Is it possible to get the declaration name of a class (dynamically) and pass it as a parameter in the constructor to set the name variable in the class itself?
Example:
public class Foo
{
public string name;
public Foo()
{
name = GetClassName();
}
}
public class SomeOtherClass
{
Foo className = new Foo();
Console.WriteLine(foo.name);
}
As result I would expect it to write: "className".
No. That is not possible. There is no way to pass in a variable name without using a parameter.
This is the closest you can get:
Foo className = new Foo(nameof(className));
That sounds like a weird requirement. A variable is nothing but a reference to an object. The name of that reference has by no means anything to do with what this variable reference. Thus the actual referenced object doesn´t know anything about its references. In fact you may have even multiple references to the same Foo-instance. So how should the instance know to which variable you refer to? So what should happen in the following example:
var f = new Foo();
var b = f;
Now you have two references to the same instance of Foo. The instance can´t know which of hose is the right, unless you provide that information to it by using a parameter (e.g. to your constructor). The thing gets even worse if you have a factory creating your Foo-instance:
void CreateFoo()
{
return new Foo();
}
// ...
var f = CreateFoo();
Now you have a further indirection, the constructor of Foo can surely not bubble though all layers in your call-stack until it reaches some assignement where it may get the actual name. In fact it´s possible that you don´t even assign your instance to anything - although this is merely a good idea:
CreateFoo(); // create an instance and throw it away
Anyway if you want to set a member of an instance to some value, you should provide that value to the instance. The answer by Patrick shows you how to do so.
Related
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 am serializing objects to XML using System.Xml.Serialization and this requires me to have parameterless constructors.
I am therefore trying to use Object Initialization Syntax to assign values to certain properties and then use the constructor logic to format these values as needs be before I serialize the objects to XML.
My problem is that the constructor runs before the properties are assigned their values. A simplified example is below:
class Program
{
static void Main(string[] args)
{
Foo myFoo = new Foo() { HelloWorld = "Beer", HelloWorldAgain = "More beer" };
Console.WriteLine(myFoo.HelloWorld);
Console.WriteLine(myFoo.HelloWorldAgain);
Console.ReadLine();
}
}
public class Foo : Bar
{
public string HelloWorld { get; set; }
public Foo()
{
Console.WriteLine("Foo Was Initialized");
Console.WriteLine(HelloWorld);
}
}
public abstract class Bar
{
public string HelloWorldAgain { get; set; }
public Bar()
{
Console.WriteLine("Bar was initialized");
Console.WriteLine(HelloWorldAgain);
}
}
This results in the following output:
As you can see the constructor logic runs, and then the properties are assigned values. I need this to work the other way around.
Is this possible?
Serialization requires you to have a parameterless constructor, but does not limit you to that one constructor.
Keep the no-arg constructor for deserialization, but add another constructor that takes your values and does the required initialization when you need to instantiate the class in code.
Object initialization syntax is just shorthand for setting properties after construction.
No. Object initialization syntax is just a shortcut. When you write:
Foo foo = new Foo { HelloWorld = "Beer" };
The compiler just rewrites this to something very close to what happens if you write:
Foo foo = new Foo();
foo.HelloWorld = "Beer";
If the parameters are required in order for your object to exist, you should put them as arguments in the constructor.
No it's not possible, cause in order to be able to initialize a property of the object, object has already exist. It's existance is guranteed by ctor running before all other stuff.
If we are talking about "not running default constructor", like a conceptual question.
You can do that by using a static property. In that case Foo() will not be called. But it's naturally out of the current question subject.
Arguably, the syntax is deceptive. What is happening here?
var myFoo = new Foo();
myFoo.HelloWorld = "Beer";
myFoo.HelloWorldAgain = "MoreBeer";
That's it, I'm afraid. It's simply not possible to initialise properties before the constructor has run. The constructor is the first thing that 'happens' after memory for the object is allocated and default values are assigned to fields.
BTW, you don't need the parens using the object initialisation syntax. This would be just as good (but even more deceptive):
var myFoo = new Foo { HelloWorld = "Beer", HelloWorldAgain = "MoreBeer" };
Looking at your question, it looks like what you're trying to do (modify the properties before serialisation) doesn't belong in the constructor.
Greeting for the day!
I have a question in my mind and looking for answer from some days.
If my understanding is correct then only diff between Instance and object is :-
instance means just creating a reference(copy) .
object :means when memory location is associated with the object( is a runtime entity of the class) by using the new operator
Now i want to know how to create an instance of an object.
Please give explanation with sample code
Any help will be appreciated.
Thanks
By your explanation it's not called an instance, but a reference of an object. An instance of a class is called an object. I think your question is: "What is the difference of an object and a reference variable?" I'll try to explain it with some examples:
Foo f;
I just declared a reference variable. This is not an object but only a reference that refers to an object.
f = new Foo();
Now I created a new object and assigned it to the f reference variable so every time I do something to f I refer to the Foo object. Like when I call f.Name = "MyFoo"; I refer to the foo object.
Foo otherFoo;
Now I declare another reference variable.
otherFoo = f;
What we have here now is having ONE object in the memory but TWO reference variables refering to the same object.
f.IsFoo = true;
bool isotherFooFoo = otherFoo.IsFoo;
This last line will return true because we changed the IsFoo property to true and f and otherFoo reffer to the same object.
I hope that explains you everything. :)
You don't create "an instance of an object", you create an instance of a class (or struct). An object is an instance of a class.
If you do:
Foo f = new Foo();
You create an instance of the Foo class.
In the phrase "an object is an instance of a class", the word "instance" does not really have a technical meaning that is different to the word "object", it is just a way of defining, in English, what the word "object" means. The meaning of "instance" is really meant to be the same as the meaning of "object". We can break this down as follows:
an object is an instance of a class
an object = instance of a class
an object = instance
In C# 9.0 there is a new way to initialize a class by Target-typed new expressions.
You can initialize the class like this:
Foo f = new();
Note, f is a reference to the class Foo.
We have a class ABC
Class ABC
{
string name="";
public ABC()
{
this.name = "A1";
}
public ABC(name)
{
this.name = name;
}
}
An Instance of a class can be created as:
ABC a1 = new ABC();
or
ABC a1 = new ABC("James");
You create instance of a class and not the object.
I want to understand the difference between 3 sets of snippets below:
private static FirstObject o = new FirstObject();
public class ClassA
{
}
//-----------------------------------------------------
public class ClassA
{
private static FirstObject o = new FirstObject();
}
//-----------------------------------------------------
public class ClassA
{
private static FirstObject o;
public ClassA
{
o = new FirstObject();
}
}
Please help me understand in terms of scope, memory, performance and usage of these.
Thank you.
Invalid, as you can't have a variable outside of object
The proper way - the class has a static member, which is initialized when the class is accessed for the first time
Very bad, because every time when new object is created the static object will be recreated.
The first option will not compile. A static variable in C# must be scoped to a class or struct.
The second option is the preferred mechanism.
The third option is wrong because this creates a new FirstObject each time an instance of ClassA is created, which is almost certainly not what you want.
A fourth option would be to leverage a static constructor, e.g.,
public class ClassA
{
private static FirstObject o;
static ClassA
{
o = new FirstObject();
}
}
This option is useful if there is some special construction constraints for FirstObject. In this example, though, choose option 2 over option 4. Just know that option 4 exists.
Three cases below...
Assuming a typo here missing some outer construct... "o" is declared so that it will be globally accessible, as a single object, to the entire application. It will have one common set of all properties and data. It can be access directly by "Namespace.o"
"o" is declared so that it will be globally accessible, as a single object, to the entire application, However it is only accessible through another defined instance of "ClassA". Each separate instance of ClassA will have the same, single "o" object with the same properties and data.
This doesn't look right to me, I'm assuming "ol" is supposed to "o;". Even with this the code looks like its missing something. if the Line "o = new FirstObject" is correct it is not accessible in this fashion.
I am having a dispute with another fellow programmer, over the scope of interfaces.
suppose we have the following:
public interface IFoo
{
string Bar { get; set; }
}
public class SomeFoo: IFoo
{
public string Bar { get; set; }
public SomeFoo(string bar)
{
this.Bar = bar;
}
}
public class Consumer
{
public void DoSomething()
{
SomeFoo fooClassInstance = new SomeFoo("test");
IFoo fooInterface = (IFoo)fooClassInstance;
// do something with fooInterface.
}
}
So the question is:
1. Is it possible the fooClassInstance to go out of scope before something else releases the fooInterface instance?
Some argue that the object (fooClassInstance) can go out of scope.
I believe that it cant. Sure, objects may or may not get disposed of by the GC when the GC decides that the object(s) are no longer in scope. However since Interfaces are by design an abstract contract whose members have to be implemented by the object that uses it, than an interface cannot lose its implementation so long as the interface is used. It's not like a whole another object is created of type "interface". The interface is merely a pointer to those abstract members of the implementor.
Can you guys help me resolve this dispute?
Thanks,
<bleepzter />
While there is definitely some confusion in the terms you are using (I'll let someone else tackle that issue), I think I understand what you are saying, and you are basically right.
In particular, it sounds like your coworker believes this is happening:
// Now there is this "SomeFoo" object somewhere in memory.
SomeFoo fooClassInstance = new SomeFoo("test");
// Now there is this "IFoo" object somewhere in memory.
IFoo fooInterface = (IFoo)fooClassInstance;
// Let's say down the road somewhere, fooClassInstance is set to null or a different
// object. Your coworker believes that the object it originally pointed to will then
// have no references to it and will thus be eligible for garbage collection?
If the above is an accurate representation of what your coworker thinks, then your coworker is wrong and you are right. The fooInterface variable contains a reference to the same object that fooClassInstance had a reference to. You can easily verify this simply by doing the following:
SomeFoo fooClassInstance = new SomeFoo("test");
IFoo fooInterface = (IFoo)fooClassInstance;
bool sameObject = ReferenceEquals(fooClassInstance, fooInterface);
If ReferenceEquals returns true, then the two variables are referencing the same object in memory.
If your coworker needs further convincing, try showing him/her something like this:
List<int> list = new List<int> { 1, 2, 3 };
// This cast is actually not needed; I'm just including it so that it mirrors
// your example code.
IList<int> ilist = (IList<int>)list;
// Now we remove an item from the List<int> object referenced by list.
list.Remove(3);
// Is it in ilist? No--they are the same List<int> object.
Console.WriteLine(ilist.Contains(3));
// How about we remove an item using ilist, now?
ilist.Remove(2);
// Is it in list? Nope--same object.
Console.WriteLine(list.Contains(2));
// And here's one last thing to note: the type of a VARIABLE is not the same
// as the type of the OBJECT it references. ilist may be typed as IList<int>,
// but it points to an object that is truly a List<int>.
Console.WriteLine(ilist.GetType());
I think you are muddling the line between instance an reference. fooClassInstance can go out of scope (that is, it can no longer be referenced) even if the instance of the object still exists (because fooInterface still holds a reference to it).
For example, if you do the following, fooClassInstance will not be available after the braces, but fooInterface will.
public void DoSomething()
{
IFoo fooInterface;
{
SomeFoo fooClassInstance = new SomeFoo("test");
fooInterface = (IFoo)fooClassInstance;
}
// do something with fooInterface, but NOT with fooClassInstance
}
Uh, what do you mean by "lose its implementation"? That makes no sense. A type implements an interface, it cannot "unimplement" an interface.
As far as your code example goes, the lifetime of the object allocated ends the instant it no longer has any roots (aka, is no longer reachable by the GC). A reference to it is a reference to it, regardless of the type of the reference (i.e. if the reference type is of some derived or parent type, it doesn't matter).
ISomething Sample() {
Something s1 = new Something();
s2.DoSomething(); // Assuming s is the only reference to s, then it no longer is
// rooted after this expression
Something s2 = new Something();
ISomething is1 = s2;
s2 = null;
is1.DoSomething(); // The reference remains valid and the lifetime of the
// object created continues until we release all
// remaining references to it.
return is1;
}
It sounds like you are correct, but I feel like we're missing part of the discussion since we don't know what you're doing with the object after you create (and cast) it.
In your example:
SomeFoo fooClassInstance = new SomeFoo("test");
At this point, you have one reference to the SomeFoo object referenced by fooClassInstance.
IFoo fooInterface = (IFoo)fooClassInstance;
At this point, you have two references to the SomeFoo object (referenced by both fooClassInstance and fooInterface).
So yes, depending on how you use it, fooClassInstance could go out of scope. But there is still a reference to it (fooInterface) so it wouldn't be garbage collected. Also, the fooInterface reference could be cast back to SomeFoo.