In C#, is there way to enforce that a class MUST have a parameterless constructor?
If you're talking about generic constraints, yes:
class SomeContainer<T> where T : new() {
...
}
If you're talking about inheritance. It is not possible to require that every class that implements your interface or inherits your base class has a parameterless constructor.
The best you can do is use reflection in your base constructor to throw an exception (at runtime), like this:
abstract class MyBase {
protected MyBase() {
if (GetType().GetConstructor(Type.EmptyTypes) == null)
throw new InvalidProgramException();
}
}
If you're talking about a single class, yes; just put one in.
Generics can enforce this, but we aren't always using generics ;p
If you are using unit tests, you could use reflection to find all the types that meet the pattern you want to have parameterless constructors (for example, everything derived from MyBaseObject, or everything in the Foo.Bar namespace), and verify that way (by finding the parameterless constructor).
If you want to assert this at runtime too (perhaps in #DEBUG), things like static constructors can be useful points to inject extra type checks.
It depends what you mean.
For example, you can constrain a generic type parameter in a class or a method to have a parameter-less constructor with the new keyword, but there's not a real method of limiting an actual class definition beyond that.
public void DoSomething<T>() where T : new() { }
As mentioned in the official MSDN documentation, the C# compiler automatically generates a parameterless constructor that initializes all member variables to default values. If you wish to enforce your own implementation, you can simply do this:
class BaseClass
{
BaseClass() { // Implementation of parameterless constructor }
}
If you're referring to generic constraints, then refer to SLaks' post.
References
http://msdn.microsoft.com/en-us/library/ace5hbzh.aspx
I've encountered a problem like this a number of times, I think there's a requirement for abstract/interface constructors. The trouble is when you're using Activator.CreateInstance or some other technique to instantiate a type which you may not implement (pretty common IoC). Life would be a whole lot easier if you could force a developer to implement a constructor with the right params - even if the purpose is just to pass the params to the base constructor.
The new() constraint has helped the problem a bit since 2.0, but it still doesn't solve the problem when using not using generics, or if you do want specific arguments (and don't want to mess about with the awkward ConstructorInfo, which can't be statically checked.)
Related
I have a base class Character which has several classes deriving from it. The base class has various fields and methods.
All of my derived classes use the same base class constructor, but if I don't redefine the constructor in my derived classes I get the error:
Error: Class "child class" doesn't contain a constructor which takes this number of arguments
I don't want to redefine the constructor in every derived class because if the constructor changes, I have to change it in every single class which, forgive any misunderstanding, goes against the idea of only writing code once?
You can use the following syntax to call the base class constructor from the classes that derive from it:
public DerivedClass() : base() {
// Do additional work here otherwise you can leave it empty
}
This will call the base constructor first, then it will perform any additional statements, if any, in this derived constructor.
Note that if the base constructor takes arguments you can do this:
public DerivedClass(int parameter1, string parameter2)
: base(parameter1, parameter2) {
// DerivedClass parameter types have to match base class types
// Do additional work here otherwise you can leave it empty
}
You can find more information about constructors in the following page:
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/using-constructors
In a derived class, if a base-class constructor is not called explicitly by using the base keyword, the default constructor, if there is one, is called implicitly.
You do have to redeclare constructors, because they're effectively not inherited. It makes sense if you think of constructors as being a bit like static methods in some respects.
In particular, you wouldn't want all constructors to be automatically inherited - after all, that would mean that every class would have a parameterless constructor, as object itself does.
If you just want to call the base class constructor though, you don't need to write any code in the body of the constructor - just pass the arguments up to the base class as per Waleed's post.
If your base class starts requiring more information, it's natural that you should have to change all derived classes - and indeed anything calling the constructors of those classes - because they have to provide the information. I know it can seem like a pain, but it's just a natural consequence of what constructors do.
I had the same problem, and I solved it by replacing my constructor with a factory method like this:
A is the parent class.
public static T getChild<T>(int number) where T:A, new()
{
T child = new T();
T._number = number;
return child;
}
You can create a Child class with
Child b = A.getChild<Child>(2);
A kind of alternative could be to rely on a Dependency Injection container to initialize your objects, that way the that reference to the base class (could be the call to the base constructor or another initializer method) would "externalized" to the DI container.
I don't know if it makes sense to your case or not
Imagine the following two classes:
class A
{
public A()
{
}
}
class B : A
{
public B()
{
}
}
Is it possible for me to define A, or alternatively an interface, in a way that forces class B to have a parameterless constructor? Or, more generalized, a constructor (or static method) that is able to create an instance of type B with a given signature?
I do not want to restrict class B to only be constructible using that signature, but I want to be sure that class B can be constructed with this signature (be it parameterless, or specifying certain parameters).
To be clear: I am not searching for a solution that would require me to use Reflection or any other method to figure that out at runtime (I don't have a problem with it, but it would make the code less readable, and generally seems like a bad idea in this case).
Is there a way to accomplish this?
I wrote a blog post that goes more in-depth about what I am trying to achieve here
There is no interface or base type that you can apply to the type to ensure it has a parameterless constructor. The only context in which you can make such a contraint is generic constraints:
public static void Foo<T>()
where T : new() {}
In such a case the only types that can be used with Foo must have a parameterless constructor.
You can define factory for instantiating objects of type A (and derived types):
interface IFactory<T> where T : A
{
T Create(int i);
T Create(string s);
// and so on...
}
and require factory implementation, when you want to create an object.
This will make sure calling code in compile time, that it tries to create an object with the given set of parameters.
Of course, there's nothing preventing from NotImplementedException from concrete IFactory<T> implementation at run-time.
This is a followup, since I did a little bit of research and at least managed to come up with an answer that is somewhat satisfying.
So after digging around a while and trying to figure out how the built-in serialization/deserialization in C# works, I found out that C# has a method called GetUninitializedObject(). This method seems like a hack, since it just avoids calling the constructor of the object in the first place, but it at least gives me a way to accomplish what I originally wanted: Being able to deserialize an object of an arbitrary type. Using this, I am able to use methods on the uninitialized created objects (and forcing their implementation via an interface).
I find this to be fitting my needs, although it does not do what I originally wanted to, it allows me to implement a pattern that works for my purposes.
Best Regards
I just read http://blog.gurock.com/articles/creating-custom-exceptions-in-dotnet/
I don't know when it is written. It says:
"Since C# unfortunately doesn’t inherit constructors of base classes, this new type only has the standard constructor with no parameters and is therefore relatively useless."
This says the same in 2010: C#: inheriting constructors
Is this still true?
EDIT: Following on from answers, I'm sure there would be a way around the default parameterless constructor. Are there other reasons for lack of constructor inheritance?
Constructors have never been inheritable in the entire lifetime of the C# language. That hasn't changed in C# 5.0: at the end of section 1.6.7.1 of the C# 5.0 spec, it still says:
Unlike other members, instance constructors are not inherited, and a class has no instance constructors other than those actually declared in the class. If no instance constructor is supplied for a class, then an empty one with no parameters is automatically provided.
So it still holds true today, and I imagine it will remain so in the foreseeable future.
You have to explicitly call the constructor of the base class, unless the base class defines a default constructor. So yes they are not inherited.
Which sometimes lead to a bunch of boiler plate code where you do nothing than pass arguments from one constructor to another
public class NegativArgument : Exception {
public NegativeArgument() : this("The number given was less than zero"){}
public NegativeArgument(string message) : this(message,null){}
public NegativeArgument(string message, Exception inner) : base:(message,inner){}
}
but what if you had an Exception type that should always have the same message? how would you solve that if the constructors were inherited? The exception class has a constructor that accepts a message so creating a new Exception type would in that case get that constructor too, not inheriting constructors makes it easy
public class NegativArgument : Exception {
public NegativeArgument() : base("The number given was less than zero"){}
}
If the base class does not have a default constructor you will have a compile error if you do not explicitly call a base class constructor.
Constructors are not inherited in C#.
If they were, then every class would have a default parameterless constructor (because all classes derive from Object and Object has a default parameterless constructor).
Many classes should only be constructed with specific values; this would be impossible to ensure if every class had a default parameterless constructor.
You should call them explicitly the constructor of the base classes. They are not inheritable.
Didn't change anything about them.
Check out : Constructors (C# Programming Guide)
From the spec §1.6.7.1:
Unlike other members, instance constructors are not inherited, and a
class has no instance constructors other than those actually declared
in the class. If no instance constructor is supplied for a class, then
an empty one with no parameters is automatically provided.
http://msdn.microsoft.com/en-us/library/ms228593.aspx
This answer is based upon the section "Constructors are not inherited" near the bottom of this entry on Jon Skeet's blog.
Summary
There are many cases in which a derived class may require information beyond that contained in the base class. Jon gives the example of the FileInfo class which requires additional information to be well-defined. Namely, that of the file for which info is to be provided
Any suggested 'fix' for this would entail overriding things in a way that prevents constructing such derived objects using the inherited constructors. However, knowingly requiring derived classes to override their base classes in a way that makes them more restrictive goes against best practice. (see: this question for Jon's discussion of the Liskov Substitution principle and the importance of being able to use derived classes wherever their base can be used.)
Additionally, from just a maintenance perspective, forcing manual override of constructors would make it difficult to reason about future behavior should the base class constructors change, and would entail having to always check, and often modify, any derived classes when new constructors are added to the base. Even a few of these would be problematic; but in cases where there are dozens or more such classes (and derived classes of those classes, etc.), maintenance and QA will quickly become a nightmare.
Constructor has not a generic name but the same name as class so there is a syntax problem to enforce them in interface class. What's the syntax then for something like this:
Interface IInterface {
<class-name-of-implementer>(string param) {}
}
The problem I see with creating an initialize method is that client app of your framwork/library can just forget to call it ! Seems to me that if constructor was just named __Constructor like in some other languages that would be possible so it's not possible just because of syntax !
Syntax should not drive design but semantics.
You can't, I'm afraid.
Generally speaking, you wouldn't be able to use such a constructor anyway - how would you expect to invoke it? You can enforce a parameterless constructor on a type parameter, like this:
public class Foo<T> where T : new()
but you can't specify any required parameters.
I've previously suggested that it would be useful to be able to specify static members (including constructors) in interfaces, solely for type constraints (and to use them the members in expressions based on those constrained type parameters). See my blog post for more details.
You can't do this. Your best bet would be
interface IInterface
{
void Initialize(string param);
}
where Initialize does some initialization work (if this is what you're after).
Constructors are not allowed in interfaces.
A workaround would be to put an initialize() method in the constructor and do all the constructor related stuff here. Messy but that's all you can do.
Lets take an example in C#
public class Foo
{
public Foo() { }
public Foo(int j) { }
}
public class Bar : Foo
{
}
Now, All the public members of Foo is accessible in Bar except the constructor.
I cannot do something like
Bar bb = new Bar(1);
Why the constructors are not inheritable?
UPDATE
I do understand we can chain constructors, but I would like to know why the above construct is not valid. I am sure there should be a valid reason for it.
Constructors are not inheritable because it might cause weird and unintended behavior. More specifically, if you added a new constructor to a base class, all derived classes get an instance of that constructor. That's a bad thing in some cases, because maybe your base class specifies parameters that don't make sense for your derived classes.
A commonly given example for this is that in many languages, the base class for all objects (commonly called "Object") has a constructor with no parameters. If constructors were inherited, this would mean that all objects have a parameterless constructor, and there's no way to say "I want people who make an instance of this class to provide parameters X, Y and Z, otherwise their code shouldn't compile." For many classes, it's important that certain parameters be defined for their proper function, and making constructors non-heritable is part of the way that class authors can guarantee that some parameters are always defined.
Edit to respond to comments: Ramesh points out that if constructors were inherited as he would like them to be, he could always override base class constructors using privately declared constructors in each derived class. That is certainly true, but there it a logistical problem with this strategy. It requires that writers of derived classes have to watch base classes closely and add a private constructor if they want block inheritance of the base class constructor. Not only is this a lot of work for people writing derived classes, this kind of implicit dependency across classes is exactly the sort of thing that can cause weird behavior.
Ramesh - it's not that what you describe would be impossible to add to a language. In general it's not done because that sort of behavior could confuse people and lead to a lot of extra debugging and code writing.
Quintin Robinson provides some very worthwhile responses to this question in the comments that are definitely worth reading.
They are (via chaining), you would have to chain the constructor in your derived object.. IE:
public class Foo
{
public Foo() { }
public Foo(int j) { }
}
public class Bar : Foo
{
public Bar() : base() { }
public Bar(int j) : base(j) { }
}
The constructors in the derived objects will then chain the calls do the constructors in the base objects.
This article provides some more examples if you want further reading.
One reason why you might introduce a constructor into a class is because it makes no sense to have an instance of that class without a specific "dependency". For example, it might be a data-access class that has to have a connection to a database:
public class FooRepository
{
public FooRepository(IDbConnection connection) { ... }
}
If all the public constructors from base classes were available, then a user of your repository class would be able to use System.Object's default constructor to create an invalid instance of your class:
var badRepository = new FooRepository();
Hiding inherited constructors by default means that you can enforce dependencies without worrying about users creating "invalid" instances.
Suppose constructors were inheritable. How would you disable the inherited constructors in the many cases were they don't make sense for a subclass?
Rather than complicating the language with a mechanism to block inheritance, the language designers opted for simply making constructors not inheritable.
The Foo constructor can only know how to initialize a Foo object, so it makes no sense that it should also know how to initialize any potential subclass
public class Bar : Foo
{
public Bar(int i) : base(i) { }
}
The story the constructor tells is: "Hey base class please do whatever work you need to do to be in a good state so that I can go ahead and set up myself properly".
Constructors are not inheritable for design reasons. (Note that this is the same situation in every object-oriented language of which I know.) The simple answer is that in many cases you'd really not want the same constructors as the base class to be available. See this SO thread for some more complete explanations.
Some discussions
Joel's forum
Eric Gunnerson's blog
The basic idea is to provide as much control to the creator as possible. And you can have private bases. How'd you create the object then?
I think you can do the following:
public class Bar : Foo
{
public Bar (int i)
: base (i)
{
}
}
I may be a bit off -- but it's the general idea.
The simple answer is that the language doesn't work that way.
The real question you are asking for though is why it doesn't work that way :-) Well it is an arbitrary choice, and it follows on from C++ and Java (and very possibly many other langauges that influenced C#).
The likely reason is that the compiler will only generate a constructor that takes no arguments and simply calls the parent is that if you want more than what the compiler makes you do it yourself. This is the best choice since odds are you do more than suply calling the parent constructor.
Really, its because the parent constructor wouldn't fully initialize the child object. A constructor is kind of a personal thing in that respect. That's why most languages don't inherit constructors.