I have an C# object that hold a big list (10-100MB) and some other properties. I want to serialize the object, but I don't want to serialize the list. is there any easy way to do that?
Thanks!
Since it is tagged xml, this could be as simple as adding [XmlIgnore] to the appropriate property:
[XmlIgnore]
public List<Foo> Items {get;set;}
Then just use XmlSerializer as normal.
If it needs to be controllable (sometimes yes, sometimes no) then an alternative is to add:
public bool ShouldSerializeItems() {
// your logic here
}
This pattern is recognised by many serializers, XmlSerializer included.
If you mean by serialization loading the object from database, then I can recommend NHibernate, you simply mark, you want to enable lazy-loading on the List and your object will return, with Items set to null (and if you are still in transaction, Items will load, after and only after attempting to access it; if you are out of transaction already, you will get a nullPointer exception I think)... but basically, Lazy-loading in any its kind is certainly a principle, you should keep an eye on.
EDIT: missed the tags, shame on me
Related
Let's say we have a business object, let's call it a Foo, which contains an ordered list of Bars. We pass this Foo around as XML.
We have a class which deserializes a Foo XML node (FooXMLDeserializer) which itself uses a class which deserializes the child Bar XML nodes (BarXMLDeserializer).
Now, I'm adding some functionality to the BarXMLDeserializer that maintains some state such that if FooXMLDeserializer is called on two separate Foo nodes without reseting the BarXMLDeserializer's state, the results may be invalid. BarXMLDeserializer does not know when it has processed the final Bar in a Foo.
Is there some way that I can design the BarXMLDeserializer class to communicate to developers working on consuming classes that it has state and must be reset for each Foo?
Further info:
My change solves a minor enough problem in our code that I won't be able to convince my manager to let me spend X days redesigning the whole system to nicely handle this case.
If it matters, BarXMLDeserializer keeps is state in a BarStateTracker class which is internal to it.
Programming in C#, but looking for a more general solution.
Thanks.
Expose your serializer only as a static method:
// no public constructor, etc
var deserializer = BarXMLDeserializer.CreateNew();
Then, when you have finished deserializing data, mark a field in your object. If the field is set, throw an exception if the same instance is used to deserialize more data when the deserialize method is called.
if(IsInstanceExhausted)
throw new InvalidOperationException("You must use a fresh instance.");
They'll figure it out after their first exception. In addition, mark your class as IDisposable so that code naturally uses using statements:
using(var deserializer = BarXMLDeserializer.CreateNew())
{
}
The list goes on of additional ways. ALTERNATIVELY, you could simply design your Deserializer to clear it's state or reset after a deserialization attempt, or to clear the state at the beginning of a deserialization attempt.
In a question about Best practices for C# pattern validation, the highest voted answer
says:
I tend to perform all of my validation in the constructor. This is a must because I almost always create immutable objects.
How exactly do you create an immutable object in C#? Do you just use the readonly keyword?
How exactly would this work if you want to validate in the constructor of your Entity Framework generated model class?
Would it look like below?
public partial readonly Person
{
public Person()
}
The interesting question here is your question from the comments:
What kind of object would you have that you do not need to modify the values at some point? I'm guessing not a model class, correct? I've had to change the name of a person in my database - this wouldn't fit with this idea.
Well, consider things that are already immutable. Numbers are immutable. Once you have the number 12, it's 12. You can't change it. If you have a variable that contains 12, you can change the contents of the variable to 13, but you are changing the variable, not the number 12.
Same with strings. "abc" is "abc", and it never changes. If you have a variable that contains "abc", you can change it to "abcd", but that doesn't change "abc", that changes the variable.
What about a list? {12, "abc"} is the list that is 12 followed by "abc", and that list never changes. The list {12, "abcd"} is a different list.
And that's where things go off the rails. Because in C# you can do it either way. You can say that there is referential identity between those two lists if lists are allowed to mutate their contents without changing their identity.
You hit the nail right on the head when you talk about the "model". Are you modeling something that changes? If so, then it is possibly wise to model it with a type that changes. The benefit of that is that the characteristics of the model match the system being modeled. The down side is that it becomes very tricky to do something like a "rollback" functionality, where you "undo" a change.
That is, if you mutate {12, "abc"} to {12, "abcd"} and then want to roll back the mutation, how do you do it? If the list is immutable you just keep around both values and choose which one you want to be the "current" value. If the list is mutable then you have to have the undo logic keep around an "undo function" which knows how to undo the mutation.
As for your specific example, you certainly can create an immutable database. How do you change the name of someone in your immutable database? You don't. You create a new database that has the data you want in it. The trick with immutable types is to do so efficiently, without copying billions of bytes. Immutable data structure design requires finding clever ways to share state between two nearly-identical structures.
Declaring all fields readonly is a good step towards creating an immutable object, but this alone is not sufficient. This is because a readonly field can still be a reference to a mutable object.
In C# immutability is not enforced by the compiler. You just have to be careful.
This question has two aspects:
Immutable type when you instantiate object
Immutable type when EF instantiate object
The first aspect demands sturcture like this:
public class MyClass
{
private readonly string _myString;
public string MyString
{
get
{
return _myString;
}
}
public MyClass(string myString)
{
// do some validation here
_myString = myString;
}
}
Now the problem - EF. EF requires parameterless constructor and EF must have setters on properties. I asked very similar question here.
Your type must look like:
public class MyClass
{
private string _myString;
public string MyString
{
get
{
return _myString;
}
private set
{
_myString = value;
}
}
public MyClass(string myString)
{
// do some validation here
_myString = myString;
}
// Not sure if you can change accessibility of constructor - I can try it later
public MyClass()
{}
}
You must also inform EF about private setter of MyString property - this is configured in properties of enitity in EDMX file. Obviously there will be no validation when EF will materialize objects from DB. Also you will not be able to use methods like ObjectContext.CreateObject (you will not be able to fill the object).
Entity Object T4 template and default code generation create factory method CreateMyClass instead of constructor with paremeters. POCO T4 template doesn't generate factory method.
I didn't try this with EF Code first.
An immutable value object is a value object that cannot be changed. You cannot modify its state, you have to create new ones
Check out Eric Lippert's blog:
Kinds of Immutability
https://learn.microsoft.com/en-us/archive/blogs/ericlippert/immutability-in-c-part-one-kinds-of-immutability
Have a look at
Immutable object pattern in C# - what do you think?
How exactly would this work if you want to validate in the constructor of your Entity Framework generated model class?
It wouldn't work in this context because EF requires the properties of the entity class be public otherwise it can't instantiate it.
But you're welcome to use immutable objects further in your code.
C# 9 is coming up with new feature names as Record. Init-only properties are great if you want to make individual properties immutable. If you want the whole object to be immutable and behave like a value, then you should consider declaring it as a record:
public data class Person
{
public string FirstName { get; init; }
public string LastName { get; init; }
}
The data keyword on the class declaration marks it as a record.
Reference: https://devblogs.microsoft.com/dotnet/welcome-to-c-9-0/#records
#Eric Lippert Good comment, but in addition in answer to the question:
What kind of object would you have that you do not need to modify the
values at some point? I'm guessing not a model class, correct? I've
had to change the name of a person in my database - this wouldn't fit
with this idea.
Let's say you have a large datastructure and you want to query its information, but it's changing all the time. You need some kind of locking system to make sure that you don't say try and count the total in the system while somebody is depositing something from one place to another. (Say a warehouse management system)
And that's hard to do because these things always affect things in unexpected ways, the data changing under your feet.
What if you could freeze your large datastructure when you're not updating it, so that no memory can be altered and it is paused at a consistent state? Now when you want to change it again you have to copy the datastructure to a new place, and it's fairly large, so that's a downside, but the upside is you won't have to lock anything because the new copy of the data goes unshared until it has been updated. This means anyone at any point can read the latest copy of the datastructure, doing complex things.
So yep very useful concept if you hate dealing with concurrency issues and don't have too much data to deal with. (E.g. if 1MB of data and updating 10/sec that's 10MB of data being copied)
I've done a lot of serialization development lately, mostly for sending objects over sockets, but I've run into an interesting question: Is it possible to send just a few of the properties from an object through a serializer?
My envisioned scenario is this: You have some sort of "state" object for each client, consisting of many properties (strings, ints, bools, etc). When your client first connects, the entire state object is serialized via an Xml or Binary serializer, and sent over the socket, to be recreated on the other side. Now both client and server have identical state objects. Your server then needs to change the state, and does so by simply setting one of the state object's property. The socket (either hooked to the state's events, or part of the state object itself) could synchronize the two states by reserializing the entire object, but it seems like a single "property change" object would do.
Obviously, this could be implemented manually. But it seems like a serializer should be able to serialize just a single property, and apply it like a patch on the other side. Does anyone know if this is possible, or would I have to write the entire thing from scratch?
With XmlSerializer (and protobuf-net, for a binary equivalent, since protobuf-net adopts most of XmlSerializer's patterns) you could do this by having a method:
public bool SouldSerializeFoo() {
return fooIsDirty;
}
public string Foo {get;set;}
for each property Foo - but you'd need to maintain the "what is dirty" manually in your own code (perhaps in the set). Lots of work; I've done a diffing serializer in the past - it was a real PITA, to be honest. I should also note that the [XmlIgnore] public bool FooSpecified {get{...} set{...}} pattern does the same thing, but for what you want, ShouldSerialize* is more appropriate.
As an addition to Marc's answer, here's the MSDN docs on the ShouldSerialize* methods
Right now, I'm currently serializing a class like this:
class Session
{
String setting1;
String setting2;
...etc... (other member variables)
List<SessionAction> actionsPerformed;
}
Where SessionAction is an interface that just has one method. All implementations of the SessionAction interface have various properties describing what that specific SessionAction does.
Currently, I serialize this to a file which can be loaded again using the default .Net binary serializer. Now, I want to serialize this to a template. This template will just be the List of SessionActions serialized to a file, but upon loading it back into memory at another time, I want some properties of these SessionActions to require input from the user (which I plan to dynamically generate GUI controls on the fly depending on the property type). Right now, I'm stuck on determining the best way to do this.
Is there some way I could flag some properties so that upon using reflection, I could determine which properties need input from user? Or what are my other options? Feel free to leave comments if anything isn't clear.
For info, I don't recommend using BinaryFormatter for anything that you are storing long-term; it is very brittle between versions. It is fine for short-lived messages where you know the same version will be used for serialization and deserialization.
I would recommend any of: XmlSerializer, DataContractSerializer (3.0), or for fast binary, protobuf-net; all of these are contract-based, so much more version tolerant.
Re the question; you could use things like Nullable<T> for value-types, and null for strings etc - and ask for input for those that are null? There are other routes involving things like the ShouldSerialize* pattern, but this might upset the serialization APIs.
If you know from start what properties will have that SessionAction, you must implement IDeserializationCallback and put to those props the attribute [NonSerialized]. When you implement the OnDeserialization method you get the new values from the user.
I have a a property defined as:
[XmlArray("delete", IsNullable = true)]
[XmlArrayItem("contact", typeof(ContactEvent)),
XmlArrayItem("sms", typeof(SmsEvent))]
public List<Event> Delete { get; set; }
If the List<> Delete has no items
<delete />
is emitted. If the List<> Delete is set to null
<delete xsi:nil="true" />
is emitted. Is there a way using attributes to get the delete element not to be emitted if the collection has no items?
Greg - Perfect thanks, I didn't even read the IsNullable documentation just assumed it was signalling it as not required.
Rob Cooper - I was trying to avoid ISerializable, but Gregs suggestion works. I did run into the problem you outlined in (1), I broke a bunch of code by just returning null if the collection was zero length. To get around this I created a EventsBuilder class (the class I am serializing is called Events) that managed all the lifetime/creation of the underlying objects of the Events class that spits our Events classes for serialization.
I've had the same issue where I did not want an element outputted if the field is empty or 0.
The XML outputted could not use xsi:null="true" (by design).
I've read somewhere that if you include a property of type bool with the same name as the field you want to control but appended with 'Specified', the XMLSerializer will check the return value of this property to determine if the corresponding field should be included.
To achieve this without implementing IXMLSerializer:
public List<Event> Delete { get; set; }
[XMLIgnore]
public bool DeleteSpecified
{
get
{
bool isRendered = false;
if (Delete != null)
{
isRendered = (Delete.Count > 0);
}
return isRendered;
}
set
{
}
}
If you set IsNullable=false or just remove it (it is false by default), then the "delete" element will not be emitted. This will work only if the collection equals to null.
My guess is that there is a confusion between "nullability" in terms of .NET, and the one related to nullable elements in XML -- those that are marked by xml:nil attribute. XmlArrayAttribute.IsNullable property controls the latter.
First off, I would say ask yourself "What is Serialization?".
The XmlSerializer is doing exactly what it is supposed to be doing, persisting the current state of the object to XML. Now, I am not sure why the current behaviour is not "right" for you, since if you have initialized the List, then it is initialized.
I think you have three options here:
Add code to the Getter to return null if the collection has 0 items. This may mess up other code you have though.
Implement the IXmlSerializable interface and do all the work yourself.
If this is a common process, then you may want to look at my question "XML Serialization and Inherited Types" - Yes, I know it deals with another issue, but it shows you how to create a generic intermediary serialization class that can then be "bolted on" to allow a serilization process to be encapsulated. You could create a similar class to deal with overriding the default process for null/zero-item collections.
I hope this helps.
You could always implement IXmlSerializer and perform the serialization manually.
See http://www.codeproject.com/KB/cs/IXmlSerializable.aspx for an example.