can put performance metrics in there. You can put in consistency checks that run in the debug build.
Basically, it comes down to : code is far more powerful than data. Any technique that lets me write the code I need is a good one. Fields don't let you write code in them, properties do.

perhaps there is a use case with nested / inherited classes or perhaps where a get/set might contain logic instead of just giving back the value of the property
I personally use this even when I don't need logic on the getter or setter of a property. Using a property, even a private one, does help future-proof your code so that you can add the logic to a getter later, if required.
If I feel that a property may eventually require extra logic, I will sometimes wrap it into a private property instead of using a field, just so I don't have to change my code later.
In a semi-related case (though different than your question), I very frequently use the private setters on public properties:
public string Password
{
get;
private set;
}
This gives you a public getter, but keeps the setter private.

One good usage for private get only properties are calculated values. Several times I've had properties which are private readonly and just do a calculation over other fields in my type. It's not worthy of a method and not interesting to other classes so private property it is.

Lazy initialization is one place where they can be neat, e.g.
private Lazy<MyType> mytype = new Lazy<MyType>(/* expensive factory function */);
private MyType MyType { get { return this.mytype.Value; } }
// In C#6, you replace the last line with: private MyType MyType => myType.Value;
Then you can write: this.MyType everywhere rather than this.mytype.Value and encapsulate the fact that it is lazily instantiated in a single place.
One thing that's a shame is that C# doesn't support scoping the backing field to the property (i.e. declaring it inside the property definition) to hide it completely and ensure that it can only ever be accessed via the property.

The only one usage that I can think of
private bool IsPasswordSet
{
get
{
return !String.IsNullOrEmpty(_password);
}
}

Properties and fields are not one to one. A property is about the interface of a class (whether talking about its public or internal interface), while a field is about the class's implementation. Properties should not be seen as a way to just expose fields, they should be seen as a way to expose the intent and purpose of the class.
Just like you use properties to present a contract to your consumers on what constitutes your class, you can also present a contract to yourself for very similar reasons. So yes, I do use private properties when it makes sense. Sometimes a private property can hide away implementation details like lazy loading, the fact that a property is really a conglomeration of several fields and aspects, or that a property needs to be virtually instantiated with each call (think DateTime.Now). There are definitely times when it makes sense to enforce this even on yourself in the backend of the class.

I use them in serialization, with things like DataContractSerializer or protobuf-net which support this usage (XmlSerializer doesn't). It is useful if you need to simplify an object as part of serialization:
public SomeComplexType SomeProp { get;set;}
[DataMember(Order=1)]
private int SomePropProxy {
get { return SomeProp.ToInt32(); }
set { SomeProp = SomeComplexType.FromInt32(value); }
}

I use private properties to reduce code for accessing sub properties which often to use.
private double MonitorResolution
{
get { return this.Computer.Accesories.Monitor.Settings.Resolution; }
}
It is useful if there are many sub properties.

One thing I do all the time is store "global" variables/cache into HttpContext.Current
private static string SomeValue{
get{
if(HttpContext.Current.Items["MyClass:SomeValue"]==null){
HttpContext.Current.Items["MyClass:SomeValue"]="";
}
return HttpContext.Current.Items["MyClass:SomeValue"];
}
set{
HttpContext.Current.Items["MyClass:SomeValue"]=value;
}
}

I use them every now and then. They can make it easier to debug things when you can easily put in a breakpoint in the property or you can add a logging statement etc.
Can be also be useful if you later need to change the type of your data in some way or if you need to use reflection.

I know this question is very old but the information below was not in any of the current answers.
I can't imagine when I would ever need to be able to internally get but not set
If you are injecting your dependencies you may well want to have a Getter on a Property and not a setter as this would denote a readonly Property. In other words the Property can only be set in the constructor and cannot be changed by any other code within the class.
Also Visual Studio Professional will give information about a Property and not a field making it easier to see what your field is being used.

It is a common practice to only modify members with get/set methods, even private ones. Now, the logic behind this is so you know your get/set always behave in a particular way (for instance, firing off events) which doesn't seem to make sense since those won't be included in the property scheme... but old habits die hard.

It makes perfect sense when there is logic associated with the property set or get (think lazy initialization) and the property is used in a few places in the class.
If it's just a straight backing field? Nothing comes to mind as a good reason.

Well, as no one mentioned you can use it to validate data or to lock variables.
Validation
string _password;
string Password
{
get { return _password; }
set
{
// Validation logic.
if (value.Length < 8)
{
throw new Exception("Password too short!");
}
_password = value;
}
}
Locking
object _lock = new object();
object _lockedReference;
object LockedReference
{
get
{
lock (_lock)
{
return _lockedReference;
}
}
set
{
lock (_lock)
{
_lockedReference = value;
}
}
}
Note: When locking a reference you do not lock access to members of the referenced object.
Lazy reference: When lazy loading you may end up needing to do it async for which nowadays there is AsyncLazy. If you are on older versions than of the Visual Studio SDK 2015 or not using it you can also use AsyncEx's AsyncLazy.

One more usage would be to do some extra operations when setting value.
It happens in WPF in my case, when I display some info based on private object (which doesn't implement INotifyPropertyChanged):
private MyAggregateClass _mac;
private MyAggregateClass Mac
{
get => _mac;
set
{
if(value == _mac) return;
_mac = value;
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(DisplayInfo)));
}
}
public string DisplayInfo => _mac.SomeStringInformationToDisplayOnUI;
One could also have some private method, such as
private void SetMac(MyAggregateClass newValue)
to do that.

Some more exotic uses of explicit fields include:
you need to use ref or out with the value - perhaps because it is an Interlocked counter
it is intended to represent fundamental layout for example on a struct with explicit layout (perhaps to map to a C++ dump, or unsafe code)
historically the type has been used with BinaryFormatter with automatic field handling (changing to auto-props changes the names and thus breaks the serializer)

Various answers have mentioned using properties to implement a lazy member. And this answer discussed using properties to make live aliases. I just wanted to point out that those two concepts sometimes go together.
When using a property to make an alias of another object's public property, the laziness of that property is preserved:
[DebuggerBrowsable(DebuggerBrowsableState.Never)]
private IDbConnection Conn => foo.bar.LazyDbConnection;
On the other hand, retrieving that property in the constructor would negate the lazy aspect:
Conn = foo.bar.LazyDbConnection;

Looking into the guideline (Properties (C# Programming Guide)) it seems no one expects to use properties as private members.
Properties enable a class to expose a public way of getting and setting values, while hiding implementation or verification code.
In any case it can be interchanged by one or two methods and vice versa.
So the reason can be to spare parentheses on getting and get field syntax on setting.

Proper Coding Direct Access to the backing field of a Property C#

I have seen some code and thought that something seems wrong with it, so I would like to know if it is acceptable for good coding or not, my first thought is no.
Consider:
class MyClass
{
private string m_MySuperString;
public string MySuperString
{
get { return m_MySuperString; }
set { m_MySuperString = value; }
}
public void MyMethod()
{
if (blah != yada)
{
m_MySuperString = badabing;
}
}
public void MyOtherMethod()
{
if (blah == yada)
{
m_MySuperString = badaboom;
}
}
}
Is this kind of direct access to the Backing Field an acceptable practice or is it bad coding - or should I ask what is the point of the Property Accessor, and if this is done internally in a class with public members , access is allowed by multiple components - is it possible to have a crash - I would venture in a multi threaded application a crash should be expected.
Please any thoughts ?
I have looked at this Link on SO and others>
Why use private members then use public properties to set them?
EDIT
Let me be clear since there is good info being provided and rather respond to all answers and comments directly.
I am not asking about what properties are for, not if I can do auto implemented properties, private setters, OnValueChange notifications, logic on the properties.
My question is in regards to accessing that backing field directly - for example if you have say a mutlithreaded scenario - isn't the whole point of the synclock on the getters/setters - to control access to the backingfield ? Will this kind of code be acceptable in that scenario - just adding a syncLock to the getter and setter ?? Keep in mind the code in the constructor of myClass is an example - the code can be in any additional method - such as the updated class - Method1
END EDIT

Properties in Object Oriented Programming (OOP) help to enforce Encapsulation. The idea is that only the object itself is allowed to interact with its own data (i.e. fields). Access to the object's data from outside is only allowed through methods. In Java, for instance, you have to explicitly write get- and set-methods. Properties in C# have a special syntax that combines both of these methods in one construct, but the getters and setters are in fact methods.
This also means that an object is absolutely allowed to access its own fields directly.
There are cases, however, where property getters and setters perform additional logic. A setter might raise the PropertyChanged event or do some validation. A getter might combine several fields or yield a formatted or calculated value. If you need this additional logic to be performed, then you must access the properties instead of the fields. If a property is auto-implemented, then you have no choice (in C#), since the backing field is hidden and not accessible. (In VB it is hidden from IntelliSense but accessible from within the class.)

I recommend checking out Chapter 8, Section 1 of #JonSkeet's C# In Depth (from which I've shamelessly taken the below snippets for the purpose of education) for more information on automatically implemented properties. In short answer to your question, no, there's nothing wrong with this code.
Consider that the following snippet:
public string Name { get; set; }
is compiled as
private string <Name>k__BackingField;
public string Name
{
get { return <Name>k__BackingField; }
set { <Name>k__BackingField = value; }
}
...so the compiler is already doing the work for you that you've done above. There are ways to modify what it's doing, but those don't really answer the question. One example given in the book for thread safety is this:
//code above, plus
private static int InstanceCounter { get; set; }
private static readonly object counterLock = new object();
public InstanceCountingPerson(string name, int age) {
Name = name;
Age = age;
lock (counterLock) // safe property access
{
InstanceCounter++;
// and whatever else you have to do with the lock enabled
}
}
--Which is a pattern also referenced in this SO question. However, as pointed out there, locks are (a) potentially slow, (b) might not actually ensure their job is done because they have to be released at some point, and (c) rely on the trust system, because they sort of naively assume that anything wanting to access that object will make proper use of the lock (not always true, at least not in some of the code I've seen :D ). The advantage of the getter and setter methods is that you can enforce the pattern of using the lock (read: properly encapsulate the field, as others have suggested) for any instance of your class.
Another pattern you might consider, however, is Inversion of Control. With a Dependency Injection container, you can specify the level of thread safety you are comfortable with. If you are comfortable with everyone waiting for a singleton instance of your object, you can declare that all references to the object's interface reference the same object (and must wait for the object to become available), or you can determine that a thread-safe instance of the object should be created each time it is requested. See this SO answer for more details.
Note:
Any peer-reviewed criticism of the above ideas will be graciously accepted and added to the answer, as I'm sort of a thread safety dilettante at this point.

In the use case described , You can define this as follows using auto-implemented properties
public string MySuperString{ get; set ;}
you should use a backing filed if you need to do some input verification or the property is different than the internal fields for example
public string FullName{ get { return firstName + LastName} }
another benefit of using properties is you can define them in an interface , which is better in the long run for future features to be added

What is the difference between a property and a variable

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?

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.

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.

In C# any "variable" that has a getter and setter is referred to as a property. A variable has no getters and setters or so that is what the text books say.
My programming instructor made us have getters and setters for almost every variable that we created in Java. Even indexing variables he made us use a getter and setter if they were declared at the global class scope. I think this might have been overkill but it did get me to make getters and setters.
The real things about getters and setters are that they more than likely do more than just set an internal variable. Most setters are going to perform some type of data validation to make certain that data can be set to the variable. A getter might also check returning data for some criteria.
If your property is private and your setters and getters are public technically anyone could access your variable and change it as if they had public access to the actual variable. So, in reality, you should check your data to make certain that it is valid or some other data check.
private int myVariable;
public int myVariable
{
get
{
return myVariable;
}
set
{
if (value < 0)
{
throw new Exception("This is your exception some where else in code");
}
myVariable = value; //remember value is something that is
//declared automatically
}
}
public string FirstName { get; set; }
The above is a shorthand way of writing the following
private string firstName;
public string FirstName
{
get
{
//...code here
}
set
{
//...code here
}
}

A property is sort of a short getter and or setter. You can add logic to the set or get of the property or make them private which means that they are not accessible from the out side, where a variable is always accessible (if it is public).

Variable is, well, a variable.
Property is a special type of method that exposes that variable. And since it is a method, therefore, you can do some other things in it apart from just exposing the variable.
From MSDN:
The Property statement introduces the declaration of a property. A property can have a Get procedure (read only), a Set procedure (write only), or both (read-write). You can omit the Get and Set procedure when using an auto-implemented property. For more information, see Auto-Implemented Properties (Visual Basic).
You can use Property only at class level. This means the declaration context for a property must be a class, structure, module, or interface, and cannot be a source file, namespace, procedure, or block. For more information, see Declaration Contexts and Default Access Levels.
By default, properties use public access. You can adjust a property's access level with an access modifier on the Property statement, and you can optionally adjust one of its property procedures to a more restrictive access level.

There is a very good article (link below) about using fields/variables vs Properties from Microsoft itself. Though the article essentially talks about a FxCop violation rule, it clearly defines the difference between the two and the exact usage guidelines.
An excerpt from the article:
The primary use of a field should be as an implementation detail.
Fields should be private or internal and should be exposed by using
properties. Accessing a property is as easy as accessing a field, and
the code in a property's accessors can change as the type's features
expand without introducing breaking changes.
Refer:
https://learn.microsoft.com/en-us/previous-versions/dotnet/netframework-3.0/ms182141(v=vs.80)

In your example A is a public field on the class ABC and B is a public property on the class ABC. Specifically, B is an auto-implemented property. What this means is that "under the hood" the compiler does some of the work for you and effectively transforms your code into:
public class ABC()
{
private int b;
public int A;
public int B
{
get
{
return b;
}
set
{
b = value;
}
}
}

Variable is defined basically for accessing value from a class or into the same class according to their modifier assigned to those variable.
When we define a property there two methods created for single property so extra overhead is generated that is the drawback of property.
The advantages of properties is to get or set value into private variable of class.

Why ever use fields instead of properties?

I'm fairly new to C#, and I think properties are a wonderful thing. So wonderful, in fact, that I can't see any real advantage to using fields, instead. Even for private fields, it seems like the flexibility and modularity that properties offer can at best save you serious headaches, and at worst have no effect at all.
The only advantage I can see for fields is that you can initialize them inline. But most of the time, you want to initialize them in the constructor, anyway. If you aren't using inline initialization, is there any reason not to use properties all the time?
Edit: Some people have brought up the need to back up properties with fields (either explicitly or automatically). Let clarify my question: Is there any reason to use fields except to back up properties? I.e., is there any time that SomeType someField; is preferable to SomeType SomeProperty { get; set; }?
Edit 2: DanM, Skurmedel, and Seth all gave really useful answers. I've accepted DanM's, as it is the most complete, but if someone were to summarize their responses into a single answer, I'd be happy to accept it.

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.

Just try using a Property when using ref/out args:
someObject.SomeMethod(ref otherObject.SomeProperty);
It won't compile.

Properties are a wonderful thing -- but there is overhead associated with property access. Not necessarily a problem, but something to be aware of.
Avoiding Overuse of Property Getters and Setters
Most people don't realize that property getters and setters are similar to methods when it comes to overhead; it's mainly syntax that differentiates them. A non-virtual property getter or setter that contains no instructions other than the field access will be inlined by the compiler, but in many other cases, this isn't possible. You should carefully consider your use of properties; from inside a class, access fields directly (if possible), and never blindly call properties repeatedly without storing the value in a variable. All that said, this doesn't mean that you should use public fields!
Source: http://dotnet.sys-con.com/node/46342

If you want to have something readonly you pretty much have to use a field as there is no way to tell an automatic property to generate a read-only field.
I do this quite often.
Contrived example:
class Rectangle
{
private readonly int _width;
private readonly int _height;
public Rectangle(int width, int height)
{
_width = width;
_height = height;
}
public int Width { get { return _width; } }
public int Height { get { return _height; } }
}
This means nothing inside of Rectangle can alter the width or height after construction. If one tries to the compiler will complain.
If I instead had used an automatic property with a private setter the compiler wouldn't protect me from myself.
Another reason I see is, if a piece of data doesn't have to be exposed (stay private) why make it a property?

While I agree with what I perceive as the "intent" in David Basarab's statement : "There is no reason to publicly expose fields," I'd like to add a slightly different emphasis :
I'd modify the quote from David above to read : "There is no reason to publicly expose fields ... outside a class ... except through the conscious choice of encapsulating the fields in Properties through which access is rigorously controlled.
Properties are not simply a "veneer" of syntax over Fields "tacked onto" C# : they are a fundamental language feature designed for good reasons including :
controlling what is exposed and not exposed outside classes (encapsulation, data hiding)
allowing certain actions to be performed when a Property is accessed or set : actions that are best expressed in the Property 'get and 'set, rather than being "elevated" to externally defined methods.
Interfaces by design cannot define 'fields : but can define Properties.
Good OO Design means making conscious choices about "state" :
local variable fields : what state is private to a method and transient : local variables typically only valid within the scope of a method body, or even with as "narrow a lifespan" as within the scope of something like a 'for loop. Of course you can regard parameter variables in a method as "local" also.
class instance fields : what state is private to a class, and has independent existence for each instance of a class, but is most likely required to be used in several places in the class.
static instance fields : what state will be a property of the class only, independent of the number of instances of the class.
state deliberately and consciously exposed "outside" the class : the key idea being that there is at least one level of indirection interposed between the class and "consumers" of the data the class exposes. The "flip side" of "exposure" is, of course, the conscious intention of hiding (encapsulating, isolating) implementation code.
a. via public properties : all aspects of this well-covered in all the other answers here
b. via indexers
c. via methods
d. public static variables are usually found in utility classes, which are often static classes.
Suggest you review : MSDN on 'Fields ... MSDN on Properties ... MSDN on Indexers

I don't see why you'd use private autoproperties. What advantage is there to
private int Count {get; set;}
over
private int count

Fields and properties are not interchangeable. I guess what you're saying is accessing private fields through private properties. I do this when it makes sense but for the most part, it's not necessary. The JIT optimizer will inline access to a private field through a private property in most cases anyway. And wrapping a private field in a private property is not considered a breaking change anyway since private members are not a part of your interface.
Personally, I would never expose any protected/public instance fields. It's generally acceptable though to expose a public static field with a readonly modifier as long as the field type is itself immutable. This is often seen with SomeStruct.Empty static fields.

As others have noted, you will need a private backing field for properties anyway.
Also there is a speed advantage in using fields over properties. In 99.99 % of the cases it won't matter. But in some it might.

Fields are the only place you can store state. Properties are actually just a pair of methods with special syntax that allows them to be mapped to the get or set method depending on how they're being used: if a property modifies or accesses state, that state still has to be stored in a field.
You don't always see the fields. With C# 3 automatic properties, the field is created for you by the compiler. But it's still there. Furthermore, automatic properties have some significant limitations (e.g. no INotifyPropertyChanged support, no business logic in setters) that mean they're often inappropriate, and you need to create an explicit field and a manually defined property anyway.
As per David's answer, you're right if you're talking about an API: you almost never want to make the internal state (fields) part of the API.

The syntax for fields is a lot quicker to write than for properties, so when it's safe to use a field (private to the class) why not use it and save that extra typing? If auto-implemented properties had the nice short concise syntax and you had to do extra work to make a plain old field, people might just start use properties instead. Also, it's a convention now in C#. That's how people think, and it's what they expect to see in code. If you do something different form the normal, you will confuse everyone.
But you could ask why the syntax for fields doesn't create an auto-implemented property instead of a field, so you get the best of both worlds - properties everywhere and a concise syntax.
There's a very simple reason why we still need to have explicit fields:
C# 1.0 didn't have all these nice features that we have now, so fields were a fact of life - you couldn't really live without them. Lots and lots of code relies on fields and properties being visibly different things. It simply cannot be changed now without breaking tons of code.
I would suspect also that there are performance implications, but perhaps that can be solved by the jitter.
So we're stuck with fields forever, and since they're there and they've taken the best syntax, it makes sense to use them when it's safe to do so.

There is no reason to publicly expose fields.
If you public expose a field you can't change the source of the information, from inline defination to configuration file without refactoring.\
You could use a field to hide internal data. I rarely favor that, I only use fields when I am doing something to hide publicly and using it in a property. (i.e. I am not using Automatic property generation)

Speed. If a field gets set or read billions of times over the course of a simulation then you want to use a field and not a property to avoid the overhead och a sub routine call. Conforming to OO (DDD?) as far as possible, in these instances, I'd recommend resorting to fields only in class dedicated to representing some sort of "value" like person. Logic should be kept to a minimum. Rather, have a personcreator or a personservicer.
But if you have these issues then you're probably not programming c++ and not c#, aren't you?

There are several good (partial) answers by #Seth (fields perform better, so in a private context you might as well use that to your benefit when it makes sense), #Skurmedel (fields can be readonly), #Jenk (fields can be used for ref/out). But I'd like to add one more:
You can use the streamlined initialization syntax for setting the value of a field, but not a property. i.e.:
private int x = 7;
vs
private int x { get; set; }
// This must go in the constructor, sometimes forcing you to create
// a constructor that has no other purpose.
x = 7;

C# Automatic Properties

I'm a bit confused on the point of Automatic properties in C# e.g
public string Forename{ get; set; }
I get that you are saving code by not having to declare a private variable, but what's the point of a property when you are not using any get or set logic? Why not just use
public string Forename;
I'm not sure what the difference between these 2 statements is, I always thought you used properties if you wanted additional get/set logic?

Properties can have code put into them without breaking contract, fields can't have code put into them without changing them to properties (and breaking the interface). Properties can be read only or write only, fields can't. Properties can be data bound, fields can't.

You can write
public string Forename{ get; private set; }
to get read-only properties... Still not nearly as versatile as real properties, but it's a compromise that for some works.

I'm not sure what the difference between these 2 statements is, I always thought you used properties if you wanted additional get/set logic?
In the first case, the compiler will automatically add a field for you, and wrap the property. It's basically the equivalent to doing:
private string forename;
public string Forename
{
get
{
return this.forename;
}
set
{
this.forename = value;
}
}
There are many advantages to using properties over fields. Even if you don't need some of the specific reasons, such as databinding, this helps to future-proof your API.
The main problem is that, if you make a field, but in v2 of your application, need a property, you'll break the API. By using an automatic property up front, you have the potential to change your API at any time, with no worry about source or binary compatibility issues.

It is meant that you expect to add the logic later.
If you do so and have it as property from the beginning, you will not have to rebuild the dependent code. If you change it from a variable to a property, then you will have to.

Consider looking at some related threads about Difference Between Automatic Properties and Public Fields, Fields vs Properties, Automatic Properties - Useful or Not?, Why Not to Use Public Fields.

Public data members are evil (in that the object doesn't control modification of it's own state - It becomes a global variable). Breaks encapsulation - a tenet of OOP.
Automatic properties are there to provide encapsulation and avoid drudgery of writing boiler plate code for simple properties.
public string ID { get; set;}
You can change automatic properties to non-automatic properties in the future (e.g. you have some validation in a setter for example)... and not break existing clients.
string m_ID;
public string ID
{
get { return m_ID; }
set
{
//validate value conforms to a certain pattern via a regex match
m_ID = value;
}
}
You cannot do the same with public data attributes. Changing a data attribute to a property will force existing clients to recompile before they can interact again.

When adding auto properties the compiler will add get set logic into the application, this means that if you later add to this logic, and references to your property from external libraries will still work.
If you migrated from a public variable to a property, this would be a breaking change for other libraries that reference yours - hence, why not start with an auto property? :)

For one, you can set the property to virtual and implement logic in an inheriting class.
You can also implement logic in the same class afterwards and there won't be side-effects on any code relying on the class.

Not all properties need get/set logic. If they do, you use a private variable.
For example, in a MV-something pattern, your model would not have much logic. But you can mix and match as needed.
If you were to use a field like you suggested in place of a property, you can't for example define an interface to describe your class correctly, since interfaces cannot contain data fields.

A property is like a contract, and you can change the implemenation of a property without affecting the clients using your classes and properties. You may not have any logic today, but as business requirements change and if you want to introduce any code, properties are your safest bet. The following 2 links are excellent c# video tutorials. The first one explains the need of properties over just using fields and the second video explains different types of properties. I found them very useful.
Need for the Properties in C#
Poperties in C#, Read Only, Write Only, Read/Write, Auto Implemented

Take a look at the following code and explanation.
The most common implementation for a property is getter or a setter that simply reads and writes to a private field of the same type as a property. An automatic property declaration instructs the compiler to provide this implementation. The compiler automatically generates a private backing field.
Look into the following code:-
public class Stock
{
decimal currentPrice ; // private backing field.
public decimal CurrentPrice
{
get { return currentPrice ; }
set { currentPrice = value ; }
}
}
The same code can be rewritten as :-
public class Stock
{
public decimal CurrentPrice { get ; set ; } // The compiler will auto generate a backing field.
}
SOURCE:- C# in a Nutshell