I want to have multiple objects share a reference through a private field, such that any of the objects can assign to the field, and the updated field will be seen by other objects sharing that reference. What I was originally hoping to do was this:
class SomeObject
{
private ref DataObject _data;
public SomeObject(ref DataObject data)
{
_data = ref data; // or something similar
}
public ChangeData(DataObject newData)
{
_data = data;
// at this point, *other* SomeObject instances that were
// created with the same reference should also have _data == newData
}
}
But of course you can't use ref that way: ref is only for method parameters. And a static field won't work, since not all SomeObject instances should refer to the same object---rather, the object in question should be set in the constructor.
Obviously I could solve this by just adding a simple wrapper class. But is there a better way? Is there some kind of SharedReference<T> class that I can use?
Update, since most of the answers misunderstood what I was asking. I know that the _data field contains a reference to the original DataObject. What I want is another level of indirection. I want to be able to change which object I'm pointing to in one instance, and have the new value be picked up by other instances that were created with the same reference. As the updated code sample shows, I want to assign to _data, and effectively change the value of _data in other instances.
I don't know of any class that you can use for this, but I seems quite easy to implement your own SharedReference<T> class.
Something like this:
public sealed class SharedReference<T>
where T : class
{
public T Reference
{
get; set;
}
}
You could simply use an array of shared objects, and reassign the array elements:
class SomeObject
{
// you probably want to make this readonly
private readonly DataObject[] _data;
public SomeObject(DataObject[] data)
{
_data = data;
}
public void ChangeData(DataObject newData)
{
_data[0] = o;
}
// and you could define your own accessor property...
private DataObject Data
{
get { return _data[0]; }
set { _data[0] = value; }
}
}
Apart from that, I think you'll need to define your own 'holder' class & use that
If you simply provide a reference to the object without the ref keyword, you will get the behaviour you want. Using ref is actually passing a reference to a reference (pointer to a pointer), so unless you want to null someone else's reference, it won't be of any use to you.
Update: Sorry I didn't spot that you wanted to re-assign a completely new object into the field and have that reflected throughout. You are best actually creating either a wrapper class to contain the object state and modify that, or a common event that all instances can subscribe to such that when you want to change the object, fire the event with the new object inside it and have each instance update it's own internal reference.
Alternatively, use the Singleton pattern - everyone accesses a publicly available static reference, but unlike the traditional Singleton, you let them change the reference if they want to - so everyone can see the change. This also has the benefit that the objects don't need internal references.
Alternatively again, have the data class expose a method allowing it to consume another data class and copy its state across - like cloning.
In C#,
class SomeObject
{
private DataObject _data;
public SomeObject(DataObject data)
{
_data = data;
}
}
in fact does exactly what you want, if DataObject is a Reference Type, which is true for all classes.
Please disregard this answer since I misunderstood the initial question.Other answers here cover the topic fully.
Related
I am building a large, complex data structure with many interconnected objects. As part of this, I have a situation where I need to create an object before I know the values for all the properties. This is so I can pass a reference to that object around inside of a larger data structure, then change it later to allow all the consumers of that object to get the "updated" values.
Tracking the consumers of each object so I can go update their references means I am A) re-implementing a reference counting garbage collector and B) would require an external consumer to modify a class. For obvious reasons, I don't want to implement a garbage collector (or something similar). and allowing external consumers to modify properties means the object is mutable. Immutability is important because I know that many objects in this data structure are going to wind up in Dictionaries after the data structure has been built, but I can safely modify properties until then. Since this is going into a library that will be provided to other users, I must block interactions that will break things like invariants for Dictionaries.
I am implementing this "deferred initialization" concept for properties like this:
class MyClass
{
private AnotherClass mReference = null;
public bool Reference
{
get
{
return this.mReference
}
internal set
{
if ( this.mReference != null )
{
throw new Exception( "This value has already been initialized!" );
}
else
{
this.mReference = value;
}
}
}
}
I am allowing anything in the internal scope to assign a non-null value to this property exactly once. Once the property has been set, there's no going back.
While C# has a keyword readonly that sort of already does this, readonly constrains the member to only be assigned to via either a static initializer or a constructor in the same class. But at the time the constructor is called, I don't know what the value for this property is!
My problem is I am now using this exact pattern across multiple classes, and I would like to avoid this code duplication.
Does anyone have any suggestions for how I can reduce the code duplication? Maybe a C# keyword like readonly, or some other language feature? Or is this just not possible? Unfortunately, it is not possible to refactor the algorithms such that I can delay instantiation of all objects until all values are known. Think of bidirectional relationships with immutable objects at each end if you need an example of this scenario.
Just encapsulate the behavior in its own type:
public sealed class OneTimeAssignable<T> where T : class
{
public T Value
{
get
{
return mValue;
}
set
{
if( mValue != null ) throw new InvalidOperationException( "Value can only be assigned once." );
mValue = value;
}
}
private T mValue;
}
Now you can use it as the backing field for your properties:
class MyClass
{
private readonly OneTimeAssignable<AnotherClass> mReference = new OneTimeAssignable<AnotherClass>();
public AnotherClass Reference
{
get
{
return this.mReference.Value;
}
internal set
{
this.mReference.Value = value;
}
}
}
Right now I have it set to only work on reference types (and assuming you don't want them to ever be set to null), but it's easy enough to modify it to accept value types as well, but requires adding a bool to track whether the property has been set.
Out of curiosity: Is there a way to create a class whose reference to the class instance can never be set to null?
Like haveing only a readonly pointer that can only be set to null by the class itself.
What I have in mind:
I would want to have an easy to read /use object that either exists with data or exists without (shown by an attribute like hasData = false). It is always accessable and should never be null / point to nowhere which as a side effect gets also rid of NullReferenceExceptions for objects that are sometimes supposed to not have a value without the need of checking for null.
This feature does not exists (yet). There is big discussion of non-nullable reference types at Roslyn forum: non-nullable reference types (the one billion $ mistake). But currently you cannot restrict reference type variable from assigning null value.
You can use value type (struct) instead, but it's not stored in heap, passed by value etc.
All other options will not guarantee that someone will not assign null to variable of your reference type. But you still can use something like Null Object pattern to simplify your life (processing objects without data in same way as usual objects).
In such a case, you may want to use a struct instead of a class. Class is a reference type and therefore its default value is null, hence a variable containing an instance can be nulled (assigned null). There is no way to prevent it. On the other hand, struct is a value type and default for struct is an empty struct - i.e. a struct whose members are set to their defaults (0 for an int field, null for a field of a reference type etc.)
Example of a struct
public struct Foo
{
public int Bar;
}
And its usage (notice it is not instantiated but still it is NOT null)
Foo foo;
foo.Bar = 1;
More about structs can be found here on the MSDN sites.
As Anton mentioned, you could use a struct which cannot have a default value of null. But I am thinking you want something more like this:
public class DataObject
{
public static bool HasData
{
get
{
return myObject != null;
}
}
public static DataObject PresistentDataObject
{
get
{
return myObject;
}
}
static DataObject myObject = new DataObject();
}
This code seems like bad practice. And maybe you'd want to resort to something like dependency injection with a singleton which will avoid setting up a state class like this.
Typically, the motivation behind such a question drives the qualities/properties of a solution.
Here, I suppose, the motivation is to avoid runtime exceptions of (faulty) code which tries to access a null reference as if it held an instance.
class Foo<T> {
T data; // might be null or hold an instance...
Foo<T>() {
data = GetInstanceOfTInMysteriousWays(); // might return null...
}
bool DoSomething() {
return data.Value > 5; // might throw an exception...
}
// ... more members...
}
To prevent this type of errors, you could borrow from C#'s cousin language F#.
If the function T GetInstanceOfTInMysteriousWays<T>() by design and contract is permitted to either return an instance or a null value, a better design of that function would be to have it return not T but an Option<T>. Then, the type of Foo.data would not be T but Option<T> and the user code in DoSomething() could not simply access member data.Value. Thus, this common pattern of bugs would be eliminated.
// Improved code from above
class Foo<T> {
Option<T> data; // Option is a struct type and cannot be null...
Foo<T>() {
data = GetInstanceOfTInMysteriousWays();
}
bool DoSomething() {
if (data.IsSome() ) {
return data.TryGetValue().Value > 5;
}
return false;
}
}
Now the only question is, where to find that Option type in C#? There are several github projects creating such a type (google is your friend). You could also consider to link the F# core library and use the Option type defined there, maybe along with a little helper as is shown in this gist snippet.
If your class is named foo, then you would have at least one constructor (possibly more). In that constructor you would assign the variable to false.
public foo(){
hasData = false;
}
As #Anton points out, this only works if the variable is instansiated.
foo f = new foo();
it would still be null if you assigned it as null:
foo f = null;
I'm not sure, I understand the question correctly or not. Let me add
some points here:
Hope that you misunderstand the term Instance, If you create an instance of the class then it will not be null, Let myClass be a class that you have created already. You are not creating any instance of the class by using myClass myClassObject. The myClassObject will became an instance of the class only when an instance of the class is assigned to it, Until then it is null which means not existing.
As others have mentioned, you'd need a struct for that.
However, we can tackle this from a different angle (in a class):
Since the variable can point to null, let's define the variable in a way that it can't be set to null:
private Class1 _c = new Class1();
public Class1 c { get { return _c; } set { if (value != null) _c = value; } }
so c will not be set to null.
A struct per your requirements:
struct Struct1
{
public bool hasData { get { return Data != null; } }
public Class1 Data;
}
I've been searching around a bit, but I can't find any way to store a reference to another variable in a certain variable.
I'm trying to make a class to undo things done by the user;
class UndoAction
{
public object var;
public object val;
public UndoAction(ref object var, object val)
{
this.var = var;
this.val = val;
}
public static List<UndoAction> history = new List<UndoAction>();
public static void AddHistory(ref object var, object val)
{
history.Add(new UndoAction(ref var, val));
}
}
I guess you can see what I'm trying to achieve here.
The problem I ran on;
this.var = var;
doesn't store the reference, but the value of the referenced 'var'.
How can I store the reference itself, so I can simply run;
this.var = val;
to "undo" an action, in my case?
Update in 2022: while this likely isn't a particularly suitable feature for implementing Undo, C# 7 introduced ref locals and ref returns, and C# 11 plans to allow these to be stored in ref fields.
Other than that:
Standard safe C# does not support this at all. The underlying framework has almost all of the necessary concepts, but they aren't exposed in the C# language. But even then, such a reference can't be stored in a field.
The best you can have is to wrap it in some class that uses delegates. This is obviously rather expensive in comparison, but unless you are modifying things in a tight loop this might be good enough:
class VarRef<T>
{
private Func<T> _get;
private Action<T> _set;
public VarRef(Func<T> #get, Action<T> #set)
{
_get = #get;
_set = #set;
}
public T Value
{
get { return _get(); }
set { _set(value); }
}
}
And then use it like this:
var myVar = ...
var myVarRef = new VarRef<T>(() => myVar, val => { myVar = val; });
...
myVarRef.Value = "47";
Console.WriteLine(myVar); // writes 47
Your going about it the wrong way.
Boxing/Unboxing isn't what you need, you need to save the object reference and the specific property, and that property's value.
Don't try to make a fully generic system that can undo creation of the human race.
Start simple.
Start by setting certain objects with an interface like... IUndoable.
Mark certain properties with an attribute like [UnduableProperty]
Use the INotifyPropertyChange interface and use the PropertyChanged event.
When a property changes, listen to it, check if it's undoable, and save its value.
Don't be afraid to use reflection but try to avoid it if you can for speed issues.
Save the object, save the property, save the value.
Create a manager object to manage the undo process.
If you are having trouble saving the value of complex structs, don't forget about ISerializable.
Good luck.
Is the following not a good practice?
public interface IMyImmutableData
{
int Data { get;}
}
public interface IMyMutableData
{
int Data { set;get;}//implements both get and set
}
public class MyData : IMyImmutableData, IMyMutableData
{
public int Data{get;set;} //implements both IMyImmutableData, IMyMutableData
}
void Main()
{
MyData myData = new MyData{Data=10};
Console.WriteLine(myData.Data);
}
The reason I ask is that resharper gives me the following warning: "possible ambiguity while accessing by this interface"
The reason I want to do the above is that when I create methods that use the MyData class, I would like to send it either as IMyMutable or IMyImmutable objects, so that users of the method know that they can expect the method to update or not update the passed in object.
I think you can ignore resharper's warning, as the ambiguity is intentional.
However, usually a wrapper class is used to provide readonly access to something, that way it can't be cast to anything that does provide more functionality.
public class MyReadonlyData : IMyReadonlyData {
private MyData instance;
public int Data {
get {
return instance.Data;
}
}
public MyReadonlyData( MyData mydata ) {
instance = mydata;
}
}
// no access to original object or setters, period.
You need to make one or both of the implementations explicit:
public int IMyImmutableData.Data { get; }
public int IMyMutableData.Data { get; set; }
When you mark one as explicit, it can only be accessed when specifically cast as that type:
MyData obj = new MyData();
obj.Data; // Doesnt exist
(obj as IMyImmutableData).Data // Exists, specifically cast as this interface
If you choose to not mark one as explicit, it will be the property chosen when cast as other appropriate types.
I think in this case your structure is fine. You don't want to explicitly implement the interfaces via separate properties, because then the Data you access via the immutable interface will actually be different than that for the mutable interface.
Also, your actual code is likely more complex, because in this case there is no ambiguity: you are accessing Data via the object itself, so interfaces need not be considered.
One solution with explicit interface implementation would be to use a common backing field, rather than auto-properties:
private int _data;
public int IMyImmutableData.Data
{
get
{
return this._data;
}
}
public int IMyMutableData.Data
{
get
{
return this._data;
}
set
{
this._data = value;
}
}
You could cast the variable and tell the compiler what exactly you mean: (resolve ambiguity)
MyData myData = new MyData{Data=10};
Console.WriteLine( ((IMyMutableData)(myData)).Data );
You need a combined interface with a "new" qualifier on the read-write interface to avoid the squawk. Further, your interfaces are poorly named. Better names would be something like "IReadableData" and "IWritableData", and "IReadWriteData". Note that while "IReadableData" does not provide any means of mutating the data, that by no stretch of the imagination implies that the data is immutable. If something is immutable it won't every be changed by anyone; that would clearly not be the case with an object of type MyData.
Suppose I have a class like this:
public class ThingManager {
List<SomeClass> ItemList;
public void AddToList (SomeClass Item)
{
ItemList.Add(Item);
}
public void ProcessListItems()
{
// go through list one item at a time, get item from list,
// modify item according to class' purpose
}
}
Assume "SomeClass" is a fairly large class containing methods and members that are quite complex (List<>s and arrays, for example) and that there may be a large quantity of them, so not copying vast amounts of data around the program is important.
Should the "AddToList" method have "ref" in it or not? And why?
It's like trying to learn pointers in C all over again ;-) (which is probably why I am getting confused, I'm trying to relate these to pointers. In C it'd be "SomeClass *Item" and a list of "SomeClass *" variables)
Since SomeClass is a class, then it is automatically passed by reference to the AddToList method (or more accurately, its reference is passed by value) so the object is not copied. You only need to use the ref keyword if you want to re-assign the object the reference points to in the AddToList method e.g. Item = new SomeClass();.
Since SomeClass is a reference type, you do not need to use the "ref" keyword. If it were a value type, "ref" might be useful.
Think of out as a way of making a parameter work as a return value.
So these are very similar:
void Foo(out int result)
{
result = 5;
}
int Foo()
{
return 5;
}
And then think of ref as a way of allowing a parameter to be both an input and an output.
So in your example, if you declared your method:
public void AddToList(ref SomeClass Item)
Then the caller would have to write something like:
SomeClass i = null;
obj.AddToList(ref i);
This would be illegal, for example:
obj.AddToList(ref new SomeClass());
They would be forced to pass a variable name, rather than an expression, so that the AddToList method can store a value in the variable. By adding the ref prefix you are allowing your method to make the passed variable point to a different object.
If you ever need to use the original value of the parameter user ref. If not, use out. For reference:
http://www.yoda.arachsys.com/csharp/parameters.html
http://msdn.microsoft.com/en-us/library/0f66670z(VS.71).aspx