Related
What is the difference in functionality between using a field with get and set methods versus using a property to attribute a value to an object through a class? For example, when setting up a value val in a class, are there any reasons to choose one of the two classes below over the other (other than length of code written and interface compatibility):
class FieldTest
{
public FieldTest()
{
}
private string val;
public void SetVal(string temp)
{
val = temp;
}
public string GetVal()
{
return val;
}
}
Versus
class PropertyTest
{
public PropertyTest()
{
}
public string val { get; set; }
}
Tested Usage in Visual Studio 2010:
class TestFunctions
{
static void Main(string[] args)
{
FieldTest Test_Fields = new FieldTest();
Test_Fields.SetVal("Test");
string temp_str = Test_Fields.GetVal();
PropertyTest Test_Property = new PropertyTest();
Test_Property.val = "test";
string temp_str_prop = Test_Property.val;
System.Windows.Forms.MessageBox.Show("Field: " + temp_str + "\n\nProperty: " + temp_str_prop);
}
}
I know only a field can use ref and out keywords, but the other advantages usually attributed to a property--encapsulation, versioning, etc-- seem to be the same with these two setups.
I've checked articles such as Difference between Property and Field in C# 3.0+ and What is the difference between a Field and a Property in C#?. Though they give good descriptions of the ideas behind properties and fields, I have not been able to find a specific answer to my question.
Thanks in advance for clarification.
EDIT 2015-07-29:
I believe this to be a separate question from other StackOverflow answers, such as those found here, as these answers did not seem to specifically address using fields with their own get and set methods as a replacement for a property.
My statement above, "I know only a field can use ref and out keywords..." comes from answers similar to the following (found here):
"Fields may be used for out / ref parameters, properties may not. Properties support additional
logic – this could be used to implement lazy loading among other things."
The functionality is almost identical. For "normal" code use-cases, these snippets will act exactly the same, as a property is in effect just a hidden field with two hidden methods (get and set).
However, there is a difference when it comes to reflection. Properties show up as PropertyInfo, and methods MethodInfo. You also can only bind to properties (in WPF/WinRT). Serialization also only works against properties. Both of these (and doubtlessly others) fail because they use reflection to find the members to act against.
So depending on your use case, they are the same. Generally speaking, I would stick with properties.
In the .NET world properties are how you attribute data to objects. Methods are typically actions associated with the objects. Fields usually store internal (private) object instance state.
Under the hood, read/write property accessors get compiled to get and set methods.
Additionally, many technologies do not work with methods. Data Annotations, Entity Framework, and serialization are a few that pop instantly to mind.
I would always vote for properties rather than getter and setter.
First of all - using Property is neat and clean. The code is more clear, less junky and easy to understand.
If you use Automatic Property you just need one line of code for one Property where you need at least 6 for a getter and setter approach. So if your class has 20 attributes then total 120 lines of codes? Oh Man!!!
but the other advantages usually attributed to a property--encapsulation, versioning, etc-- seem to be the same with these two setups. => I disagree, consider a scenario where you want to force all implementation of an interface with an attribute to be readonly. That is easily doable with a readonly property in the interface. Now try that with getter and setter. Frankly you can't.
Then there comes Serialization. You cannot serialize a computed value unless that is a property. Methods are never serialized.
Let's take a look at your second code:
class PropertyTest
{
public PropertyTest()
{
}
public string val { get; set; }
}
As said in the Auto-Implemented Properties page on MSDN, when you declare an auto-implemented property like in your example, the compiler creates a private, anonymous backing field that can only be accessed through the property's get and set accessors.
In other words, it would be like writing this code:
public class PropertyTest
{
public PropertyTest()
{
}
private string _val;
public string val
{
get { return _val; }
set { val = value; }
}
}
So, properties are a way to encapsulate fields. As you can see on MSDN, too:
A property is a member that provides a flexible mechanism to read,
write, or compute the value of a private field. Properties can be used
as if they are public data members, but they are actually special
methods called accessors. This enables data to be accessed easily and
still helps promote the safety and flexibility of methods.
In my opinion, you should always prefer to use the property implementation than the getter/setter methods. Not because it seems cleaner and flexible to make things like compute values, but it is actually easier to implement (you write less code on auto-implemented properties).
We could say that there are almost no difference from the properties than the getter/setter methods too, if we look at the part where MSDN says "but they are actually special methods called accessors". But again, we have the example of brainless coder above, we have Framework behaviours that encourages us to use properties: methods can not be serialized, while properties can.
I'm creating a custom attribute in C# and I want to do different things based on whether the attribute is applied to a method versus a property. At first I was going to do new StackTrace().GetFrame(1).GetMethod() in my custom attribute constructor to see what method called the attribute constructor, but now I'm unsure what that will give me. What if the attribute was applied to a property? Would GetMethod() return a MethodBase instance for that property? Is there a different way of getting the member to which an attribute was applied in C#?
[AttributeUsage(AttributeTargets.Method | AttributeTargets.Property,
AllowMultiple = true)]
public class MyCustomAttribute : Attribute
Update: okay, I might have been asking the wrong question. From within a custom attribute class, how do I get the member (or the class containing the member) to which my custom attribute was applied? Aaronaught suggested against walking up the stack to find the class member to which my attribute was applied, but how else would I get this information from within the constructor of my attribute?
Attributes provide metadata and don't know anything about the thing (class, member, etc.) they are decorating. On the other hand, the thing being decorated can ask for the attributes it is decorated with.
If you must know the type of the thing being decorated you will need to explicitly pass it to your attribute in its constructor.
[AttributeUsage(AttributeTargets.Method | AttributeTargets.Property,
AllowMultiple = true)]
public class MyCustomAttribute : Attribute
{
Type type;
public MyCustomAttribute(Type type)
{
this.type = type;
}
}
Since there seems to be a lot of confusion with respect to how the stack frames and methods work, here is a simple demonstration:
static void Main(string[] args)
{
MyClass c = new MyClass();
c.Name = "MyTest";
Console.ReadLine();
}
class MyClass
{
private string name;
void TestMethod()
{
StackTrace st = new StackTrace();
StackFrame currentFrame = st.GetFrame(1);
MethodBase method = currentFrame.GetMethod();
Console.WriteLine(method.Name);
}
public string Name
{
get { return name; }
set
{
TestMethod();
name = value;
}
}
}
The output of this program will be:
set_Name
Properties in C# are a form of syntactic sugar. They compile down to getter and setter methods in the IL, and it's possible that some .NET languages might not even recognize them as properties - property resolution is done entirely by convention, there aren't really any rules in the IL spec.
Now, let's say for the moment that you had a really good reason for a program to want to examine its own stack (and there are precious few practical reasons to do so). Why in the world would you want it to behave differently for properties and methods?
The whole rationale behind attributes is that they are a kind of metadata. If you want a different behaviour, code it into the attribute. If an attribute can mean two different things depending on whether it's applied to a method or property - then you should have two attributes. Set the target on the first to AttributeTargets.Method and the second to AttributeTargets.Property. Simple.
But once again, walking your own stack to pick up some attributes from the calling method is dangerous at best. In a way, you are freezing your program's design, making it far more difficult for anybody to extend or refactor. This is not the way attributes are normally used. A more appropriate example, would be something like a validation attribute:
public class Customer
{
[Required]
public string Name { get; set; }
}
Then your validator code, which knows nothing about the actual entity being passed in, can do this:
public void Validate(object o)
{
Type t = o.GetType();
foreach (var prop in
t.GetProperties(BindingFlags.Instance | BindingFlags.Public))
{
if (Attribute.IsDefined(prop, typeof(RequiredAttribute)))
{
object value = prop.GetValue(o, null);
if (value == null)
throw new RequiredFieldException(prop.Name);
}
}
}
In other words, you're examining the attributes of an instance that was given to you but which you don't necessarily know anything about the type of. XML attributes, Data Contract attributes, even Attribute attributes - almost all attributes in the .NET Framework are used this way, to implement some functionality that is dynamic with respect to the type of an instance but not with respect to the state of the program or what happens to be on the stack. It is very unlikely that you are actually in control of this at the point where you create the stack trace.
So I'm going to recommend again that you don't use the stack-walking approach unless you have an extremely good reason to do so which you haven't told us about yet. Otherwise you are likely to find yourself in a world of hurt.
If you absolutely must (don't say we didn't warn you), then use two attributes, one that can apply to methods and one that can apply to properties. I think you'll find that to be much easier to work with than a single super-attribute.
GetMethod will always return you the function name. If it is a property, you will get either get_PropertyName or set_PropertyName.
A property is basically a type of method, so when you implement a property, the compiler creates two separate functions in the resulting MSIL, a get_ and a a set_ methods. This is why in the stack trace you receive these names.
custom attributes are activated by some code calling the GetCustomAttributes method on the ICustomAttributeProvider (reflection object) that represents the location where the attribute is applied. So in the case of a property, some code would obtain the PropertyInfo for the property and then call GetCustomAttributes on that.
If you want to build out some validation framework you would need to write the code that inspects types & members for custom attributes. You could for example have an interface that attributes implement to participate in your validation framework. Could be as simple as the following:
public interface ICustomValidationAttribute
{
void Attach(ICustomAttributeProvider foundOn);
}
Your code could look for this inteface on (for example) a Type:
var validators = type.GetCustomAttributes(typeof(ICustomValidationAttribute), true);
foreach (ICustomValidationAttribute validator in validators)
{
validator.Attach(type);
}
(presumably you would walk the whole reflection graph and do this for each ICustomAttributeProvider). For an example of a similar approach in action in the .net FX you can look at WCF's 'behaviors' (IServiceBehavior, IOperationBehavior, etc).
Update: the .net FX does have a sort-of general purpose, but basically undocumented interception framework in the form of ContextBoundObject and ContextAttribute. You can search the web for some examples of using it for AOP.
Consider the following attribute.
internal class NiceAttribute : Attribute
{
private string _stuff;
public string Stuff
{
set { _stuff = value; }
}
}
When I try to use the attribute [Nice(Stuff = "test")] the compiler gives the following error.
'Stuff' is not a valid named attribute argument. Named attribute
arguments must be fields which are not readonly, static, or const, or
read-write properties which are public and not static.
What is the rational behind the requirement for the property to be readable?
Update
I will try to sketch my use case for having write-only properties on attributes.
interface ISettingsBuilder
{
Settings GetSettings();
}
class SettingsAttribute : Attribute, ISettingsBuilder
{
private readonly IDictionary<string, object> _settings =
new Dictionary<string, object>();
public Settings GetSettings()
{
// Use _settings to create an immutable instance of Settings
}
public string Stuff
{
set { _settings["Stuff"] = value; }
}
// More properties ...
}
There may be other implementations of ISettingsBuilder. For example one that offers a nice API to build settings through code.
I ended up with implementing my getters by throwing a NotImplementedException.
public string Stuff
{
get { throw new NotImplementedException(); }
set { _settings["Stuff"] = value; }
}
Can you think of a nicer way to do something like this?
I suspect the compiler is using a slightly misguided check to see whether you are accessing a private property here
Edit "we" have now located the actual source. For informational purposes, here is the full breakdown, but feel free to skip to the bottom.
(note how bug should be filed against the Mono compiler. I'll think that one over for a while)
Compiler Error CS0617
http://msdn.microsoft.com/en-us/library/978b3z1b(v=vs.71).aspx
'reference' is not a valid named attribute argument. Named attribute arguments must be fields which are not readonly, static or const, or read-write properties which are not static.
An attempt was made to access a private member of an attribute class.
It could seem that it is using some kind of lookup (akin to reflection) to make sure that the getter is accessible and if it isn't, concludes that it must be private.
Which, of course, it doesn't need to be :)
Mono Compatibility:
For fun, observe that the mono compiler has no problem whatsoever accepting this attribute: https://ideone.com/45fCX
Because of Reflection:
Of course it could be that the compiler requires attribute parameters to have reflectable values. If the property wasn't publicly readable, you could only use reflection to 'observe' that the attribute is present, not with what parameter it was initialized.
I don't know exactly, why such a design choicde would have been made, but it does make sense if you factor in reflection usage.
Update #Arun posted the relevant quote that confirms this conjecture (thanks!):
Accessing Attributes Through Reflection Once attributes have been associated with program elements, reflection can be used to query their existence and values. The main reflection methods to query attributes are contained in the System.Reflection.MemberInfo class (GetCustomAttributes family of methods).
So the reason must be: Attribute parameters must have reflectible values
Prize question: How does that work with positional parameters? How would one reflect those?
This link is a reference from Visual Studio 2003, but I guess it hardly changed.
Relevant portion from that link:
Accessing Attributes Through Reflection
Once attributes have been associated with program elements, reflection can be used to query their existence and values. The main reflection methods to query attributes are contained in the System.Reflection.MemberInfo class (GetCustomAttributes family of methods).
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;
Seen a few examples of code where this happens:
public class Foo
{
string[] m_workID;
public string[] WorkID
{
get
{
return m_workID;
}
private set
{
m_workID = value;
}
}
}
What's the point of this?
Since the use m_workID unnescessary.
In general, the point is to separate implementation (the field) from API (the property).
Later on you can, should you wish, put logic, logging etc in the property without breaking either source or binary compatibility - but more importantly you're saying what your type is willing to do, rather than how it's going to do it.
I have an article giving more benefits of using properties instead of public fields.
In C# 3 you can make all of this a lot simpler with automatically implemented properties:
public class Foo
{
public string[] WorkID { get; private set; }
}
At that point you still have a public getter and a private setter, but the backing field (and property implementation) is generated for you behind the scenes. At any point you can change this to a "normal" fully-implemented property with a backing field, and you'll still have binary and source compatibility. (Compatibility of serialized objects is a different matter, mind you.)
Additionally, in this case you can't mirror the behaviour you want (the ability to read the value publicly but write it privately) with a field - you could have a readonly field, but then you could only write to it within the constructor. Personally I wish there were a similar shorthand for this:
public class Foo
{
private readonly int id;
public int Id { get { return id; } }
...
}
as I like immutable types, but that's a different matter.
In another different matter, it's generally not a good idea to expose arrays like this anyway - even though callers can't change which array WorkID refers to, they can change the contents of the array, which is probably not what you want.
In the example you've given you could get away without the property setter, just setting the field directly within the same class, but it would mean that if you ever wanted to add logging etc you'd have to find all those writes.
A property by itself doesn't provide anywhere to put the data - you need the field (m_workID) for storage, but it entirely correct to hide that behind a property for many, many reasons. In C# 3.0 you can reduce this to:
public string[] WorkID {get; private set;}
Which will do much of the same. Note that exposing an array itself may be problematic, as there is no mechanism for protecting data in an array - at least with an IList<string> you could (if needed) add extra code to sanity check things, or could make it immutable. I'm not saying this needs fixing, but it is something to watch.
In addition to the Object Oriented philosophy of data encapsulation, it helps when you need to do something every time your property is read/write.
You can have to perform a log, a validation, or any another method call later in your development.
If your property is public, you'll have to look around all your code to find and modify your code. And what if your code is used as a library by someone else ?
If your property is private with appropriate get/set methods, then you change the get/set and that's all.
You can use C# 3.0 auto properties feature to save time typing:
public class Foo
{
public string[] WorkID
{
get; private set;
}
}
In addition properties gives you lot of advantages in comparison to fields:
properties can be virtual
properties hide implementation details (not all properties are just trivial variable accessors)
properties can contain validation and logging code and raise change events
interfaces cannot contains fields but properties
A lot of times you only want to provide read access to a field. By using a property you can provide this access. As you mention you may want to perform operations before the field is accessed (lazy loading, e.g.). You have a lot of code in there that just isn't necessary anymore unless you're still working in .Net 2.0-.