Prevent calling Public constructors except for XML Serializer

Question

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Why XML-Serializable class need a parameterless constructor

Does anyone know if it is possible to prevent calling of a public constructor from outside a range of assemblies?

What I'm doing is coming up with a class library that has XML serializable classes. These classes make no sense if constructed without setting various properties so I want to prevent that state.

I'm wondering if there is a way to prevent anyone calling my public constructor, and keep it there only for serialization?

Why do you want to keep a constructor public, when you want to prevent it from being called? — Narendra N, Jul 15 '09 at 11:29
Is it required that the constructors be public, or could you make them internal? — Kenan E. K., Jul 15 '09 at 11:20
Apparently, they claim here they don't - http://stackoverflow.com/questions/267724/why-xml-serializable-class-need-a-parameterless-constructor — Kenan E. K., Jul 15 '09 at 11:28
Check this answer: [http://stackoverflow.com/questions/267724/why-xml-serializable-class-need-a-parameterless-constructor](http://stackoverflow.com/questions/267724/why-xml-serializable-class-need-a-parameterless-constructor). You may create internal or private constructor and still use XML serialization. — empi, Jul 15 '09 at 11:26
I did not know that. I've always thought it had to be public, but I just tried it with an internal one, and it worked. — John Saunders, Jul 15 '09 at 11:31
John. I'd be interested to know how you got that to work? All of my unit tests fail if I attempt to use anything but Public constructors. — Ian, Jul 15 '09 at 11:36
I also tried it with private and it works. Could you post the exception? — empi, Jul 15 '09 at 11:39
System.MissingMethodException: No parameterless constructor defined for this object.. — Ian, Jul 15 '09 at 11:39
I just added a constructor, then tried it. I just realized I didn't try to deserialize. Hold five. — John Saunders, Jul 15 '09 at 11:40
hmm, this may be a slightly different error though, because I use element = (T)Activator.CreateInstance(elementType) as part of my custom serialization... — Ian, Jul 15 '09 at 11:40
In XML Serialization? Odd. At any rate, deserialization works fine with the internal parameterless constructor. — John Saunders, Jul 15 '09 at 11:44
ah. Thanks empi. Answered both my original, and the question I never asked but needed to know. :) — Ian, Jul 15 '09 at 11:46
If your actual aim is to make it impossible to instantiate an object of your class except by calling certain constructors, [unfortunately that's impossible to ensure](http://msdn.microsoft.com/en-us/library/system.runtime.serialization.formatterservices.getuninitializedobject.aspx). — Daniel Earwicker, Jul 15 '09 at 11:55
If you do not explicitly call any constructor in the base class, the parameterless constructor will be called implicitly. There's no way around it, you cannot instantiate a class without a constructor being called. — joe, Jul 15 '09 at 11:21

score 1 · Accepted Answer · answered Jul 15 '09 at 11:25

1

Typically, I would create a public constructor with parameters that will make the object valid and create a public parameterless constructor with valid default values.

public foo() {
  this.bar = -1;
}

public foo(int bar) {
  this.bar = bar;
}

Private/internal constructor can be used depending on your situation, and you can create a Factory pattern that deals with object creation for external code.

answered Jul 15 '09 at 11:25

Adrian Godong

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+1 just because I agree that normally you should initalize sensible values, however in this situation the values are other classes, which need to be correct. – Ian Jul 15 '09 at 11:47

score 0 · Answer 2 · answered Jul 15 '09 at 11:22

To achieve what you are asking, you should probably make the constructor private, and then use a getInstance method to return the object if the correct assembly calls getInstance. You can do a check inside the getInstance method if you pass the object or assembly that called the function as one of its parameters.

score 0 · Answer 3 · answered Jul 15 '09 at 11:29

0

I don't know if it would work in your case, but you could check the calling assembly:

if (Assembly.GetCallingAssembly().GetName().Name == "Forbidden.Assembly")
{
    throw new Exception(...);
}

answered Jul 15 '09 at 11:29

Philippe Leybaert

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Prevent calling Public constructors except for XML Serializer

3 Answers3