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What does the Auto Property give me over a simple Field? [duplicate]

We're often told we should protect encapsulation by making getter and setter methods (properties in C#) for class fields, instead of exposing the fields to the outside world.
But there are many times when a field is just there to hold a value and doesn't require any computation to get or set. For these we would all do this number:
public class Book
{
private string _title;
public string Title
{
get => _title;
set => _title = value;
}
}
Well, I have a confession, I couldn't bear writing all that (really, it wasn't having to write it, it was having to look at it), so I went rogue and used public fields.
Then along comes C# 3.0 and I see they added automatic properties:
public class Book
{
public string Title { get; set; }
}
Which is tidier, and I'm thankful for it, but really, what's so different than just making a public field?
public class Book
{
public string Title;
}

In a related question I had some time ago, there was a link to a posting on Jeff's blog, explaining some differences.
Properties vs. Public Variables
Reflection works differently on variables vs. properties, so if you rely on reflection, it's easier to use all properties.
You can't databind against a variable.
Changing a variable to a property is a breaking change. For example:
TryGetTitle(out book.Title); // requires a variable

Ignoring the API issues, the thing I find most valuable about using a property is debugging.
The CLR debugger does not support data break points (most native debuggers do). Hence it's not possible to set a break point on the read or write of a particular field on a class. This is very limiting in certain debugging scenarios.
Because properties are implemented as very thin methods, it is possible to set breakpoints on the read and write of their values. This gives them a big leg up over fields.

Changing from a field to a property breaks the contract (e.g. requires all referencing code to be recompiled). So when you have an interaction point with other classes - any public (and generally protected) member, you want to plan for future growth. Do so by always using properties.
It's nothing to make it an auto-property today, and 3 months down the line realize you want to make it lazy-loaded, and put a null check in the getter. If you had used a field, this is a recompile change at best and impossible at worst, depending on who & what else relies on your assemblies.

Just because no one mentioned it: You can't define fields on Interfaces. So, if you have to implement a specific interface which defines properties, auto-properties sometimes are a really nice feature.

A huge difference that is often overlooked and is not mentioned in any other answer: overriding. You can declare properties virtual and override them whereas you cannot do the same for public member fields.

It's all about versioning and API stability. There is no difference, in version 1 - but later, if you decide you need to make this a property with some type of error checking in version 2, you don't have to change your API- no code changes, anywhere, other than the definition of the property.

Another advantage of auto-implemented properties over public fields is that you can make set accessors private or protected, providing the class of objects where it was defined better control than that of public fields.

There is nothing wrong in making a field public. But remember creating getter/setter with private fields is no encapsulation. IMO, If you do not care about other features of a Property, you might as well make it public.

Trivial properties like these make me sad. They are the worst kind of cargo culting and the hatred for public fields in C# needs to stop. The biggest argument against public fields is future-proofing: If you later decide you need to add extra logic to the getter and setter, then you will have to do a huge refactor in any other code that uses the field. This is certainly true in other languages like C++ and Java where the semantics for calling a getter and setter method are very different from those for setting and getting a field. However, in C#, the semantics for accessing a property are exactly the same as those for accessing a field, so 99% of your code should be completely unaffected by this.
The one example I have seen of changing a field into a property actually being a breaking change at the source level is something like:
TryGetTitle(out book.Title); // requires a variable
To this I have to ask, why TF are you passing some other class's field as a reference? Depending on that not being a property seems like the real coding failure here. Assuming that you can directly write to data in another class that you know nothing about is bad practice. Make your own local variable and set book.Title from that. Any code that does something like this deserves to break.
Other arguments I have seen against it:
Changing a field to a property breaks binary compatibility and requires any code that uses it to be recompiled: This is a concern iff you are writing code for distribution as a closed-source library. In that case, yes, make sure none of your user-facing classes have public fields and use trivial properties as needed. If however you are like 99% of C# developers and writing code purely for internal consumption within your project, then why is recompilation a big concern? Just about any other change you make is going to require recompilation too, and so what if it does? Last I checked, it is no longer 1995, we have fast computers with fast compilers and incremental linkers, even larger recompilations shouldn't need more than a few minutes, and it has been quite some time since I have been able to use "my code's compiling" as an excuse for swordfighting through the office.
You can't databind against a variable: Great, when you need to do that, make it into a property.
Properties have features that make them better for debugging like reflection and setting breakpoints: Great, one you need to use one of those things, make it into a property. When you're done debugging and ready to release, if you don't still need those functionalities, change it back into a field.
Properties allow you to override behavior in derived classes: Great, if you are making a base class where you think such a scenario is likely, then make the appropriate members into properties. If you're not sure, leave it as a field and you can change it later. Yes, that will probably require some recompilation, but again, so what?
So in summary, yes there are some legitimate uses for trivial properties, but unless you are making a closed source library for public release, fields are easy enough to convert into properties when needed, and an irrational fear of public fields is just some object oriented dogma that we would do well to rid ourselves of.

For me, the absolute deal breaker for not using public fields was the lack of IntelliSense, showing the references:
Which is not available for fields.

If you decide later to check that the title is unique, by comparing to a collection or a database, you can do that in the property without changing any code that depends on it.
If you go with just a public attribute then you will have less flexibility.
The extra flexibility without breaking the contract is what is most important to me about using properties, and, until I actually need the flexibility, auto-generation makes the most sense.

One thing you can do with Fields but not with Properties (or didn't used to be able to ... I'll come to that in a moment) is that Fields can be designated as readonly whereas Properties cannot. So Fields give you a clear way of indicating your intention that a variable is there to be set (from within the constructor) at object-instantiation time only and should not be changed thereafter. Yes, you can set a Property to have a private setter, but that just says "this is not to be changed from outside the class", which is not the same as "this is not to be changed after instantiation" - you can still change it post-instantiation from within the class. And yes you can set the backing field of your property to be readonly, but that moves post-instantiation attempts to change it to being run-time errors rather than compile-time errors. So readonly Fields did something useful which Properties cannot.
However, that changes with C# 9, whereby we get this helpful syntax for Properties:
public string Height { get; init; }
which says "this can get used from outside of the class but it may only be set when the object is initialized", whereupon the readonly advantage of Fields disappears.

One thing I find very useful as well as all the code and testing reasons is that if it is a property vs a field is that the Visual Studio IDE shows you the references for a property but not a field.

My pov after did some researches
Validation.
Allow overriding the accessor to change the behaviour of a property.
Debugging purpose. We'll be able to know when and what the property change by setting a breakpoint in the accessor.
We can have a field set-only. For instance, public set() and private get(). This is not possible with the public field.
It really gives us more possibility and extensibility.

Properties with empty accessors

Though I'm of course familiar with auto-properties, I just ran across this at work, which seems to be a distinctly different beast:
public SomeType SomeProp
{
get
{
return someField;
}
set
{
}
}
I was surprised it even compiled, and I imagine it must be a bug: the property seems to allow setting, but doing so does absolutely nothing.
Is there a use for this construct? Is it like those "Close Door" buttons in elevators that don't do anything, but make the user feel good?

Why would you expect it not to compile? The setter is just a void method with a single parameter, effectively. You can write broken methods perfectly easily without expecting the compiler to notice - and the same is true of properties.
I can't easily imagine any case where this would be deliberate, however, other than for "partial" implementations - e.g. to demonstrate language features, or if you're testing something that does set a property, but you don't care what the test sets it to. (I'd personally still usually at least record that the property had been set.)

You often see this when a result needs to be serialized in a web service or using an XML or binary serializer.
It's lazy and sloppy, but it happens often. This leaves the object with the "appearance" that the property is settable. If it's done to implement an interface and allow compilation, then the developer who did it needs to be beaten liberally about the head and shoulders with a blunt object, as he just broke the interface. If there is a valid reason that it can't be implemented, then the developer needs to kick it back up to the architect for review. You don't just leave empty stubbed methods behind when implementing an interface. If you don't have a technique defined for implementation at the moment, then at least throw a new NotImplementedException so the unit tests will catch it.
As far as serialization: ReadOnly properties don't get included in regular serialization, and that can leave the property unavailable to a web service client. (ref: Read-Only Properties Cannot Be Exposed by XML Web Services.) This is one of the reasons we should all be moving to WCF and DataContracts. If you accept this class as an input type for a method through WCF, then again retrieve the blunt object.

This doesn't seem useful by itself but consider an interface that required classes to have a SomeProp and you need to implement this interface in your class but have SomeProp only readable and not writeable.
public interface IQuestion
{
public int AnwserToLife { get; set; } //leave out 'set' for read-only
}
public class HitchHiker : IQuestion
{
public int AnwserToLife
{
get
{
return 42;
}
set
{
//never changes
}
}
}

There are a few use cases, where this would be a necessary workaround, some of which I have already encountered "in the wild".
E.g.: The property is a remains from old times, no longer of use, but some other part of the app has never been updated (Source lost? Third party?) and insists on setting the property. I have seen that in old code, that required plugins to set a isDirty property after updating some dataset, when the implementation changed to observe the dataset on itself, the isDirty property became useless, but couldn't be put away, because other code still wants to set it.

I would recommend avoiding this kind of programming. It compiles, because there is no reason it shouldn't, but if the interface requires you to implement a setter method, then there are two options:
Either the setter is redundant and the property should be made read-only, or
There exists a part of your code which will set this value and falsely assume that it worked.

Why do we use blank get; set; accessors in C#? [duplicate]

This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
c#: why have empty get set properties instead of using a public member variable?
string name;
vs
string name {get; set;}
Assuming your get and set are blank as above, what's the point in specifying them?

It encapsulates the compiler generated field, and provides you, the class or struct developer the ability to update it internally later without breaking your API by simply modifying the get/set part that you care about.
For instance, suddenly never want to return null? You can do that by simply changing the empty get to get { return storedName ?? ""; }. Of course, it means you suddenly need to manually control the variable, but that's a small price to pay for the flexibility.
The first use is an example of a field declaration. The second use is an example of an auto-implemented property.
It is generally bad practice to provide direct access to a field. However, the .NET team noticed that a lot of getters/setters are basically just that. For example, consider the following:
// C#
public string Name
{
get { return name; }
set { name = value; }
}
// Without properties (or a Java implementation)
public void setName(String name)
{
this.name = name;
}
public String getName()
{
return name;
}
Either way, that's a lot verbosity to really just expose a field. However, it is regularly the case that, as a developer, you need to go back and change how a field is handled internally, but you do not want to break or even affect other code if you can get away with it.
That is why using direct access to fields is bad. If you provide direct access to fields, but need to change something about using the field, then all code that uses that field must change as well. If you use a property (or even a method), then you can change the internal code and potentially not effect external code.
Consider the following example:
public string Name
{
get;
set;
}
Later you decide that you need to raise a changing and changed event around the setter. If you exposed a field, then it's time for a potentially big rewrite. If you used properties (or a method), then you can just add the logic there. You suddenly lose the benefit of auto-implementing properties, but you gained the ability to refactor your class without breaking existing code.
private string name;
public event NameChangingEventHandler NameChanging;
public event NameChangedEventHandler NameChanged;
public string Name
{
get { return name; }
set
{
OnNameChanging(/*...*/);
name = value;
OnNameChanged(/*...*/);
}
}
protected virtual void OnNameChanging(/*...*/) { }
protected virtual void OnNameChanged(/*...*/) { }
All of that maintains your public API and requires no work from users of the class (the rest of your code, or external developers using of your API). Breaking changes are not always avoidable, but avoiding direct access to fields is a good step to try to ensure that it won't happen. Auto-implemented properties are a quick, and easy way to do it.
(Unrelated: lost power while typing this and I am very happy that my browser saved most of it!)

The first one is actually a Field, but the second one is an Auto-Implemented property. The difference between them has already been discussed.

The first, assuming it's declared in class scope, is a field name. It's accessed as a field. The second is a property. A Blank get/set is known as an auto-property.

You might need to actually do something in your accessors in the future. Changing a field (which is what your first declaration is) to a property is a breaking change, so specifying accessors in advance is a small investment in the future.

Being able to add logic to a field's accessors without breaking compatibility is the standard explanation, and it's certainly a big one if you're writing a library or an application that's split among several assemblies that might be updated independently. I think it's something that one could dismiss as less of a concern if you're working on any sort of "all-in-one" software, though, since it'll all be recompiled anyway.
But even then, there's still another very compelling reason to only expose properties in your public interfaces: Even if you never need to make any internal updates, using fields can still lead to other problems on down the line because many portions of the .NET framework strongly prefer properties to fields. WPF, for example, does not generally support binding to fields. You can get around that by doing fancy things like implementing ICustomTypeDescriptor, but it's just so much easier to simply type {get; set;}.

string name {get; set;}
This is called auto implemented property. Actually, C# creates variable starting with _ itself, so on get, that variable value is fetched and on set, that variable value is set. Its just like normal properties. Where as string name; is just a field.

The first is a variable, the second is a (shorthanded) property

Properties are very nice, but as a general rule, objects shouldn't expose state to the public; they should be a black box from the perspective of outsiders. And you especially shouldn't state to direct change. State should change as a side effect of asking the object instance to do something useful in the problem domain.
If you are going to expose state, expose it as a read-only property (e.g. public widget Foo { get ; private set ; }).

Change code based on existing attribute?

I'm writing a library which is shared between .net and silverlight. I have several places where I am doing this, to satisfy the silverlight deserialization (which can't access private members):
[DataMember (IsRequired = true)]
public Object MyProperty { get;
#if SILVERLIGHT
internal
#else
private
#endif
set; }
I know the rules for this, which are that if the setter is private and SILVERLIGHT is defined then the setter should be internal.
Could I use an aspect oriented framework like postsharp to help me reduce this code so that I don't need to specify anything and it will inspect the property, if it has the DataMember attribute and the setter is private, then make the setter internal instead?
Or is there some other technique I could use for this?
EDIT
There seems to be some confusion. My goal is to avoid having the compiler directives at all, but to still have code which is generated with a private member in .net and with a member that can be set by the DataContractDeserializer in Silverlight, which can't access private members. If possible I'd like to automatically modify the property in a silverlight build so it is internal, whilst not having anything other than the DataMember attribute in the source.
Ideally I see the solution being something like:
Write an aspect which checks every property or field.
If the property/field has the [DataMember] attribute then
If the silverlight compiler directive exists then
if the setter is private make it internal (for properties) or if it is declared as private make it internal (for fields)
but I'm not sure which bits of that it would be possible to do using a tool like post sharp.

The other answers either attack the merit of the problem or present alternative approaches that do not directly answer the question. The question was whether there was a way to change the visibility of the setter for a property with a DataMember attribute after it is compiled, to support two versions (.NET and Silverlight).
I suspect the PostSharp SDK would support this. However, this is a problem I had to solve while developing Afterthought, as I needed to change the visibility of anonymous static delegates generated by the C# compiler (normally private until I made them internal). Afterthought itself does not currently support your scenario directly, but it leverages the open source Microsoft CCI libraries, which do. The IL Mutator example shows how to use the CCI libraries to load a compiled assembly and modify it by creating a mutable copy. The example is actually much more complex than your scenario, as you will not be modifying IL, just the visibility of a setter.
This is an example of changing the visibility of a method from within a mutator in CCI:
public override MethodDefinition Mutate(MethodDefinition methodDef)
{
// Automatically make all private static methods to have internal scope
if (methodDef.IsStatic && methodDef.Visibility == TypeMemberVisibility.Private)
methodDef.Visibility = TypeMemberVisibility.Assembly;
This is a slightly simplified excerpt from the Afterthought amender. There are also examples in the same class for how to determine if a method is a setter (starts with set_, HideBySig, etc). You would just need to create a mutator, override the method in this example, verify that the method is a property setter with a DataMember attribute on the containing property definition, and change the visibility.

Simple answer is NO. PostSharp is a Post-Compile framework so you cannot use your compiler directives (as you're trying to do in your question). You can use PostSharp to
Introduce properties into the class with the accessors desired (which doesn't give you access at design time) Directives can be applied to the aspect at design time to determine which property to inject though
Or use reflection to change the accessor (I dont think you can do that)
An alternative is to use T4 templates to generate these classes for you
Edit: Example of property injection
[Serializable]
[MulticastAttributeUsage(MulticastTargets.Class, Inheritance = MulticastInheritance.Strict)]
public class PropInj : InstanceLevelAspect
{
#if SILVERLIGHT
[IntroduceMember(OverrideAction = MemberOverrideAction.Ignore, IsVirtual=true, Visibility=Visibility.FamilyAndAssembly)]
public string MyProperty { get; set; }
#else
[IntroduceMember(OverrideAction = MemberOverrideAction.Ignore, IsVirtual = true, Visibility = Visibility.Private)]
public string MyProperty { get; set; }
#endif
}
[PropInj]
public class test
{
//public int MyProperty { get; set; }
public test()
{
}
}
but really you need to rethink your design.

What is the difference between public int i and public int i {get; set;} (what is the difference between automatic property and a public member?) [duplicate]

This question already has answers here:
Closed 12 years ago.
Possible Duplicates:
c#: why have empty get set properties instead of using a public member variable?
C#: Public Fields versus Automatic Properties
I am using "automatic" properties in my code,
and I wonder what is the actual difference between
this code:
public class foo{
public int i;
}
and
public class foo{
public int i {get; set;}
}
I know there is a difference, as sine 3rd parties that I've used missed the public members but found them once adding the {get; set;}.
AS there is no private field behind that, what is going behind the scene ?

A private field gets generated by the compiler when using automatic properties.
When you declare a property as shown in the following example, the compiler creates a private, anonymous backing field that can only be accessed through the property's get and set accessors.
In regards to the difference between the two examples - the first one exposes the field directly for manipulation. This is considered bad practice (think information hiding, loss of encapsulation).
With the second example, you must use the getter and setter and you can add any kind of validation and other logic around these actions.
See this blog post:
If I have a field with no special behavior, should I write a "just in case" property (with trivial get/set), or should I expose a public field?
The reason that the library design guidelines suggest you write a property here is that it is important that libraries be easily versioned. If you put a property in there ahead of time, you can change the property implementation without requiring users to recompile their code.

The first is a field and could be described as POD. The second is a property and allow for derived classes to overload and Shadow while the first does not. Also the second is a nicety since the complier silently creates a backing store.

That's an auto property, not an anonymous property. There is, in fact, a private backing field for it, it's just generated automatically by the compiler and isn't available to you at compile time. If you run your class through something like Reflector (or examine it at runtime with reflection), you'll see the backing field.
To answer your question of "What's the difference?", the obvious answer is that one is a field, whereas one is a property. The advantage to using auto properties is that it gives you the flexibility to move to traditional properties later, should the need arise, without changing your API. As far as third party code being able to "reach" one but not the other, that would be a question best answered by the other developer. That being said, most API's are designed to work on properties, not fields (since conventional wisdom is that you do not expose fields outside of the declaring class). If the third-party library is reflectively scanning your class, then it's likely only looking for properties.
The important thing to remember is that:
private string backingField;
public string Data
{
get { return backingField; }
set { backingField = value; }
}
and
public string Data { get; set; }
Are compiled to essentially the same code. The only substantive difference is the name of the backing field.