Refactoring INotifyPropertyChanged Setters - c#

I am sick to death of writing INPC setters like this:
public string Label
{
get {return _label;}
set
{
if (_label == value) return;
_label = value;
NotifyPropertyChanged(() => Label);
}
}
I'd like to refactor the setting of the field the same way I did INPC; I'd like to pass in the Expression<Func<T>> (and probably the backing field) into something like this:
public string Label
{
get {return _label;}
set
{
SetProperty(() => Label, ref _label, value);
}
}
... and here's the implementation I've come up with in the base class:
public virtual void SetProperty<T>(Expression<Func<T>> expression, ref T field, T value)
{
if (Equals(field, value)) return;
field = value;
NotifyPropertyChanged(expression);
}
... and it seems to work - it compiles, at least <grin />. My question is: am I missing something here? is passing by ref going to accomplish what I'm trying to do?

Yes, it will. You're still going to be a little sick of writing private backing fields and repetitive parts though:
private int _foo;
public int Foo { get{return _foo;}, set { SetProperty("Foo", ref _foo, value); } }
you can't easily escape from that.
INPC works based solely on three things:
needs a NAME of the property
needs to raise the event
and optionally needs to actually store the value, somehow, somewhere
The value-storage is actually not so important. It's important so that the property itself actually behaves, but it's not important for INPC itself. INPC is only about name+eventraising.
Your approach is OK, I'm often tempted to do the same. Writing INPCs implementation is .. soo boring. Technically, what you "invented" (sorry, you're not the first one, I saw at least a dozen similar implementations:) ) isn't really not much of a difference, you just extracted some common code to a method. Not rocket science, but still few lines saved. There is a small "expense" of not being able to tighty control when/how the event is raised. But not much of a pain though, as you can always write manual event-rising and custom conditions when needed.
However, there are some things worth noting:
if (Equals(field, value)) is a good start, but with custom objects it will may require you to override Equals and GetHashCode in many data classes, and it's not always a good idea, especially in WPF and bindings. When passing a Equals/HashCode-overriding dataobject to WPF you must take care and be prepared to see that sometimes the WPF will get confused and sometimes may not update bindings properly (i.e. it may fail to notice that an object was replaced with a new one and leave some bindings bounds to old instance). It's good to make an overload to SetProperty that will taking a IEqualityComparer<T> instead; cosmetics though
in old .Net versions, it was possible to automatically get the NAME from the callstack, but it was very expensive. That's for example why many frameworks (like i.e. XPO) deliberately ceased to do it at some point and required you to provide the name manually. However, since .Net 4.5, the "caller information" is much cheaper. See: this article on [CallerMemberName] and friends and you will be able to remove the "string propertyName". It will cost a bit, but it may be handy for you.
speaking of frameworks, please see Caliburn Micro and it's PropertyChangedBase class. You may find it very useful (Caliburn is available on nuget) or, at least, see the implementation. It uses above-mentioned caller information as default, and also has some nice features like boolean flag to temporarily disabling notifications (IsNotifying). Very useful in some cases.
There is also commercial PostSharp utility, that is able to .. automatically generate all INPC implementations. You attach the PostSharp util as a postbuild step, and it reads attributes from your properties, and if it finds out that some property is marked as to-be-a-INPC, then it rewrites IL of your assembly and adds the INPC implementation to it. What's the gain? [INPC] public int Foo {get;set;} and done. However, please note, PostSharp is commercial utility. You may be able to do such thing yourself if you find an IL weaver, probably based on Cecil from Mono. Or maybe you will find a free util for that too. I don't remember much more, sorry.
EDIT:
Now you mentioned it - you quoted SetProperty(()=> Label, somePocoClass.Label, value) which uses a expressions to find the name of property in a compile-safe way (no strings! refactor/rename works). If you decide to stick to ref, then with some extra work, you can get the ()=>Label expression, analyze it and extract the name, then transform&rewrite that into val => _label = val delegate, and cache that delegate per-field, and then use that cached delegate instead using ref parameter, resulting in: set{ SetProperty(()=>Label, value); }
That's almost as "cool" as it could ever get wit current C# specs. But: Such SetProperty gets really complex, transforming expressions isn't light but can be made almost one-time cost, but delegate cache is not "for free" and you have to cache them, because calculating transformed delegates onthefly is way too heavy. I've tried that, I've seen others trying that, and from my observations, most of people quits. And many coworkers see it as magic and won't try to touch that if something works not right. Cool, fun, but you shouldn't need C# gurus for debugging/patching simple feature like INPC.
EDIT2:
Even simpler way - get some T4 templates to generate the code for you. This is also very popular way to handle that. Or rather, was, like 3-4 years ago. I know some people around who really love T4. Somehow, I not. I felt like writing PHP or first versions of ASP-not-Net.. but don't care about my opinion here - see them and try them for yourself. Many people like it.
By the way, sorry for being chaotic - I've just remembered some bits about IL weaving:
for starters, see this article, very informative, covers all the basics plus more - dynamic proxies, il weaving by Cecil, (...)
the "free PostSharp-like ILweaver" - it's called Fody, on GitHub. See for example this article on INPC auto-implementation via Fody

If you are using .NET 4.5 You can use the CallerMemberName attribute to get the caller, you then don't need to pass in a expression.
Here is the base method I use for INPC
public virtual bool SetProperty<T>(ref T field, T value, [CallerMemberName] string propertyName = "")
{
if (Equals(field, value))
return false;
field = value;
OnNotifyPropertyChanged(propertyName);
return true;
}
This is used like
public string Label
{
get {return _label;}
set
{
bool changed = SetProperty(ref _label, value);
if(changed)
{
//Some other code you want to run only on change.
}
}
}
One very nice feature of this pattern is it is compatible with ReSharper's NotifyPropertyChangedInvocator attribute so you can have automatic rt.Click -> "To property with change notification".

Related

Is there a good way to get the current property name?

I know there are ways, but is there actually a good way?
At the moment I have a decent Attribute based framework which is working really well with the exception of the verbosity of:
public Variable<int> Value { get { return Get.Int(MethodBase.CurrentMethod()); } }
Picture that, * a few hundred (with setters often too). Is there anyway to get the same result but more concise?
I've considered:
public Variable<int> Value { get { return Get.Int("Value"); } }
and tried:
public Variable<int> Value { get { return Get.Int(() => Value); } }
But as there's many, many of these variables (there is method in this madness, I promise) I'm concerned about obscure bugs arising from the second Value (or string) not matching the first.
So my question is, can anyone think of a neater way to write the above? All I need is the current property name.
My favourite "solution" so far has been:
protected Func<MethodBase> Self = MethodBase.GetCurrentMethod;
public Variable<int> Value { get { return Get.Int(Self()); } }
Which is short, sweet, and pretty to look at.
But alas the JIT kills rules that out as an option. Is there perhaps a way I can rewrite GetCurrentMethod in C#, under a different name? It's a pity StackCrawlMark (which I believe is required) is internal to System.
Alternatively, are there any tools out there that are compatible with ReSharper but perhaps let me view the code through goggles, reducing the verbose MethodBase.GetCurrentMethod to something shorter and sweeter, whilst still compiling exactly the same? (I'd really rather not have a pre-compile fiddling step, but am open to suggestions - one that reduced it all to the property name would be nice).
In short, no. There are various attempts at this, including stack inspection (GetCurrentMethod) and lambda expressions, but ultimately they are all a bit inefficient.
Unless you need need obfuscation, I would just use a string - Get.Int("Value") - this is very unlikely to be a genuine maintenance problem.
The stack inspection approach suffers from potential brittleness if it inlines; if you need obfuscation, I would probably go with Expression. Or just mark the method as "don't obfuscate this name".
Another approach would be to have an abstract base type, and use meta-programming to create the concrete type with the right data/code at runtime, based on inspecting your attributes.

Difference between Property and Method [duplicate]

This question already has answers here:
Exposing Member Objects As Properties or Methods in .NET
(7 answers)
Closed 9 years ago.
Which one is better to use when it come to return value for example
public int EmployeeAge
{
get{return intEmployeeAge};
}
And
public int EmployeeAge()
{
return intEmployeeAge;
}
Which one is better and why? And what is best programming practice to use when we have secnario like above ?
Properties are a useful way of expressing a feature of an object, allowing get/set in a common way that can be used by APIs like data-binding, reflection and serialization. So for simple values of the object, properties are handy. Properties can't take arguments, should not have significant side-effects*, and should return quickly and repeatably. Also, there is no such thing as an "extension property" (to mirror an extension method) nor a generic property.
(*=lazy loading etc isn't uncommon, however)
Methods (C# doesn't have functions) are better for expressing things that either change the state, or which have an expectation of taking some time and not necessarily being reproducible. They don't tend to work in binding / serialization etc.
Note that properties are actually just a special way of writing methods. There is little functional difference. It is all about expressing intent. The one thing you don't want to expose, however, is fields (the actual intEmployeeAge instance variable).
So I would have:
public int EmployeeAge { get{return intEmployeeAge}; }
or just (if on the Employee object):
public int Age { get{return intEmployeeAge}; }
Of course... then the question becomes "in what unit?" I assume that is years?
If all you need to do is return a value, use a property.
If you need to do something before returning a value, use a function.
Properties holds object data
Functions defines object behavior
Take a look at -> Property Usage Guidelines
Which one is better and why? And what is best programming practice to use when we have
secnario like above ?
I write in C# however I prefer to use Get/Set functions, for me it's just better way to express what I can get from object and how I can change it's state (and this methods are grouped by alphabet in Intelisense which is also nice). However, if team prefers other conventions it's not a problem but when I work on my own projects it's just easier to read API.
e.g
Obejct1 o = new Object1();
o.P1;
o.P2;
o.P3;
from looking to API you can't say what you change in a public API or what it a read only property, unless you use IDE that shows you a small icon showing actually what you can do.
Object1 o = new Object1();
o.GetP1();
o.SetP2();
o.SetP3();
One can easily find from API how data can be changed by type's clients.
A method returns values after work is completed and a value is the result of the work being done. I don't think this is what you are doing.
A property (accessor) is meant for returning variables, which seems to be what you're trying to achieve:
As per MSDN:
The accessor of a property contains
the executable statements associated
with getting (reading or computing) or
setting (writing) the property. The
accessor declarations can contain a
get accessor, a set accessor, or both.
The declarations take the following
forms:
public int EmployeeAge
{
get;
set;
}
Have a look here, as it gives a very good description on the uses of these.
Property is a way explore the internal data element of a class in a simple manner. We can implement a properties with the type-safe get and set method.Property is implicitly called using calling convention.Property works on compile and runtime.
Method is a block of code that contain a series of statements.Method is explicitly called.
Methods works on runtime.
I'm a little late to this party, but I'll just mention another surprising difference between a property and a parameterless "get" method. As #MarcGravell notes, lazy loading is a common pattern when using properties, but beware of the Heisenberg Watch Window gotcha!
I think this has a lot to do with the culture you are programming in. As I see it, the C# / .NET culture would prefer to use a property for this case.
My advice: Try to be consistent with the main libraries you are using.
BUT: Be wary of using properties (or functions that serve the same purpose, as in your example above), as they are often a sign of bad design. You want to tell your objects to do stuff, as opposed to asking them for information. Don't be religious about this, though, just be aware of this as a code smell.

Why should I avoid using Properties in C#?

In his excellent book, CLR Via C#, Jeffrey Richter said that he doesn't like properties, and recommends not to use them. He gave some reason, but I don't really understand. Can anyone explain to me why I should or should not use properties?
In C# 3.0, with automatic properties, does this change?
As a reference, I added Jeffrey Richter's opinions:
• A property may be read-only or write-only; field access is always readable and writable.
If you define a property, it is best to offer both get and set accessor methods.
• A property method may throw an exception; field access never throws an exception.
• A property cannot be passed as an out or ref parameter to a method; a field can. For
example, the following code will not compile:
using System;
public sealed class SomeType
{
private static String Name
{
get { return null; }
set {}
}
static void MethodWithOutParam(out String n) { n = null; }
public static void Main()
{
// For the line of code below, the C# compiler emits the following:
// error CS0206: A property or indexer may not
// be passed as an out or ref parameter
MethodWithOutParam(out Name);
}
}
• A property method can take a long time to execute; field access always completes immediately.
A common reason to use properties is to perform thread synchronization, which
can stop the thread forever, and therefore, a property should not be used if thread synchronization
is required. In that situation, a method is preferred. Also, if your class can be
accessed remotely (for example, your class is derived from System.MashalByRefObject),
calling the property method will be very slow, and therefore, a method is preferred to a
property. In my opinion, classes derived from MarshalByRefObject should never use
properties.
• If called multiple times in a row, a property method may return a different value each
time; a field returns the same value each time. The System.DateTime class has a readonly
Now property that returns the current date and time. Each time you query this
property, it will return a different value. This is a mistake, and Microsoft wishes that they
could fix the class by making Now a method instead of a property.
• A property method may cause observable side effects; field access never does. In other
words, a user of a type should be able to set various properties defined by a type in any
order he or she chooses without noticing any different behavior in the type.
• A property method may require additional memory or return a reference to something
that is not actually part of the object's state, so modifying the returned object has no
effect on the original object; querying a field always returns a reference to an object that
is guaranteed to be part of the original object's state. Working with a property that
returns a copy can be very confusing to developers, and this characteristic is frequently
not documented.
Jeff's reason for disliking properties is because they look like fields - so developers who don't understand the difference will treat them as if they're fields, assuming that they'll be cheap to execute etc.
Personally I disagree with him on this particular point - I find properties make the client code much simpler to read than the equivalent method calls. I agree that developers need to know that properties are basically methods in disguise - but I think that educating developers about that is better than making code harder to read using methods. (In particular, having seen Java code with several getters and setters being called in the same statement, I know that the equivalent C# code would be a lot simpler to read. The Law of Demeter is all very well in theory, but sometimes foo.Name.Length really is the right thing to use...)
(And no, automatically implemented properties don't really change any of this.)
This is slightly like the arguments against using extension methods - I can understand the reasoning, but the practical benefit (when used sparingly) outweighs the downside in my view.
Well, lets take his arguments one by one:
A property may be read-only or
write-only; field access is always
readable and writable.
This is a win for properties, since you have more fine-grained control of access.
A property method may throw an
exception; field access never throws
an exception.
While this is mostly true, you can very well call a method on a not initialized object field, and have an exception thrown.
• A property cannot be passed as an
out or ref parameter to a method; a
field can.
Fair.
• A property method can take a long
time to execute; field access always
completes immediately.
It can also take very little time.
• If called multiple times in a row, a
property method may return a different
value each time; a field returns the
same value each time.
Not true. How do you know the field's value has not changed (possibly by another thread)?
The System.DateTime class has a
readonly Now property that returns the
current date and time. Each time you
query this property, it will return a
different value. This is a mistake,
and Microsoft wishes that they could
fix the class by making Now a method
instead of a property.
If it is a mistake it's a minor one.
• A property method may cause
observable side effects; field access
never does. In other words, a user of
a type should be able to set various
properties defined by a type in any
order he or she chooses without
noticing any different behavior in the
type.
Fair.
• A property method may require
additional memory or return a
reference to something that is not
actually part of the object's state,
so modifying the returned object has
no effect on the original object;
querying a field always returns a
reference to an object that is
guaranteed to be part of the original
object's state. Working with a
property that returns a copy can be
very confusing to developers, and this
characteristic is frequently not
documented.
Most of the protestations could be said for Java's getters and setters too --and we had them for quite a while without such problems in practice.
I think most of the problems could be solved by better syntax highlighting (i.e differentiating properties from fields) so the programmer knows what to expect.
I haven't read the book, and you haven't quoted the part of it you don't understand, so I'll have to guess.
Some people dislike properties because they can make your code do surprising things.
If I type Foo.Bar, people reading it will normally expect that this is simply accessing a member field of the Foo class. It's a cheap, almost free, operation, and it's deterministic. I can call it over and over, and get the same result every time.
Instead, with properties, it might actually be a function call. It might be an infinite loop. It might open a database connection. It might return different values every time I access it.
It is a similar argument to why Linus hates C++. Your code can act surprising to the reader. He hates operator overloading: a + b doesn't necessarily mean simple addition. It may mean some hugely complicated operation, just like C# properties. It may have side effects. It may do anything.
Honestly, I think this is a weak argument. Both languages are full of things like this. (Should we avoid operator overloading in C# as well? After all, the same argument can be used there)
Properties allow abstraction. We can pretend that something is a regular field, and use it as if it was one, and not have to worry about what goes on behind the scenes.
That's usually considered a good thing, but it obviously relies on the programmer writing meaningful abstractions. Your properties should behave like fields. They shouldn't have side effects, they shouldn't perform expensive or unsafe operations. We want to be able to think of them as fields.
However, I have another reason to find them less than perfect. They can not be passed by reference to other functions.
Fields can be passed as ref, allowing a called function to access it directly. Functions can be passed as delegates, allowing a called function to access it directly.
Properties... can't.
That sucks.
But that doesn't mean properties are evil or shouldn't be used. For many purposes, they're great.
Back in 2009, this advice merely seemed like bellyaching of the Who Moved My Cheese variety. Today, it's almost laughably obsolete.
One very important point that many answers seem to tiptoe around but don't quite address head on is that these purported "dangers" of properties are an intentional part of the framework design!
Yes, properties can:
Specify different access modifiers for the getter and setter. This is an advantage over fields. A common pattern is to have a public getter and a protected or internal setter, a very useful inheritance technique which isn't achievable by fields alone.
Throw an exception. To date, this remains one of the most effective methods of validation, especially when working with UI frameworks that involve data-binding concepts. It's much more difficult to ensure that an object remains in a valid state when working with fields.
Take a long time to execute. The valid comparison here is with methods, which take equally long - not fields. No basis is given for the statement "a method is preferred" other than one author's personal preference.
Return different values from its getter on subsequent executions. This almost seems like a joke in such close proximity to the point extolling the virtues of ref/out parameters with fields, whose value of a field after a ref/out call is pretty much guaranteed to be different from its previous value, and unpredictably so.
If we're talking about the specific (and practically academic) case of single-threaded access with no afferent couplings, it's fairly well understood that it's just bad property design to have visible-state-changing side-effects, and maybe my memory is fading, but I just can't seem to recall any examples of folks using DateTime.Now and expecting the same value to come out every time. At least not any instances where they wouldn't have screwed it up just as badly with a hypothetical DateTime.Now().
Cause observable side effects - which is of course precisely the reason properties were invented as a language feature in the first place. Microsoft's own Property Design guidelines indicate that setter order shouldn't matter, as to do otherwise would imply temporal coupling. Certainly, you can't achieve temporal coupling with fields alone, but that's only because you can't cause any meaningful behaviour at all to happen with fields alone, until some method is executed.
Property accessors can actually help prevent certain types of temporal coupling by forcing the object into a valid state before any action is taken - for example, if a class has a StartDate and an EndDate, then setting the EndDate before the StartDate could force the StartDate back as well. This is true even in multi-threaded or asynchronous environments, including the obvious example of an event-driven user interface.
Other things that properties can do which fields can't include:
Lazy loading, one of the most effective ways of preventing initialization-order errors.
Change Notifications, which are pretty much the entire basis for the MVVM architecture.
Inheritance, for example defining an abstract Type or Name so derived classes can provide interesting but nevertheless constant metadata about themselves.
Interception, thanks to the above.
Indexers, which everyone who has ever had to work with COM interop and the inevitable spew of Item(i) calls will recognize as a wonderful thing.
Work with PropertyDescriptor which is essential for creating designers and for XAML frameworks in general.
Richter is clearly a prolific author and knows a lot about the CLR and C#, but I have to say, it seems like when he originally wrote this advice (I'm not sure if it's in his more recent revisions - I sincerely hope not) that he just didn't want to let go of old habits and was having trouble accepting the conventions of C# (vs. C++, for example).
What I mean by this is, his "properties considered harmful" argument essentially boils down to a single statement: Properties look like fields, but they might not act like fields. And the problem with the statement is, it isn't true, or at best it's highly misleading. Properties don't look like fields - at least, they aren't supposed to look like fields.
There are two very strong coding conventions in C# with similar conventions shared by other CLR languages, and FXCop will scream at you if you don't follow them:
Fields should always be private, never public.
Fields should be declared in camelCase. Properties are PascalCase.
Thus, there is no ambiguity over whether Foo.Bar = 42 is a property accessor or a field accessor. It's a property accessor and should be treated like any other method - it might be slow, it might throw an exception, etc. That's the nature of Abstraction - it's entirely up to the discretion of the declaring class how to react. Class designers should apply the principle of least surprise but callers should not assume anything about a property except that it does what it says on the tin. That's on purpose.
The alternative to properties is getter/setter methods everywhere. That's the Java approach, and it's been controversial since the beginning. It's fine if that's your bag, but it's just not how we roll in the .NET camp. We try, at least within the confines of a statically-typed system, to avoid what Fowler calls Syntactic Noise. We don't want extra parentheses, extra get/set warts, or extra method signatures - not if we can avoid them without any loss of clarity.
Say whatever you like, but foo.Bar.Baz = quux.Answers[42] is always going to be a lot easier to read than foo.getBar().setBaz(quux.getAnswers().getItem(42)). And when you're reading thousands of lines of this a day, it makes a difference.
(And if your natural response to the above paragraph is to say, "sure it's hard to read, but it would be easier if you split it up in multiple lines", then I'm sorry to say that you have completely missed the point.)
I don't see any reasons why you shouldn't use Properties in general.
Automatic properties in C# 3+ only simplify syntax a bit (a la syntatic sugar).
It's just one person's opinion. I've read quite a few c# books and I've yet to see anyone else saying "don't use properties".
I personally think properties are one of the best things about c#. They allow you to expose state via whatever mechanism you like. You can lazily instantiate the first time something is used and you can do validation on setting a value etc. When using and writing them, I just think of properties as setters and getters which a much nicer syntax.
As for the caveats with properties, there are a couple. One is probably a misuse of properties, the other can be subtle.
Firstly, properties are types of methods. It can be surprising if you place complicated logic in a property because most users of a class will expect the property to be fairly lightweight.
E.g.
public class WorkerUsingMethod
{
// Explicitly obvious that calculation is being done here
public int CalculateResult()
{
return ExpensiveLongRunningCalculation();
}
}
public class WorkerUsingProperty
{
// Not at all obvious. Looks like it may just be returning a cached result.
public int Result
{
get { return ExpensiveLongRunningCalculation(); }
}
}
I find that using methods for these cases helps to make a distinction.
Secondly, and more importantly, properties can have side-effects if you evaluate them while debugging.
Say you have some property like this:
public int Result
{
get
{
m_numberQueries++;
return m_result;
}
}
Now suppose you have an exception that occurs when too many queries are made. Guess what happens when you start debugging and rollover the property in the debugger? Bad things. Avoid doing this! Looking at the property changes the state of the program.
These are the only caveats I have. I think the benefits of properties far outweigh the problems.
That reason must have been given within a very specific context. It's usually the other way round - it is recomended to use properties as they give you a level of abstraction enabling you to change behaviour of a class without affecting its clients...
I can't help picking on the details of Jeffrey Richter's opinions:
A property may be read-only or write-only; field access is always readable and writable.
Wrong: Fields can marked read-only so only the object's constructor can write to them.
A property method may throw an exception; field access never throws an exception.
Wrong: The implementation of a class can change the access modifier of a field from public to private. Attempts to read private fields at runtime will always result in an exception.
I don't agree with Jeffrey Richter, but I can guess why he doesn't like properties (I haven't read his book).
Even though, properties are just like methods (implementation-wise), as a user of a class, I expect that its properties behave "more or less" like a public field, e.g:
there's no time-consuming operation going on inside the property getter/setter
the property getter has no side effects (calling it multiple times, does not change the result)
Unfortunately, I have seen properties which did not behave that way. But the problem are not the properties themselves, but the people who implemented them. So it just requires some education.
The argument assumes that properties are bad because they look like fields, but can do surprising actions. This assumption is invalidated by .NET programmers' expectactions:
Properties don't look like fields. Fields look like properties.
• A property method may throw an exception; field access never throws an exception.
So, a field is like a property that is guaranteed to never throw an exception.
• A property cannot be passed as an out or ref parameter to a method; a field can.
So, a field is like a property, but it has additional capabilities: passing to a ref/out accepting methods.
• A property method can take a long time to execute; field access always completes immediately. [...]
So, a field is like a fast property.
• If called multiple times in a row, a property method may return a different value each time; a field returns the same value each time. The System.DateTime class has a readonly Now property that returns the current date and time.
So, a field is like a property that is guaranteed to return the same value unless the field was set to a different value.
• A property method may cause observable side effects; field access never does.
Again, a field is a property that is guaranteed to not do that.
• A property method may require additional memory or return a reference to something that is not actually part of the object's state, so modifying the returned object has no effect on the original object; querying a field always returns a reference to an object that is guaranteed to be part of the original object's state. Working with a property that returns a copy can be very confusing to developers, and this characteristic is frequently not documented.
This one may be surprising, but not because it's a property that does this. Rather, it's that barely anyone returns mutable copies in properties, so that 0.1% case is surprising.
There is a time when I consider not using properties, and that is in writing .Net Compact Framework code. The CF JIT compiler does not perform the same optimisation as the desktop JIT compiler and does not optimise simple property accessors, so in this case adding a simple property causes a small amount of code bloat over using a public field. Usually this wouldn't be an issue, but almost always in the Compact Framework world you are up against tight memory constraints, so even tiny savings like this do count.
You shouldn't avoid using them but you should use them with qualification and care, for the reasons given by other contributors.
I once saw a property called something like Customers that internally opened an out-of-process call to a database and read the customer list. The client code had a 'for (int i to Customers.Count)' which was causing a separate call to the database on each iteration and for the access of the selected Customer. That's an egregious example that demonstrates the principle of keeping the property very light - rarely more than a internal field access.
One argument FOR using properties is that they allow you to validate the value being set. Another is that the value of of the property may be a derived value, not a single field, like TotalValue = amount * quantity.
Personally I only use properties when creating simple get / set methods. I stray away from it when coming to complicated data structures.
I haven't read his book but I agree with him.
It's quite a contradiction on the usual paradigm.
They look like a field but had many sideeffects. Exceptions, performance, sideeffects, read/write access difference, etc.
If you care about readability and reliability i think you'd care about this as well.
In my experience i never gained anything from properties and only felt like using them when i was lazy.
But many times i encountered people using properties as if they were fields inadvertently creating all sort of issues.
Ive worked professionally on many languages for quite a time and never needed them, not even once.
Of course other people might find them useful and that's fine too.
Depends on your user case and what you value.
I personally believe they are just 'features ' that you can choose to use or not.
Invoking methods instead of properties greatly reduces the readability of the invoking code. In J#, for example, using ADO.NET was a nightmare because Java doesn't support properties and indexers (which are essentially properties with arguments). The resulting code was extremely ugly, with empty parentheses method calls all over the place.
The support for properties and indexers is one of the basic advantages of C# over Java.

Why GetHashCode is not a property like HashCode in .NET

Why GetHashCode is not a property like HashCode in .NET?
Probably because it requires computation, and exposing it as a propery might imply that the hashcode is already available for free.
Edit:
Guidelines on this: Properties versus Methods
"The operation is expensive enough that you want to communicate to the user that they should consider caching the result."
Perhaps GetHashCode is expensive enough in some cases.
I don't think there's any good reason. Any implemention of GetHashCode should be fast enought to put into a property. That said, there are plenty of design flaws in the .Net framework, some small, some serious. This seems like a small one.
Often it is not possible to define a HashCode for a class that makes since:
e.g. the objects of the class don’t
have a well defined concept of
identity.
Therefore it is common to make the GetHashCode() method throw a NotImplementedException. This would course all sort of problem if HashCode was a property, as most people (and debuggers) assume it is always valid to get the value of a property
Besides that a property is nothing else than a getter and a setter method, from a design perspective a property should never contain any computations other than initializing or validation, eg:
private object _obj;
public object Obj
{
get
{
if(_obj == null)
{
_obj = new object();
}
return _obj;
}
set
{
if(value == badvalue)
{
throw new ArgumentException("value");
}
_obj = value;
}
}
GetHashCode() does not contain extensive computations, but it could contain such long running operations (just from the fact that it could compute the hashcode of an object in a complex manner), this is why its a method instead of a property.
properties should only be used if the computation behind them is really fast or cached
besides most of the time the only logic in properties should be validation
You have to remember that the .NET Framework is designed to be accessed by a wide variety of languages.
In theory you could create a compiler that is incapable of correctly overriding properties. While that would make for a pretty crappy compiler, it would not necessarily be illegal. (Remember properties are just methods with some meta data)

Have trivial properties ever saved your bacon?

There's a lot of advice out there that you shouldn't expose your fields publically, and instead use trivial properties. I see it over & over.
I understand the arguments, but I don't think it's good advice in most cases.
Does anyone have an example of a time when it really mattered? When writing a trivial property made something important possible in the future (or when failing to use one got them in to real trouble)?
EDIT: The DataBinding argument is correct, but not very interesting. It's a bug in the DataBinding code that it won't accept public fields. So, we have to write properties to work around that bug, not because properties are a wise class design choice.
EDIT: To be clear, I'm looking for real-world examples, not theory. A time when it really mattered.
EDIT: The ability to set a breakpoint on the setter seems valuable. Designing my code for the debugger is unfortunate: I'd rather the debugger get smarter, but given the debugger we have, I'll take this ability. Good stuff.
It may be hard to make code work in an uncertain future, but that's no excuse to be lazy. Coding a property over a field is convention and it's pragmatic. Call it defensive programming.
Other people will also complain that there's a speed issue, but the JIT'er is smart enough to make it just about as fast as exposing a public field. Fast enough that I'll never notice.
Some non-trivial things that come to mind
A public field is totally public, you can not impose read-only or write-only semantics
A property can have have different get versus set accessibility (e.g. public get, internal set)
You can not override a field, but you can have virtual properties.
Your class has no control over the public field
Your class can control the property. It can limit setting to allowable range of values, flag that the state was changed, and even lazy-load the value.
Reflection semantics differ. A public field is not a property.
No databinding, as others point out. (It's only a bug to you. - I can understand Why .net framework designers do not support patterns they are not in favour of.)
You can not put a field on an interface, but you can put a property on an interface.
Your property doesn't even need to store data. You can create a facade and dispatch to a contained object.
You only type an extra 13 characters for correctness. That hardly seems like speculative generality. There is a semantic difference, and if nothing else, a property has a different semantic meaning and is far more flexible than a public field.
public string Name { get; set; }
public string name;
I do recall one time when first using .net I coded a few classes as just fields, and then I needed to have them as properties for some reason, and it was a complete waste of time when I could have just done it right the first time.
So what reasons do you have for not following convention? Why do you feel the need to swim upstream? What has it saved you by not doing this?
I've had a trivial property save me a couple of times when debugging. .Net doesn't support the concept of a data break point (read or write). Occasionally when debugging a very complex scenario it was important to track read/writes to a particular property. This is easy with a property but impossible with a field.
If you're not working in a production environment, it's simple to refactor a field -> property for the purpose of debugging. Occasionally though you hit bugs that only reproduce in a production environment that is difficult to patch with a new binary. A property can save you here.
It's a fairly constrained scenario though.
I used to think the same thing, Jay. Why use a property if it's only there to provide direct access to a private member? If you can describe it as an autoproperty, having a property at all rather than a field seemed kind of silly. Even if you ever need to change the implementation, you could always just refactor into a real property later and any dependent code would still work, right?. Well, maybe not.
You see, I've recently seen the light on trivial properties, so maybe now I can help you do the same.
What finally convinced me was the fairly obvious point (in retrospect) that properties in .Net are just syntactic sugar for getter and setter methods, and those methods have a different name from the property itself. Code in the same assembly will still work, because you have to recompile it at the same time anyway. But any code in a different assembly that links to yours will fail if you refactor a field to a property, unless it's recompiled against your new version at the same time. If it's a property from the get-go, everything is still good.
I'll answer your question with another one: have you ever really benefited from not making all your types and members public? I suspect I haven't directly prevented any bugs by doing so. However, I've encapsulated my types properly, only exposing what's meant to be exposed. Properties are similar - good design more than anything else. I think of properties as conceptually different from fields; they're part of the contract rather than being fundamentally part of the implementation. Thinking about them as properties rather than fields helps me to think more clearly about my design, leading to better code.
Oh, and I've benefited occasionally from not breaking source compatibility, being able to set breakpoints, log access etc.
Part of the idea is that those properties may not be trivial in the future - if you bound external code to a field and later wanted to wrap that in a property, all of the dependent code would have to change, and you may not be able to do that, especially in the case where you are a control designer or have libraries that you can't control, etc.
Not to mention there are certain .NET practices that will not allow you to use a field - databinding particularly.
I am sure there are other good reasons as well. Why does it matter to you? Use an automatic property and be done with it. Seems like something not worth being concerned about to me...
It's much easier to debug a problem involving a field if it is wrapped by a property accessor. Placing breakpoints in the accessor can quite quickly help with finding re-entrancy and other issues that otherwise might not be caught. By marshaling all access to the field through an accessor, you can ascertain exactly who is changing what and when.
In .NET, from my understanding, you cannot databind to public fields; but only to properties. Thus, if you want to do databinding, you have no choice.
I once had fields that I wanted to expose from a windows from project which allowed the stats for the program (TotalItems and SuccessfulItems).
Later I decided to display the stats on the form and was able to add a call in the setter that updated the display when the property changed.
Obviously, if you're not creating a shared class library, and you're not using DataBinding, then using a field will cause you no problems whatsoever.
But if you're creating a shared class library, you'd be foolish IMHO to do otherwise than follow the guidelines, for the usual three reasons:
consumers of your shared class library may want to use DataBinding.
consumers of your shared class might want binary compatibility, which is not possible if you switch from a field to a property.
the principal of least surprise implies you should be consistent with other shared class libraries including the .NET Framework itself.
IMHO there is no such thing as a trivial property as people have been calling them. Via the way things such as databinding work, Microsoft has implied that any bit of data that is a part of the public interface of an object should be a property. I don't think they meant it merely to be a convention like it is in some other languages where property syntax is more about convention and convenience.
A more interesting question may be: "When should I use a public field instead of a property", or "When has a public field instead of a public property saved your bacon?"
Fields which are of structure types allow direct access to the members thereof, while properties of such types do not. Consequently, if Thing.Boz were a field of type Point, code which wants to modify its X value could simply say Thing.Boz.X += 5;; if Thing.Boz were a mutable property, it would be necessary to use var tmp = Thing.Boz; tmp.X += 5; Thing.Boz = tmp;. The ability to write things more cleanly with the exposed field is often, but not always, a blessing.
If it will always be possible for Boz to be a field, modifying its members directly will be cleaner, faster, and better than copying it to a temporary variable, modifying that, and copying it back. If the type of Boz exposes its mutable fields directly (as structures should) rather than wrapping them in trivial wrappers, it will also be possible to use things like Interlocked methods on them--something that's simply impossible with properties. There's really only one disadvantage to using fields in that way: if it's ever necessary to replace the field with a property, code which relied upon the thing being a field will break, and may be hard to fix.
In short, I would posit that in cases where one isn't concerned about being able to swap in different versions of code without having to recompile any consumers, the biggest effect of using properties rather than fields is to prevent consumers of the code from writing code which would take advantage of (and rely upon) the semantics of exposed fields which are of structure types.
Incidentally, an alternative to exposing a field of a structure type would be to expose an ActOnXXX method. For example:
delegate void ActionByRef<T1>(ref T1 p1);
delegate void ActionByRef<T1,T2>(ref T1 p1, ref T2 p2);
delegate void ActionByRef<T1,T2,T3>(ref T1 p1, ref T2 p2, ref T3 p3);
// Method within the type that defines property `Boz`
void ActOnBoz<T1>(ActionByRef<Point, T1> proc, ref T1 p1)
{
proc(ref _Boz, ref p1); // _Boz is the private backing field
}
Code which wanted to add some local variable q to Thing.Boz.X could call Thing.ActOnBoz((ref Point pt, ref int x) => {pt.X += x;}, ref q); and have the action performed directly on Thing._Boz, even though the field is not exposed.

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