When you have automatic properties, C# compiler asks you to call the this constructor on any constructor you have, to make sure everything is initialized before you access them.
If you don't use automatic properties, but simply declare the values, you can avoid using the this constructor.
What's the overhead of using this on constructors in structs? Is it the same as double initializing the values?
Would you recommend not using it, if performance was a top concern for this particular type?
I would recommend not using automatic properties at all for structs, as it means they'll be mutable - if only privately.
Use readonly fields, and public properties to provide access to them where appropriate. Mutable structures are almost always a bad idea, and have all kinds of nasty little niggles.
Do you definitely need to create your own value type in the first place though? In my experience it's very rare to find a good reason to create a struct rather than a class. It may be that you've got one, but it's worth checking.
Back to your original question: if you care about performance, measure it. Always. In this case it's really easy - you can write the struct using an automatic property and then reimplement it without. You could use a #if block to keep both options available. Then you can measure typical situations and see whether the difference is significant. Of course, I think the design implications are likely to be more important anyway :)
Yes, the values will be initialized twice and without profiling it is difficult to say whether or not this performance hit would be significant.
The default constructor of a struct initializes all members to their default values. After this happens your constructor will run in which you undoubtedly set the values of those properties again.
I would imagine this would be no different than the CLR's practice of initializing all fields of a reference type upon instantiation.
The reason the C# compiler requires you to chain to the default constructor (i.e. append : this() to your constructor declaration) when auto-implemented properties are used is because all variables need to be assigned before exiting the constructor. Now, auto-implemented properties mess this up a bit in that they don't allow you to directly access the variables that back the properties. The method the compiler uses to get around this is to automatically assign all the variables to their default values, and to insure this, you must chain to the default constructor. It's not a particularly clever method, but it does the job well enough.
So indeed, this will mean that some variables will end up getting initialised twice. However, I don't think this will be a big performance problem. I would be very surprised it the compiler (or at very least the JIT) didn't simply remove the first initialisation statement for any variable that is set twice in your constructor. A quick benchmark should confirm this for you, though I'm quite sure you will get the suspected results. (If you by chance don't, and you absolutely need the tiny performance boost that avoidance of duplicate initialisation offers, you can just define your properties the normal way, i.e. with backing variables.)
To be honest, my advice would be not even to bother with auto-implemented properties in structures. It's perfectly acceptable just to use public variables in lieu of them, and they offer no less functionality than auto-implemented properties. Classes are a different situation of course, but I really wouldn't hesitate to use public variables in structs. (Any complex properties can be defined normally, if you need them.)
Hope that helps.
Don't use automatic properties with structure types. Simply expose fields directly. If a struct has an exposed public field Foo of type Bar, the fact that Foo is an exposed field of type Bar (information readily available from Intellisense) tells one pretty much everything there is to know about it. By contrast, the fact that a struct Foo has an exposed read-write property of Boz does not say anything about whether writing to Boz will mutate a field in the struct, or whether it will mutate some object to which Boz holds a reference. Exposing fields directly will offer cleaner semantics, and often also result in faster-running code.
Related
Wrapping a struct or class field in a property forces all accesses to that field to go through "getter" and "setter" methods. This allows for the possibility of adding logic for validation, lazy initialization, etc. Further, in the case of class fields, it allows for the possibility that one might have logic which applies to some instances but not others; if the properties are not virtual, it may be difficult to implement such logic efficiently (e.g. one would might have to define a static VerySpecialInstance and have the property getter say if (this == VerySpecialInstance) GetSpecialProperty(); else GetOrdinaryProperty();) but it could be done.
If, however, the semantics of a struct (e.g. System.Drawing.Point) dictate that a particular read-write property may be written with any value which is legal for its type, writing will have no side effect other than to change its value, it will always return the last value written (if any), and if not written it will read as the default value for its type; and if code which uses the type will likely rely upon such assumptions, I'm unclear on what possible benefit would be served by using a read-write property rather than a field to hold the value.
The fact that Microsoft uses properties rather than fields for things like Point.X etc. has historically caused confusion since MyList[3].X = 4; would be translated to MyList[3].Set_X(4), and without looking inside the definition of Set_X it's not possible to tell whether that method would achieve its desired effect without changing any fields of the struct in question; today's C# compiler will guess that it wouldn't work, and will forbid that construct even though there are some struct types where property setters would in fact work just fine. If X been a field rather than a property, and if Microsoft had said that the two safe ways to mutate a struct are either to access the fields directly or to pass the struct as a ref parameter to a mutating method (which, if it's a static method of the struct type, could access public fields), such guesswork would not be necessary.
Given that using exposed struct fields rather than read-write properties improves both performance and semantic clarity, what reasons exist to make struct fields private and wrap them in properties? Data binding requires properties, but I don't think it works with structure types anyway (if one makes a copy of a struct and then sets some property of the original to one value and the corresponding property of the duplicate to another, what value should be reported to the bound object?) Are there some benefits of struct properties of which I'm unaware?
Personally, I think the 'ideal' struct in many cases would simply be a list of exposed public fields, and a constructor whose parameters are simply the initial values of those fields, in order. Such a struct would offer optimal performance and predictable semantics (behaving identically to all other such structs, aside from the types and names of the fields). Is there any reason to favor read-write properties in cases where there isn't anything they could do anything other than simply read and write an underlying field?
Don't see any benefit on immutable struct of using read/write properties, except point you wrote about: wrapping the logic inside setter and/or getter of the property, and maintaining general guideline across your code base (benefit for maintainance and readability point of view) .
I personally when define a struct almost always use raw public fields and no properties, for simplicity and easy consumption of my type (for the problems on immutable types you wrote already in question)
Hope this helps.
Rico Mariani wrote a good MSDN blog article on this very topic.
Reasons to use public fields rather than getters and setters include:
There are no values the field cannot be allowed to have.
The client is expected to edit it.
To be able to write things such as object.X.Y = Z.
To making a strong promise that the value is just a value and there are no side-effects associated with it (and won't be in the future either).
Some people find this very controversial. I suspect this is because the case listed rarely or never come up in the kind of software they write, but they don't realise that in other application areas they come up a great deal.
(This is a copy of an answer I provided here, but I thought the information is useful enough to be repeated here.)
Coming from a C++ background and trying to learn C#, one of the most frustrating language omissions that I've come across is an equivalent to the const keyword.
So, I have been attempting to settle on a pattern that I can use to achieve const correctness in C#.
This answer has an interesting suggestion: create a read only interface for all of your types. But as Matt Cruikshank pointed out in the comments, this becomes problematic if your class has collections or other rich types. Particularly if you do not have control over the type, and can't make it implement a read-only interface.
Do any patterns or solutions exist that can handle rich types and collections, or are we forced in C# to simply make copies? Is it better to just give up on const correctness in C# altogether?
Can you get immutability in C#? Sure, if you design for it. You can do creative things with interfaces, and so on, to only expose the get of properties and none of the mutable methods.
That said, keep in mind there is nothing that prevents a crafty user from casting it back to the actual type (of course, same could be said of C++, you can cast away const-ness).
ISomeReadOnlyInterface readOnly = new SomeFullObject();
// hah, take that read-only interface!
((SomeFullObject)readOnly).SomeMutatingMethod();
Same with collections. Even if you return a ReadOnlyCollection (which prevents mutating behaviors on the collection itself) the data in the collection is still mutable (as long as the type allows it of course).
So I'm afraid there's really no simple answer here. There's no "flip-a-switch" const that gives you what C++ does.
It's really up to you, you can:
Design your types to be immutable and return iterators (or other read only sequences) instead of mutable collections.
Return new copies each time so that if they alter them it's no biggie.
Just return the actual data and leave tampering behavior as "undefined".
etc...
The latter is what collections like Dictionary<TKey, TValue> do. There's nothing that says you can't make the key type a mutable type (but woe if you do), and the MSDN is pretty clear that if you alter the key in such a way that it's hash code changes, it's on your own neck...
For my own work, I tend to keep it simple unless there is actually a big concern my class may be altered in a way that would cause side-effects. For example, if I'm storing web service results in a cache, I'll return a copy of the cached item instead so that if a user modifies the result they won't inadvertently modify the cached value.
So, long and the short of it is that I wouldn't worry about const-correctness of every type you return, that's just way too much. I'd only worry about things that you return that, if altered, could create a side-effect to other users.
From this Answer, I came to know that KeyValuePair are immutables.
I browsed through the docs, but could not find any information regarding immutable behavior.
I was wondering how to determine if a type is immutable or not?
I don't think there's a standard way to do this, since there is no official concept of immutability in C#. The only way I can think of is looking at certain things, indicating a higher probability:
1) All properties of the type have a private set
2) All fields are const/readonly or private
3) There are no methods with obvious/known side effects
4) Also, being a struct generally is a good indication (if it is BCL type or by someone with guidelines for this)
Something like an ImmutabeAttribute would be nice. There are some thoughts here (somewhere down in the comments), but I haven't seen one in "real life" yet.
The first indication would be that the documentation for the property in the overview says "Gets the key in the key/value pair."
The second more definite indication would be in the description of the property itself:
"This property is read/only."
I don't think you can find "proof" of immutability by just looking at the docs, but there are several strong indicators:
It's a struct (why does this matter?)
It has no settable public properties (both are read-only)
It has no obvious mutator methods
For definitive proof I recommend downloading the BCL's reference source from Microsoft or using an IL decompiler to show you how a type would look like in code.
A KeyValuePair<T1,T2> is a struct which, absent Reflection, can only be mutated outside its constructor by copying the contents of another KeyValuePair<T1,T2> which holds the desired values. Note that the statement:
MyKeyValuePair = new KeyValuePair(1,2);
like all similar constructor invocations on structures, actually works by creating a new temporary instance of KeyValuePair<int,int> (happens before the constructor itself executes), setting the field values of that instance (done by the constructor), copying all public and private fields of that new temporary instance to MyKeyValuePair, and then discarding the temporary instance.
Consider the following code:
static KeyValuePair MyKeyValuePair; // Field in some class
// Thread1
MyKeyValuePair = new KeyValuePair(1,1);
// ***
MyKeyValuePair = new KeyValuePair(2,2);
// Thread2
st = MyKeyValuePair.ToString();
Because MyKeyValuePair is precisely four bytes in length, the second statement in Thread1 will update both fields simultaneously. Despite that, if the second statement in Thread1 executes between Thread2's evaluation of MyKeyValuePair.Key.ToString() and MyKeyValuePair.Value.ToString(), the second ToString() will act upon the new mutated value of the structure, even though the first already-completed ToString()operated upon the value before the mutation.
All non-trivial structs, regardless of how they are declared, have the same immutability rules for their fields: code which can change a struct can change its fields; code which cannot change a struct cannot change its fields. Some structs may force one to go through hoops to change one of their fields, but designing struct types to be "immutable" is neither necessary nor sufficient to ensure the immutability of instances. There are a few reasonable uses of "immutable" struct types, but such use cases if anything require more care than is necessary for structs with exposed public fields.
I read all the questions related to this topic, and they all give reasons why a default constructor on a struct is not available in C#, but I have not yet found anyone who suggests a general course of action when confronted with this situation.
The obvious solution is to simply convert the struct to a class and deal with the consequences.
Are there other options to keep it as a struct?
I ran into this situation with one of our internal commerce API objects. The designer converted it from a class to a struct, and now the default constructor (which was private before) leaves the object in an invalid state.
I thought that if we're going to keep the object as a struct, a mechanism for checking the validity of the state should be introduced (something like an IsValid property). I was met with much resistance, and an explanation of "whoever uses the API should not use the default constructor," a comment which certainly raised my eyebrows. (Note: the object in question is constructed "properly" through static factory methods, and all other constructors are internal.)
Is everyone simply converting their structs to classes in this situation without a second thought?
Edit: I would like to see some suggestions about how to keep this type of object as a struct -- the object in question above is much better suited as a struct than as a class.
For a struct, you design the type so the default constructed instance (fields all zero) is a valid state. You don't [shall not] arbitrarily use struct instead of class without a good reason - there's nothing wrong with using an immutable reference type.
My suggestions:
Make sure the reason for using a struct is valid (a [real] profiler revealed significant performance problems resulting from heavy allocation of a very lightweight object).
Design the type so the default constructed instance is valid.
If the type's design is dictated by native/COM interop constraints, wrap the functionality and don't expose the struct outside the wrapper (private nested type). That way you can easily document and verify proper use of the constrained type requirements.
The reason for this is that a struct (an instance of System.ValueType) is treated specially by the CLR: it is initialized with all the fields being 0 (or default). You don't really even need to create one - just declare it. This is why default constructors are required.
You can get around this in two ways:
Create a property like IsValid to indicate if it is a valid struct, like you indicate and
in .Net 2.0 consider using Nullable<T> to allow an uninitialized (null) struct.
Changing the struct to a class can have some very subtle consequences (in terms of memory usage and object identity which come up more in a multithreaded environment), and non-so-subtle but hard to debug NullReferenceExceptions for uninitialized objects.
The reason why there is no possibility to define a default constructor is illustrated by the following expression:
new MyStruct[1000];
You've got 3 options here
calling the default constructor 1000 times, or
creating corrupt data (note that a struct can contain references; if you don't initialize or blank out the reference, you could potentially access arbitrary memory), or
blank the allocated memory out with zeroes (at the byte level).
.NET does the same for both structs and classes: fields and array elements are blanked out with zeroes. This also gets more consistent behavior between structs and classes and no unsafe code. It also allows the .NET framework not to specialize something like new byte[1000].
And that's the default constructor for structs .NET demands and takes care of itself: zero out all bytes.
Now, to handle this, you've got a couple of options:
Add an Am-I-Initialized property to the struct (like HasValue on Nullable).
Allow the zeroed out struct to be a valid value (like 0 is a valid value for a decimal).
I am wondering how immutability is defined? If the values aren't exposed as public, so can't be modified, then it's enough?
Can the values be modified inside the type, not by the customer of the type?
Or can one only set them inside a constructor? If so, in the cases of double initialization (using the this keyword on structs, etc) is still ok for immutable types?
How can I guarantee that the type is 100% immutable?
If the values aren't exposed as public, so can't be modified, then it's enough?
No, because you need read access.
Can the values be modified inside the type, not by the customer of the type?
No, because that's still mutation.
Or can one only set them inside a constructor?
Ding ding ding! With the additional point that immutable types often have methods that construct and return new instances, and also often have extra constructors marked internal specifically for use by those methods.
How can I guarantee that the type is 100% immutable?
In .Net it's tricky to get a guarantee like this, because you can use reflection to modify (mutate) private members.
The previous posters have already stated that you should assign values to your fields in the constructor and then keep your hands off them. But that is sometimes easier said than done. Let's say that your immutable object exposes a property of the type List<string>. Is that list allowed to change? And if not, how will you control it?
Eric Lippert has written a series of posts in his blog about immutability in C# that you might find interesting: you find the first part here.
One thing that I think might be missed in all these answers is that I think that an object can be considered immutable even if its internal state changes - as long as those internal changes are not visible to the 'client' code.
For example, the System.String class is immutable, but I think it would be permitted to cache the hash code for an instance so the hash is only calculated on the first call to GetHashCode(). Note that as far as I know, the System.String class does not do this, but I think it could and still be considered immutable. Of course any of these changes would have to be handled in a thread-safe manner (in keeping with the non-observable aspect of the changes).
To be honest though, I can't think of many reasons one might want or need this type of 'invisible mutability'.
Here is the definition of immutability from Wikipedia (link)
"In object-oriented and functional programming, an immutable object is an object whose state cannot be modified after it is created."
Essentially, once the object is created, none of its properties can be changed. An example is the String class. Once a String object is created it cannot be changed. Any operation done to it actually creates a new String object.
Lots of questions there. I'll try to answer each of them individually:
"I am wondering how immutability is defined?" - Straight from the Wikipedia page (and a perfectly accurate/concise definition)
An immutable object is an object whose state cannot be modified after it is created
"If the values aren't exposed as public, so can't be modified, then it's enough?" - Not quite. It can't be modified in any way whatsoever, so you've got to insure that methods/functions don't change the state of the object, and if performing operations, always return a new instance.
"Can the values be modified inside the type, not by the customer of the type?" - Technically, it can't be modified either inside or by a consumer of the type. In pratice, types such as System.String (a reference type for the matter) exist that can be considered mutable for almost all practical purposes, though not in theory.
"Or can one only set them inside a constructor?" - Yes, in theory that's the only place where state (variables) can be set.
"If so, in the cases of double initialization (using the this keyword on structs, etc) is still ok for immutable types?" - Yes, that's still perfectly fine, because it's all part of the initialisation (creation) process, and the instance isn't returned until it has finished.
"How can I guarantee that the type is 100% immutable?" - The following conditions should insure that. (Someone please point out if I'm missing one.)
Don't expose any variables. They should all be kept private (not even protected is acceptable, since derived classes can then modify state).
Don't allow any instance methods to modify state (variables). This should only be done in the constructor, while methods should create new instances using a particular constructor if they require to return a "modified" object.
All members that are exposed (as read-only) or objects returned by methods must themselves be immutable.
Note: you can't insure the immutability of derived types, since they can define new variables. This is a reason for marking any type you wan't to make sure it immutable as sealed so that no derived class can be considered to be of your base immutable type anywhere in code.
Hope that helps.
I've learned that immutability is when you set everything in the constructor and cannot modify it later on during the lifetime of the object.
The definition of immutability can be located on Google .
Example:
immutable - literally, not able to change.
www.filosofia.net/materiales/rec/glosaen.htm
In terms of immutable data structures, the typical definition is write-once-read-many, in other words, as you say, once created, it cannot be changed.
There are some cases which are slightly in the gray area. For instance, .NET strings are considered immutable, because they can't change, however, StringBuilder internally modifies a String object.
An immutable is essentially a class that forces itself to be final from within its own code. Once it is there, nothing can be changed. In my knowledge, things are set in the constructor and then that's it. I don't see how something could be immutable otherwise.
There's unfortunately no immutable keywords in c#/vb.net, though it has been debated, but if there's no autoproperties and all fields are declared with the readonly (readonly fields can only bet assigned in the constructor) modfier and that all fields is declared of an immutable type you will have assured your self immutability.
An immutable object is one whose observable state can never be changed by any plausible sequence of code execution. An immutable type is one which guarantees that any instances exposed to the outside world will be immutable (this requirement is often stated as requiring that the object's state may only be set in its constructor; this isn't strictly necessary in the case of objects with private constructors, nor is it sufficient in the case of objects which call outside methods on themselves during construction).
A point which other answers have neglected, however, is a definition of an object's state. If Foo is a class, the state of a List<Foo> consists of the sequence of object identities contained therein. If the only reference to a particular List<Foo> instance is held by code which will neither cause that sequence to be changed, nor expose it to code that might do so, then that instance will be immutable, regardless of whether the Foo objects referred to therein are mutable or immutable.
To use an analogy, if one has a list of automobile VINs (Vehicle Identification Numbers) printed on tamper-evident paper, the list itself would be immutable even though cars aren't. Even if the list contains ten red cars today, it might contain ten blue cars tomorrow; they would still, however, be the same ten cars.