If I have a C# class with only readonly fields, but the fields' types are NOT immutable, is the class considered immutable?
Is this class immutable?
public class Foo
{
private readonly int[] _blah;
public Foo(int[] blah)
{
_blah = blah;
}
public int[] Blah { get { return _blah; } }
}
_blah is not immutable, since I can change the members of the array, though the array member variable can never change.
So, is a class immutable if its fields are all readonly, or is a class immutable only if its fields are not only readonly, but also immutable themselves?
It will really depend on who you ask or what you mean. Some might say that the class itself is immutable because its direct members are immutable. Some might say that the class and all its members (and their properties, etc) -- basically, the entire object graph -- must be immutable in order to be considered immutable.
private field arrays marked as readonly are immutable, but that doesn't mean you can't replace the indexes with different values. A way to solve this is to return a copy of the array or an enumeration of the array
The objects of the array may or may not be immutable. Whether you make them immutable or you return clones of the them or whatever, that's really up to you.
In your situation, the objects are integers (which are immutable), but your array itself isn't (again, depending what you define as immutable). If all you want to guarantee is the private field can't be altered and don't care about the indexes, then you're fine. But, if you want the indexes to be locked in then you need to expose your array in another way.
Also, a nice collection to look into is the ReadOnlyCollection<T>. It is a collection that holds a reference to the original collection (it wraps it), so that the indexes can't be changed.
Also, point in case... already you have varying answers to the degree of what "immutable" actually means.
Instances of this class are not immutable, because you can mutate their contents. E.g. foo.Blah[0] = 42;. But if you changed the Blah property to IEnumerable<int> or IReadOnlyList<int>, the instances would be considered immutable.
This class is not immutable since it can be inherited and have other things added it.
I think its immutable if it follows three principals:
Make the fields private readonly.
Provide a public property get accesor.
If the class is not longer needed to be inherited, make it sealed.
These are not hard and fast rules with a strict definition of immutable. They are just guidelines.
Related
Say I have:
class foo
{
private List<T> bar;
public IEnumerable<T> GetBar();
}
where GetBar() should return that very same List, without exposing internals (i.e. no other class should be able to remove elements from the internal list, reorder it etc.) and avoiding excessive copying. What is the standard way to do this?
If you want to return an immutable list, return bar.AsReadOnly().
Typically the member that exposes this readonly wrapper would be of type IList<T>. I would only downcast to IEnumerable<T> if I wanted to indicate to the consumer that the implementation might use lazy enumeration.
Personally I'd make it a property, and return the same readonly wrapper on each invocation:
class Foo
{
private List<T> bar;
private IList<T> readonlyBar;
public Foo()
{
bar = ...;
readonlyBar = bar.AsReadOnly();
}
public IList<T> Bar
{
get { return readonlyBar; }
}
}
You can return an instance of ReadOnlyCollection from your method:
return new ReadOnlyCollection(items);
A property which encapsulates part of an object's mutable state may, if the type is a mutable class, represent any of these:
A live mutable view of the property's state
A mutable view of a detached copy of the property's state
A mutable view of a state that may or may not remain attached to the property's state
If the type is read-only, but not guaranteed to be immutable, it may represent any of these:
A guaranteed-"live" read-only view of the property's state
An immutable copy of the state as of the time it was called
A view of a state that may start out being attached to the property's state, but may or may not remain attached.
While the type of the determined object may be used to distinguish the first set of three behaviors from the latter set, I know of no standard convention to indicate, in the absence of a suitable guaranteed-immutable return type, which of the behaviors within the appropriate set applies. Since there is no guaranteed-immutable implementation of IEnumerable<T> that would seem applicable here, your best bet is probably to use Joe's code, but also document your behavior as #3. Unless you can think of some reason clients would benefit from #2, documenting your behavior as #3 will give you the most flexibility to change your implementation in future should that be necessary. For example, if thread-safe access ends up being necessary in the future, your class could implement an internal method which locks the collection long enough to copy it to an array and returns that array (possibly wrapped in ReadOnlyCollection<T>, though it probably wouldn't matter) cast as IEnumerable<T>. Such an approach wouldn't work if the property was supposed to represent a live view.
I have a generic Vector<T> class and a generic Matrix<T> class and I was wondering if it would be a good idea to have both classes implement an interface.
Basically, I'm implementing two algorithms: AlgorithmA and AlgorithmB, both of which perform very similar operations (reset, average...etc) but with different algorithms and act on different structures:
AlgorithmA uses Vector<double> while AlgorithmB uses Matrix<Complex>.
The design I have so far:
abstract class AlgorithmArray
{
// Operator overloading
}
class AlgorithmAArray : AlgorithmArray
{
private Vector<double> _vector;
// Overrides
}
class AlgorithmBArray : AlgorithmArray
{
private Matrix<Complex> _matrix;
// Overrides
}
I would prefer to have AlgorithmAArray derive from Vector<T> and also implement an interface 'IAlgorithmArray' (instead of the abstract class). Anyway, these algorithms are then used to simulate transmission/receiving between two locations:
public class CommunicationParameters
{
private AlgorithmArray _transmission;
private AlgorithmArray _receiving;
public void Compute()
{
if(_transmission != null)
_transmission.Compute();
if(_receiving != null)
_receiving.Compute()
}
}
Are there better ways to approach my problem ?
Note: The base class AlgorithmArray duplicates many of the operator/cloning...etc methods and I feel this could be avoided, perhaps using generics ?
Thanks !
I would suggest making two Algorithm classes that can take any data structure as a parameter and do their thing. I don't see a need for all this OOP inheritance, it just adds complexity.
Interfaces would allow routines which only read or only write vectors/matrices to accept vectors/matrices of a subtype or supertype of the expected vector/matrix type. I'm not sure that would generally be useful with matrices, but it could be handy with some applications of vectors.
Another advantage of interfaces over classes, which might be more applicable to your situation, would be that they could allow smooth interoperation between mutable, immutable, and copy-on-write objects (the latter requiring an extra level of indirection). This could be useful if you have lots of vectors or matrices which are going to be copies of one another, but a few of them will end up getting modified. Methods AsImmutable, AsNewMutable, and AsPossiblyExistingMutable can be useful for that. The first method (if invoked on a mutable object) will either create a new immutable object whose contents match those of its subject at the time of the call, or (if invoked on an immutable) object, simply return its subject. The second will create a new mutable object regardless of whether the existing object is mutable or immutable. The third method will return its subject if mutable, or else create a new mutable object; it should generally only be used in cases where the holder of an object would know that, if the object is mutable, it holds the only reference.
For example, if I have a private field _thing of type IReadableVector<Foo>, my setter could set it to value.AsImmutable() and my getter could return _thing.AsImmutable(). My mutating method would set _thing = _thing.AsPossiblyExistingMutable() before calling mutating methods on it. If I haven't tried to mutate _thing since I received it, it would be an immutable object (to which other objects might also hold references). The first time I mutate it, it would be copied to a new mutable object. Subsequent mutations, however, could keep using the same mutable object, since it would never get exposed to any outside code.
PS--There are arguments both for and against having IImmutableVector<T> and IImmutableMatrix<T> as interfaces, versus only having ImmutableVector<T> and ImmutableMatrix<T> classes. On the one hand, if they're interfaces, it's possible to have useful implementations which don't need to actually store all the elements. For example, one could have a classes like AllMatchingVector<T> which inherits IImmutableVector<T> but just contains one T and a number indicating its length; its indexed getter would simply return that element regardless of the specified index, or DiagonalMatrix<T> which simply holds an IImmutableVector<T> for the contents of its diagonal, and a T which would be returned everywhere else; especially for large vectors/matrices, such classes could save memory. On the other hand, there would be no way of ensuring that nobody implemented one of those interfaces with a class which was not, in fact, immutable. My personal feeling is that it's fine to use interfaces for that. After all, few people complain that SortedDictionary<T> will fail if a class implements IComparable<T> in a fashion that doesn't yield an immutable sorting relation. Nonetheless, a lot of people disagree with such a concept.
Is it correct that it is not possible to change the value of an immutable object?
I have two scenarios regarding readonly that I want to understand:
What if I have a collection and mark it as readonly, like the following. Can I still call _items.Add?
private readonly ICollection<MyItem> _items;
And also for the following variable, if later on I call _metadata.Change which will change the internal values of a couple member variable in the Metadata instance. Is _metadata still immutable?
private readonly Metadata _metadata;
For both variables above, I totally understand that I can't directly assign new values to them outside of initializer and constructors.
I suggest you to read the series of blog posts by Eric Lippert. The first part is Immutability in C# Part One: Kinds of Immutability. Very informative and helpful, as always. The series describes what does it mean for a variable to be readonly, immutable etc. in details.
Generally, readonly means only that you can't re-assign a field outside the constructor. The field itself can be modified as long as it stays the same instance. So yes, you can add elements to the collection stored in readonly field.
About mutability, this is more complex and it depends a bit what kind of mutability you consider. When Metadata internal values are references and those references itself (the instances it point to) doesn't change, you could say Metadata stays not mutated. But it is mutated logically. See Eric's posts for more insights.
Marking a field as read-only only means that you cannot change the value of that field. It has no bearing on the internal state of the object there. In your examples, while you would not be able to assign a new metadata object to the _metadata field, nor a new ICollection to the _items field (outside of a constructor that is), you can change the internal values of the existing objects stored in those fields.
An immutable object is one that cannot be changed, once created. In C# strings are immutable. If you look at the string manipulation routines you can see that all of them return a new, modified, string and leaves the original string unchanged.
This facilitates the string handling considerably. When you have a reference to a string, you can be sure that noone else will unexpectedly change it under your feet.
readonly is something else. It means that the reference cannot be changed, once set and that it can only be set during object construction. In your examples you can change the contents of _items or the properties of _metadata, but you cannot assign another ICollection<MyItem> to the _items member or another Metadata instance to _metadata.
readonly is set on the reference to an object. Immutability is a property of the object itself. These can be freely combined. To make sure that a property is not changed in any way, it should be a readonly reference to an immutable object.
There is nothing preventing you from mutating an object stored in a readonly field. So you CAN call _items.Add() and metadata._Change() outside the constructor/initializer. readonly only prevents you from assigning new values to those fields after construction.
private readonly ICollection<MyItem> _items;
Does not prevent items from being added. This simply prevents _items from being re-assigned. The same is true for _metadata. Accessible members of _metadata can be changed - _metadata cannot be re-assigned.
Yes.
Readonly doesn't equal to immutable. You can still call _metadata.Change.
The readonly keyword applies to a variable - this means you can't assign another value to it, but you can alter its internal state. That's why you can alter a collection's items but you can't assign a new collection to the variable.
In languages like C++ there are quite a few different uses of the keyword "const", it is partly easy for developers to pass constant parameters and also constant pointers of constant values.
That is not easy in C#.
We need to build immutable class by definition, which means Once an instance is created, there is no way it can be changed programmatically.
Java has a little easier way than C# because of the keyword final and its usages.
Lets consider this example and see how immutable it is..
public sealed class ImmutableFoo
{
private ImmutableFoo()
{
}
public string SomeValue { get; private set; }
//more properties go here
public sealed class Builder
{
private readonly ImmutableFoo _instanceFoo = new ImmutableFoo();
public Builder SetSomeValue(string someValue)
{
_instanceFoo.SomeValue = someValue;
return this;
}
/// Set more properties go here
///
public ImmutableFoo Build()
{
return _instanceFoo;
}
}
}
You can use it like this
public class Program
{
public static void Main(string[] args)
{
ImmutableFoo foo = new ImmutableFoo.Builder()
.SetSomeValue("Assil is my name")
.Build();
Console.WriteLine(foo.SomeValue);
Console.WriteLine("Hit enter to terminate");
Console.ReadLine();
}
}
I have a type with about 40 properties (all value types) that represents a type of transaction for my business. An instance of this class corresponds to a row in my database. I would like to keep my class immutable since it will only ever be used for read operations, but I am not sure how to go about setting 40 properties during initialization.
Typically I use constructor initialization for immutable types, but I would like to avoid writing a constructor with 40 parameters. The setters for my properties are currently private though I am willing to change with good enough reason. Is there a common way to handle this situation or a better way to approach the problem?
Your problem isn't so much a constructor with 40 arguments, but a class with 40 fields.
I'd recommend breaking this down. Are any of the fields related in some way? If so, group them into a common object (e.g. EmailInfo), then have your big object just reference the grouping objects.
// Instead of this:
foo.EmailHeader
foo.EmailSubject
foo.Email...
// Do this:
foo.Email.Header
foo.Email.Subject
Once your class has fewer direct properties, creating a constructor that takes those grouping objects isn't so terrible.
Quick point. You mentioned your setters on the object are private. If that is the case then your object is not immutable (otherwise setters couldn't exist). At best your object is read only.
For a true immutable object there is no choice but to have the constructor take in all of the values necessary to initialize the object. The best way to reduce the number of parameters in the constructor is to group the values into bigger objects which are then passed to the constructor. Although I wouldn't do that unless the values are otherwise logically related.
If your immutable type does truly need the 40 values and they are otherwise unrelated, the best approach is to have a constructor with 40 values. That or further break down the big immutable object.
I like the approach of using a mutable object to instantiate an immutable object; the mutable object is just for tidy passing of options. One example of this in the .NET framework is ProcessStartInfo.
class XInfo {
public int A;
public int B;
}
class X {
public X (XInfo i) {
// you can transform the data/layout from i any way you need
..
}
}
new X(new XInfo() {
A = 42
})
While I'll hold my tongue about the '40 properties', I find the above approach works pretty well. An added bonus is the XInfo and the internal structure used in X can be entirely different, as long as you can provide a sane mapping.
If i go by your words "but I am not sure how to go about setting 40 properties during initialization.", it appears that your problem is a class with too many fields/properties.
Doesnt seem to be a problem of making it immutable, because you already know how to do that.
I would suggest (like others), Refactor and Extract Class.
As an alternative, you can make your class derive from freezable, I think this may be the solution you are searching for. You can Instatiate the object, set the values, then set it frozen. Once you set it frozen the class is 'read only'.
I would suggest putting the parameters into one or more structures, and having the object hold those structures. Nesting objects would be possible, but would add more overhead than nesting structures.
As an alternative, you could create an abstract base class with "readonly mustoverride" versions of all your properties. From this, derive a mutable and immutable object class. The immutable one can accept the base class in its constructor, and use all the readonly properties to build the new object. The mutable class can provide a means of writing the properties using methods, read-write properties with different names from the readonly versions, etc.
What is meant by immutable type and immutable property in C# ? can you give simple example?
An immutable type is a type of which its properties can only be set at initialization. Once an object is created, nothing can be changed anymore. An immutable property is simply a read-only property.
In the following example, ImmutableType is an immutable type with one property Test. Test is a read-only property. It can only be set at construction.
class ImmutableType
{
private readonly string _test;
public string Test
{
get { return _test; }
}
public ImmutableType(string test)
{
_test = test;
}
}
See also: The Wikipedia article, and some Stack Overflow questions on the topic.
In addition to #fretje's answer above, in C#6 and later, getter-only auto properties have now been implemented, which allows for immutable auto-properties without the need for the additional explicit private readonly backing field. The equivalent code would be abbreviated to:
class ImmutableType
{
public string Test
{
get; // No Set at all, not even a private set.
}
public ImmutableType(string test)
{
Test = test; // The compiler understands this and initializes the backing field
}
}
Note that private set only provides a restricted encapsulation of change to the property from within the same class, and thus isn't truly immutable:
public string Test
{
get;
private set; // Not immutable, since this is still mutable from within the class
}
More about immutability
As others have said, an immutable Property is a property which cannot change once it has been set. Setting of the 'only' value is done during construction.
An immutable Type is a type where all (externally visible) properties and fields are immutable - for example the "Record" Type originally scheduled for C#7 (hopefully now 8) would have been an immutable type. Other examples of Immutable Types are Tuples, and all anonymous classes.
Immutable fields should be qualified with the readonly keyword in C# - this is enforced by the compiler to ensure that no other code attempts to change the field outside of the constructor.
Wherever possible, immutability of fields, variables and properties is regarded as good practice, as this greatly reduces the surface area for bugs (since fields represent the state of an object, preventing change to fields reduces the number of states).
The benefit of immutability is especially important in multi-threaded programs, where two or more threads concurrently access the same object. Since multiple concurrent reading threads can safely read the value of a field or property, the programmer doesn't need to be concerned about thread safety issues relating to change to the field by other threads (because change to the property is prohibited)
One common drawback of immutability when dealing with complex objects consisting of multiple composed objects, is that the entire graph needs to be built 'in one go', which can lead to messy code. A common solution here is to use the Builder pattern as a scaffold, which allows a transient, mutable representation to be built in steps, and then the final, immutable object is obtained in the final .Build() step.
fretje is correct. The most popular example of an immutable type is a string object in C#. This is the entire reason that StringBuilder exists.
There is no clear definition for immutability in C#:
It usually entails all public fields to be readonly and all public properties to have init setter or no setter.
It may also extend this to private members (although it is common to use mutable private members to cache values ie the hashcode)
It may also mean all members are themselves of immutable types. As otherwise a consumer can still modify the object by calling a member method
It may also mean all methods always return the same output when called with the same arguments. In particular an immutable GetHashCode() means the object is safe to use as a key in dictionaries etc
A stronger and often more useful concept is that of - 'Data' an Immutable type which also has value semantics (usually by inheriting from IEquatable<T>)
Here is an example of a 'Data' type that satisfies all of the demands above:
record MyData(int age, DateTime JoinDate);
(see project F for more on how Data types are useful)