Why XML-Serializable class need a parameterless constructor

Question

I m writing code to do Xml serialization  With below function   public static string SerializeToXml object obj        XmlSerializer serializer   new XmlSerializer obj GetType         using  StringWriter writer   new StringWriter                  serializer Serialize writer  obj           return writer ToString              If the argument is a instance of class without parameterless constructor  it will throw a exception      Unhandled Exception    System InvalidOperationException    CSharpConsole Foo cannot be serialized   because it does not have a   parameterless constructor     at   System Xml Serialization TypeDesc CheckSupported     at   System Xml Serialization TypeScope GetTypeDesc Type   type  MemberInfo sourc e  Boolean   directReference  Boolean throwOnError    at   System Xml Serialization ModelScope GetTypeModel Type   type  Boolean direct Reference     at   System Xml Serialization XmlReflectionImporter ImportTypeMapping Type   type   XmlRootAttribute root  String   defaultNamespace     at   System Xml Serialization XmlSerializer  ctor Type   type  String defaultName space     at   System Xml Serialization XmlSerializer  ctor Type   type    Why must there be a parameterless constructor in order to allow xml serialization to succeed   EDIT  thanks for cfeduke s answer  The parameterless constructor can be private or internal

User · Accepted Answer

During an object s de-serialization  the class responsible for de-serializing an object creates an instance of the serialized class and then proceeds to populate the serialized fields and properties only after acquiring an instance to populate   You can make your constructor private or internal if you want  just so long as it s parameterless

User · Answer

This is a limitation of XmlSerializer  Note that BinaryFormatter and DataContractSerializer do not require this - they can create an uninitialized object out of the ether and initialize it during deserialization   Since you are using xml  you might consider using DataContractSerializer and marking your class with  DataContract   DataMember   but note that this changes the schema  for example  there is no equivalent of  XmlAttribute  - everything becomes elements    Update  if you really want to know  BinaryFormatter et al use FormatterServices GetUninitializedObject   to create the object without invoking the constructor  Probably dangerous  I don t recommend using it too often  -p See also the remarks on MSDN      Because the new instance of the object   is initialized to zero and no   constructors are run  the object might   not represent a state that is regarded   as valid by that object  The current   method should only be used for   deserialization when the user intends   to immediately populate all fields  It   does not create an uninitialized   string  since creating an empty   instance of an immutable type serves   no purpose    I have my own serialization engine  but I don t intend making it use FormatterServices  I quite like knowing that a constructor  any constructor  has actually executed

User · Answer

The answer is  for no good reason whatsoever   Contrary to its name  the XmlSerializer class is used not only for serialization  but also for deserialization  It performs certain checks on your class to make sure that it will work  and some of those checks are only pertinent to deserialization  but it performs them all anyway  because it does not know what you intend to do later on   The check that your class fails to pass is one of the checks that are only pertinent to deserialization   Here is what happens    During deserialization  the XmlSerializer class will need to create instances of your type   In order to create an instance of a type  a constructor of that type needs to be invoked   If you did not declare a constructor  the compiler has already supplied a default parameterless constructor  but if you did declare a constructor  then that s the only constructor available  So  if the constructor that you declared accepts parameters  then the only way to instantiate your class is by invoking that constructor which accepts parameters  However  XmlSerializer is not capable of invoking any constructor except a parameterless constructor  because it does not know what parameters to pass to constructors that accept parameters   So  it checks to see if your class has a parameterless constructor  and since it does not  it fails    So  if the XmlSerializer class had been written in such a way as to only perform the checks pertinent to serialization  then your class would pass  because there is absolutely nothing about serialization that makes it necessary to have a parameterless constructor    As others have already pointed out  the quick solution to your problem is to simply add a parameterless constructor  Unfortunately  it is also a dirty solution  because it means that you cannot have any readonly members initialized from constructor parameters   In addition to all this  the XmlSerializer class could have been written in such a way as to allow even deserialization of classes without parameterless constructors   All it would take would be to make use of  The Factory Method Design Pattern   Wikipedia   From the looks of it  Microsoft decided that this design pattern is far too advanced for DotNet programmers  who apparently should not be unnecessarily confused with such things  So  DotNet programmers should better stick to parameterless constructors  according to Microsoft

User · Answer

This is a limitation of XmlSerializer  Note that BinaryFormatter and DataContractSerializer do not require this - they can create an uninitialized object out of the ether and initialize it during deserialization   Since you are using xml  you might consider using DataContractSerializer and marking your class with  DataContract   DataMember   but note that this changes the schema  for example  there is no equivalent of  XmlAttribute  - everything becomes elements    Update  if you really want to know  BinaryFormatter et al use FormatterServices GetUninitializedObject   to create the object without invoking the constructor  Probably dangerous  I don t recommend using it too often  -p See also the remarks on MSDN      Because the new instance of the object   is initialized to zero and no   constructors are run  the object might   not represent a state that is regarded   as valid by that object  The current   method should only be used for   deserialization when the user intends   to immediately populate all fields  It   does not create an uninitialized   string  since creating an empty   instance of an immutable type serves   no purpose    I have my own serialization engine  but I don t intend making it use FormatterServices  I quite like knowing that a constructor  any constructor  has actually executed

User · Answer

First of all  this what is written in documentation  I think it is one of your class fields  not the main one - and how you want deserialiser to construct it back w o parameterless construction     I think there is a workaround to make constructor private

User · Answer

This is a limitation of XmlSerializer  Note that BinaryFormatter and DataContractSerializer do not require this - they can create an uninitialized object out of the ether and initialize it during deserialization   Since you are using xml  you might consider using DataContractSerializer and marking your class with  DataContract   DataMember   but note that this changes the schema  for example  there is no equivalent of  XmlAttribute  - everything becomes elements    Update  if you really want to know  BinaryFormatter et al use FormatterServices GetUninitializedObject   to create the object without invoking the constructor  Probably dangerous  I don t recommend using it too often  -p See also the remarks on MSDN      Because the new instance of the object   is initialized to zero and no   constructors are run  the object might   not represent a state that is regarded   as valid by that object  The current   method should only be used for   deserialization when the user intends   to immediately populate all fields  It   does not create an uninitialized   string  since creating an empty   instance of an immutable type serves   no purpose    I have my own serialization engine  but I don t intend making it use FormatterServices  I quite like knowing that a constructor  any constructor  has actually executed

User · Answer

This is a limitation of XmlSerializer  Note that BinaryFormatter and DataContractSerializer do not require this - they can create an uninitialized object out of the ether and initialize it during deserialization   Since you are using xml  you might consider using DataContractSerializer and marking your class with  DataContract   DataMember   but note that this changes the schema  for example  there is no equivalent of  XmlAttribute  - everything becomes elements    Update  if you really want to know  BinaryFormatter et al use FormatterServices GetUninitializedObject   to create the object without invoking the constructor  Probably dangerous  I don t recommend using it too often  -p See also the remarks on MSDN      Because the new instance of the object   is initialized to zero and no   constructors are run  the object might   not represent a state that is regarded   as valid by that object  The current   method should only be used for   deserialization when the user intends   to immediately populate all fields  It   does not create an uninitialized   string  since creating an empty   instance of an immutable type serves   no purpose    I have my own serialization engine  but I don t intend making it use FormatterServices  I quite like knowing that a constructor  any constructor  has actually executed

User · Answer

First of all  this what is written in documentation  I think it is one of your class fields  not the main one - and how you want deserialiser to construct it back w o parameterless construction     I think there is a workaround to make constructor private

User · Answer

First of all  this what is written in documentation  I think it is one of your class fields  not the main one - and how you want deserialiser to construct it back w o parameterless construction     I think there is a workaround to make constructor private

User · Answer

First of all  this what is written in documentation  I think it is one of your class fields  not the main one - and how you want deserialiser to construct it back w o parameterless construction     I think there is a workaround to make constructor private

User · Answer

The answer is  for no good reason whatsoever   Contrary to its name  the XmlSerializer class is used not only for serialization  but also for deserialization  It performs certain checks on your class to make sure that it will work  and some of those checks are only pertinent to deserialization  but it performs them all anyway  because it does not know what you intend to do later on   The check that your class fails to pass is one of the checks that are only pertinent to deserialization   Here is what happens    During deserialization  the XmlSerializer class will need to create instances of your type   In order to create an instance of a type  a constructor of that type needs to be invoked   If you did not declare a constructor  the compiler has already supplied a default parameterless constructor  but if you did declare a constructor  then that s the only constructor available  So  if the constructor that you declared accepts parameters  then the only way to instantiate your class is by invoking that constructor which accepts parameters  However  XmlSerializer is not capable of invoking any constructor except a parameterless constructor  because it does not know what parameters to pass to constructors that accept parameters   So  it checks to see if your class has a parameterless constructor  and since it does not  it fails    So  if the XmlSerializer class had been written in such a way as to only perform the checks pertinent to serialization  then your class would pass  because there is absolutely nothing about serialization that makes it necessary to have a parameterless constructor    As others have already pointed out  the quick solution to your problem is to simply add a parameterless constructor  Unfortunately  it is also a dirty solution  because it means that you cannot have any readonly members initialized from constructor parameters   In addition to all this  the XmlSerializer class could have been written in such a way as to allow even deserialization of classes without parameterless constructors   All it would take would be to make use of  The Factory Method Design Pattern   Wikipedia   From the looks of it  Microsoft decided that this design pattern is far too advanced for DotNet programmers  who apparently should not be unnecessarily confused with such things  So  DotNet programmers should better stick to parameterless constructors  according to Microsoft

[c#] Why XML-Serializable class need a parameterless constructor

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