I have a class library project. I have a property there like below. It's not a read only property
private Int32 ABC ;
public Int32 ABCD
{
set
{
this.ABC = value;
}
get
{
return this.ABC;
}
}
My question is, Is it necessary to declare the private variable for the same and the getter / setter ?
EDIT
I mean could it be like below ?
public Int32 ABCD {}
Automatic property declarations require the get; set; statements in the braces (which is what you had in your original question):
public Int32 ABCD { get; set; }
An empty property block isn't valid.
Use auto-implemented properties introduced in C# 3.0 (VS 2008) or later.
http://msdn.microsoft.com/en-us/library/bb384054.aspx
public Int32 Abcd {get;set;}
The compiler creates the backing field for you. However it is anonymous and cannot be accessed. If later on you find the need to access the backing field, you can explicitly declare it and reimplement the getter and setter using that field without breaking your interface.
Use auto implemented property, It interduced from 2008 :
public Int ABCD
{
set;
get;
}
But difference is default value, i.e in property with backing field you can set a default value in variable but in this case default is .net defaults. (in fact you should initiate it in constructor for default value).
If you do the second choice, C# will create the first one for you.
Is it necessary to declare the private variable for the same and the getter / setter ?
If you mean is it necessary to use the private variable then yes, it is (unless you use the automatic method mentioned by BoltClock).You should keep in mind that it is a full code block, so you can do pretty much anything you like in there - although you should do it within reason, as you don't want anything that is going to slow down the access to the properties too much.
For example, there can be side effects to changing a property, i.e. another property may have to be updated, or you may have to notify that other properties have changed as well.
If you are not notifying or doing anything else, then the automatic getter/setter method is the quicker to develop (that is not claiming it is the fastest (or slowest) to execute though).
you can do like this
public string ABCD { get;set;}
Related
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 ; }).
Is there any difference between these two classes from API/backward compatibility point of view:
Case A:
class Employee
{
public string Title { get; set; }
}
Case B:
class Employee
{
public string Title;
}
May I change from case B to case A without braking backward compatibility ?
Case C:
class Employee
{
public string Title { get { return T; } set { T = value; } }
private string T;
}
May I change from case B to case C too without breaking backward compatibility ?
Switching between A and C is fine. From a public point of view they are identical. An automatic property is still a normal property. It just spares you the work of defining a getter, setter and backing field manually.
B is different from both of them since a field is not a property.
In particular reflection distinguishes them(listing properties vs listing fields). For example typical property editors or some serializers only list properties but not public fields.
And of course only the field can be passed around as ref or out parameter. So such code breaks when you switch to a property.
For normal code they are usually source compatible, but not binary compatible. i.e. if you change between field and property without recompiling a dependent assembly it will break. But if you recompile both it will work is most cases.
This means a public field is not that bad in application code(but I'd still avoid it), but for library code you should really use a property.
Case A and C are identical (in case A backing field is genreated automatically by compiler).
No you can't change from B to either A or C without breaking backward compatibility - as you are changing field to proerties.
No, generally you can't.
You for example can't pass references to properties, only to fields (you can't pass a value of property by reference). The are also differences if some of the code using the class used reflection.
They are not the same: B uses a public field, A uses a public property. If any of the consumers of your class uses reflection to get to the field or passes the public field by reference, they will break, same goes for C.
Your two examples are not the same. The following two are the same:
class Employee
{
public string Title {get; set;}
}
and
class Employee
{
private string _title;
public string Title
{
get {return _title;}
set {_title = value;}
}
}
If you keep the names the same, then it shouldn't break backwards compatibility.
All the calls will be the same syntax-wise.
Problems may occur if you do it backwards. If you had a private variable and a property, the private variable may have been called in the class somewhere.
I want to make something clear that other people seem to be glossing over. While changing from B to C would not break a solution, this only works for things that are compiled together. If you are making an API or library which you want to reference in other projects and which you are not going to include in the solution (for example you are distributing the dll file to clients), changing from B to C will break clients using the API while changing from A to C will not.
Yes, you can, because you have declared the three classes private :-) :-)
Just kidding. Now... If you use StyleCop (or other similar "programs") they will mark all the public (and protected) (not-readonly nor const) fields as "bad things". This is because you can't change ANYTHING of them without breaking compatibility. The "right" thing to use a public field is not to use it, but to use a property (backed by a private field, or perhaps an internal field (but I'm not sure of this)). Now... A and C are "equivalent" against a "well behaved" user of the class (a "well behaved" user of the class uses only the public "interface" and doesn't use reflection).
To name a difference between A and B. You can pass ref and out of a field. You can't do it for a property (this is the "main" difference). The use of reflection on fields and property is different (it's OK to use public fields and properties through reflection)...
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.
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 { }
}
In C#, if I declare an auto-implemented property, why do I have to declare BOTH the get and set part?
i.e.
public string ThisWorks { get; set; }
public string ThisDoesnt { get; }
Isn't this just syntactic sugar - i.e. the compiler inserts a private field for the property? So why the problem?
Curious.
If you didn't have a setter - then how would you ever set the property?
Incidentally, you can specify the accessibility, eg:
public string Foo
{
get;
private set;
}
Without a setter, you would never be able to provide a value - as you don't have any way of specifying the backing variable's name.
I've requested a readonly automatic property, declared like this:
public string ReadonlyProperty { get; readonly set; }
which would create a readonly backing variable, a property with only a getter, and translate all calls to the setter into direct access to the variable. You could only call the setter within the constructor - just like for normal readonly variables.
We'll see whether this request does any good... it's a real shame it's not in there at the moment, as it makes it harder to implement immutable types than mutable types :(
An auto-implemented property has no accessible private store, so you would have no way to set the value without a setter, making it totally useless.
You need a set - otherwise, how does your auto-implemented property get its value? When auto-implementing the property, you have to have a set accessor to at least give it a value during construction.
Interestingly, the new Roslyn compiler in Visual Studio 2015 now allows this, even if the project is configured to use C# version 5.