could someone explain me what's the idea behind using Auto-Implemented Properties c#?
public class Customer
{
public int ID { get; set; }
public string Name { get; set; }
}
I get the motivation to use properties for private field, so we can determine how one can access a private field. But here - it's just like defining the field to be public from the first place. no?
Is there a difference between defining a field to be "public const" or define it to have a get-only property ?
A public automatic property is not the same as a public field, they are not binary compatible. If you implement a public field and later on want to add some logic, you will have to change it into a property and thereby introduce a breaking change (because of the binary incompatibility). This is the reason why many conventions state that you should never expose public fields but rather use properties.
So, automatic properties are just a convenient starting point for any simple non-private class value member, allowing one to add logic later on while keeping binary compatibility.
Properties can be databound, whereas fields can not.
Automatically implemented properties are essentially syntactic sugar. Once compiled, the backing store exists. It just isn't available from the source code.
As others have stated, properties and fields are not equivalent. Fields and properties aren't compatible so changing between them is a breaking change. In addition, you cannot use data binding with fields.
Final point. Though in your case there's little functional difference between the example and a public field, you can change the visibility of one of the accessors. So, to create a read-only property using an automatic property, you may do something like:
public int ID { get; private set; }
In this case, the get accessor is public, as per the entire signature, but the set accessor is private.
I will let MSDN do the talking here....
"In C# 3.0 and later, auto-implemented properties make property-declaration more concise when no additional logic is required in the property accessors. They also enable client code to create objects. When you declare a property as shown in the following example (see MSDN article for example), the compiler creates a private, anonymous backing field that can only be accessed through the property's get and set accessors"
Probably the most advantageous difference is you can do pre/post validation, raise PropertyChanged events etc
Is there a difference between defining a field to be "public const" or define it to have a get-only property?
Yes, a get-only field must have a private field declaration. This field can be changed by the class internally, marking a field as const means it cannot be modified.
2: a public const has to be defined at compiletime, you cannot use reference objects for that. Only classes that inherit from System.ValueType (string, int, double, ...)
A const is also static whereas a property with only a getter is not (every class has it's own instance.)
Add logic to getter function. Can access values of another property
public string Status
{
get { return DeactivateDate != null ? "InActive" : "Active"; }
private set { }
}
Related
I know that when using auto-properties, the compiler creates its own backing field behind the screen. However, in many programs I read to learn from, I see people explicitly write
private int _backingField;
public int Property { get { return _backingField; } }
What is the difference between above, and below?
public int Property { get; private set; }
I understand that its obvious to use the property when you actually have side-effects in the getter or setter, but that's often not the case. Also, I understand that you have to explicitly use the backing field in the case of structs, you can't access their members via properties.
The only difference I have been able to find is that the way of calling the value is different inside the class it is defined in. Is it then simple preference, or is there something more to calling a value through its property or by directly accessing the field?
Simple conventions?
There's not much difference between those two snippets - you can't pass a property by reference, for example, but that's rarely an issue. However, if you want the field to be readonly, like this:
private readonly int _backingField;
public int Property { get { return _backingField; } }
then there's a difference. The code I've written above prevents the value from being changed elsewhere within the class, making it clear that this is really meant to be immutable. I'd really like to be able to declare a read-only field with a read-only automatically implement property, settable only within the constructor - but that's not available at the moment.
This is rather confusing, by the way:
Also, I understand that you have to explicitly use the backing field in the case of structs, you can't access their members via properties.
What do you mean? You can definitely use properties within structs. Are you talking about backing fields which are mutable structs, i.e. the difference between:
foo.someField.X = 10;
and
foo.SomeProperty.X = 10;
? If so, I normally avoid that being an issue by making my structs immutable to start with :)
I have been coding classes like this:
public class ReportViewModel
{
public string Status;
public string DataSource;
public String DataStore { get; set; }
public PageMeta PageMeta { get; set; }
public ICollection<Question> List { get; set; }
}
Note that most of the fields use { get; set; } except the first two which I let Visual Studio add for me.
What I am wondering is do I really need to use { get; set; }. It seems to me that VS2010 does not automatically add this so do I need it?
You've created a class with two public fields (Status and DataSource) and three public properties (DataStore, PageMeta and List). I would advise against having public fields - and you should actually consider whether you really need all of these to be mutable properties at all.
There are various advantages to using properties over public fields, but the main one in my mind is that a property is logically part of the API of a class, whereas a field is logically an implementation detail. The property says what callers can do - a field says how a value is stored.
{ get; set; } indicate autoimplemented properties. In .NET there is a difference between properties and fields. Normally fields should be private. They are used for some specific implementation and should in most cases be internal to the class. Properties on the other hand are used to encapsulate behavior that is exposed to the consumers.
If you want them properly exposed as properties, yes.
They are different: your first two members are fields - not properties. The others are properties with auto-implemented accessors.
When you don't add the get and set you are using a field rather than a property. Which in many cases won't make a lot of difference. However, you can't databind to a field like you can with a Property. So you would lose that.
At all depends on how this class will be used.
If this is in your code, just used in your current product, then there isn't really much difference between fields (no {get;set;}) and properties (with the {get;set;}).
However in that case they probably shouldn't be public, make them internal or private instead so that it's clear that external code shouldn't use them.
If your class is going to be used by other assemblies then you should always convert public fields to properties.
The reason is that if you want to extend properties later on (i.e. add a body to the set) then your users can just get the new DLL from you. However if you've used fields then converting them to a property will look the same in the IDE, but require your users to recompile when they get the altered DLL.
Being public tells consumers that they can rely on that member being present, being a property gives you more control of how you deliver it to them.
There is a difference. The first two are fields and the remainder are auto-properties.
The second one, the compiler generates a private backing field and some boiler-plate get/set methods. These then allow you to access the properties like they were fields, but with the advantages only available to properties.
It is always recommended to hide fields behind properties, by either making them private and writing your own property around it or using an auto-property.
There's some advantages to properties. One being that properties can be made read-only, or even write-only, or read-only with an internal write-only, etc. Since they act just like methods, you can execute any arbitrary code inside of them. This is useful for when you need to implement things like INotifyPropertyChanged or if the property is actually calculated from several fields behind it.
The other advantage is encapsulation. You aren't tying yourself directly to the fields of the class, but rather the property. So if some detail about the field changes (say it goes away and becomes calculated), by using the property you are insulating yourself from those implementation details.
You should certainly look at using properties (for now adding the { get; set; }) for all cases. They are good practice in that they provide a level of encapsulation that shields the user from implementation specific details.
You do not have to, but this just coding stundart, with its pros and cons.
Consider this link for more resources:
Property Acessors
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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.
Please explain the following error on struct constructor. If i change struct to class
the erros are gone.
public struct DealImportRequest
{
public DealRequestBase DealReq { get; set; }
public int ImportRetryCounter { get; set; }
public DealImportRequest(DealRequestBase drb)
{
DealReq = drb;
ImportRetryCounter = 0;
}
}
error CS0188: The 'this' object cannot be used before all of its fields are assigned to
error CS0843: Backing field for automatically implemented property
'DealImportRequest.DealReq' must be fully assigned before control is returned to the caller. Consider calling the default constructor from a constructor initializer.
As the error message recommends, you can resolve this by calling the default constructor from a constructor initializer.
public DealImportRequest(DealRequestBase drb) : this()
{
DealReq = drb;
ImportRetryCounter = 0;
}
From the language specification:
10.7.3 Automatically implemented properties
When a property is
specified as an automatically
implemented property, a hidden backing
field is automatically available for
the property, and the accessors are
implemented to read from and write to
that backing field. [...] Because the
backing field is inaccessible, it can
be read and written only through the
property accessors, even within the
containing type. [...] This
restriction also means that definite
assignment of struct types with
auto-implemented properties can only
be achieved using the standard
constructor of the struct, since
assigning to the property itself
requires the struct to be definitely
assigned. This means that user-defined
constructors must call the default
constructor.
The other (more verbose) alternative, of course, is to manually implement the properties and set the backing fields yourself in the constructor.
Do note that the struct you have there is mutable. This is not recommended. I suggest you either make the type a class (your compilation problems should go away immediately) or make the type immutable. The easiest way to accomplish this, assuming the code you have presented is the entire struct, would be to make the setters private (get; private set;). Of course, you should also make sure that you don't add any mutating methods to the struct afterwards that rely on private access to modify the fields. Alternatively, you could back the properties with readonly backing fields and get rid of the setters altogether.
The code you have is equivalent to the following code:
public struct DealImportRequest
{
private DealRequestBase _dr;
private int _irc;
public DealRequestBase DealReq
{
get { return _dr; }
set { _dr = value; }
}
public int ImportRetryCounter
{
get { return _irc; }
set { _irc = value; }
}
/* Note we aren't allowed to do this explicitly - this is didactic code only and isn't allowed for real*/
public DealImportRequest()
{
this._dr = default(DealRequestBase); // i.e. null or default depending on whether this is reference or value type.
this._irc = default(int); // i.e. 0
}
public DealImportRequest(DealRequestBase drb)
{
this.DealReq = drb;
this.ImportRetryCounter = 0;
}
}
Now, all I have done here is remove the syntactic sugar that:
Implements automatic properties.
Works out which members are dealt with relative to this.
Gives all structs a default no-parameter constructor.
The first two are optional (you could write them explicitly if you wished) but the third is not - we aren't allowed to write our own code for a struct's parameterless constructor, we have to go with one that works like the one in the code above being given to us automatically.
Now, looked at here, suddenly the meaning of the two errors becomes clear - your constructor is implicitly using this before it's fields are assigned (error 188) and those fields are those backing the automatic properties (error 843).
It's a combination of different automatic features that normally we don't have to think about, but in this case don't work well. We can fix this by following the advice in the error message for 843 and calling the default constructor as part of your explicit constructor:
public DealImportRequest(DealRequestBase drb)
:this()
{
DealReq = drb;
ImportRetryCounter = 0;
}
Considering this in relation to my expanded version of your code above, you can see how this solves the problem, because it calls the constructor that assigns to the backing fields before it proceeds.
I would recommend not using auto-properties with structures unless you have a good reason to use them. Wrapping a class field in a read-write property is useful because it makes it possible for an instance to control the circumstances where it may be read or written, and take action when a read or write takes place. Further, code within an object instance can identify the instance being acted upon, and may thus perform a special action only when reading and writing a particular instance. Using an auto-property in an early version of a class will make it possible for future versions of the class to use a manually-implemented property including the aforementioned benefits while retaining compatibility with already-compiled client code. Unfortunately, wrapping a struct field in a read-write property doesn't offer those same benefits because the fields of one struct instance can be copied to another without either instance having any say in the matter. If the semantics of a struct allow a property to be written with arbitrary values in most instances [as would be the case for an auto-property], then any legitimate replacement would be semantically equivalent to a field.
I know that automatic properties must define a get and set accessor method, I also know that it is possible for either of these accessors to be made invisible by means of an access modifier.
Is there a technical reason why the compiler is happy with
public object Property { get; set; }
but not
public object Property { get; }
My (possibly wrong) understanding of this code is that the compiler generates a backing field that is hidden from the calling code like so:
private object hiddenField; //hidden by compiler.
public object Property
{
get { return hiddenField; }
set { hiddenField = value;}
}
If the compiler can generate that, is there a reason that it can't omit the set accessor function based on the presence (or lack thereof) of a setter in the property declaration.
I understand that this may be an issue of feature scope rather than a technical limitation, I also freely admit that I have not yet consulted the C# language specification as yet.
[UPDATE 2]
Forgive me...I'm an idiot :P, I see now, thank you everyone for tollerating my senior moment/
Without the set accessor, there is no way to set the value, since you don't have a way to access "hiddenField".
Similarly, without a get accessor, there would be no way to get back a value you set.
Since it really becomes useless, it's not allowed.
However, you can have different accessibility on the two methods:
public object Property { get; private set; }
This provides you the ability to hide the set from outside, but still have a usable property.
public object Property { get; private set; }
will work, and it will have the semantics you expect it to.
How could you use a property such the following?
public object Property { get; }
Theoritically if you could write something like that it always returns null as it lacks to the set accessor. I think it is useless unless you set the hidden field in some way to have a static value to always return it.
From the C# spec:
Because the backing field is
inaccessible, it can be read and
written only through the property
accessors, even within the containing
type.
Leaving one of the accessors out would mean that the property would either be read-only or write-only, even within the constructor of the class/struct. Not very useful.