I have an object like so:
public class Intent
{
public List<Entity> Updates { get; set; }
}
Which I wish to serialize into XML for passing as a message using MSMQ. The list of type Entity can contain any number of instances of classes that inherit from Entity. For example, there may be:
public Person : Entity { /* ... */ }
public Vehicle : Entity { /* ... */ }
I'm using XmlMessageFormatter, which so far I have defined as:
XmlMessageFormatter _formatter =
new XmlMessageFormatter(new[] { typeof(T) });
Where T in this instance is Intent (as above).
Trouble is, when the code actually attempts to serialize the following exception occurs:
The type CoreApi.Domain.Person was not expected. Use the XmlInclude or SoapInclude attribute to specify types that are not known statically.
I believe this is because I need to tell the serializer somehow of the fact that Person is a child class of entity.
I've seen solutions that basically entail adorning Entity with multiple XmlInclude decorations, which in my scenario is unworkable as the list of inheritors of Entity is large and could grow - I don't want to constantly update this list as new inheritors are added.
I've seen other solutions that use XmlSerializer, passing in a list of known types, the trouble with this is that I somehow need to replace XmlMessageSerialiser with the XmlSerialiser instance which isn't compatible.
Related
I'm having an issue trying to convert an object to json. The error is a Newtonsoft.Json.JsonSerializationException:
Self referencing loop detected for property 'Project' with type 'System.Data.Entity.DynamicProxies.Project_F29F70EF89942F6344C5B0A3A7910EF55268857CD0ECC4A484776B2F4394EF79'. Path '[0].Categories[0]'.
The problem is that the object (it's actually a list of objects) has a property which is another object that refers back to the first object:
public partial class Project
{
...
public virtual ICollection<Category> Categories { get; set; }
...
}
public partial class Category
{
...
public virtual Project Project { get; set; }
...
}
This is all fine and dandy as far as Entity Framework is concerned, but to convert this to json would result in an infinite regress, hence the exception.
Here is my code:
public async Task<HttpResponseMessage> GetProjects()
{
var projects = _projectService.GetProjects().ToList();
string jsonString = JsonConvert.SerializeObject(projects); // <-- Offending line
return Request.CreateResponse(HttpStatusCode.OK, jsonString);
}
I've looked online for solutions to this and I found this stackoverflow post:
JSON.NET Error Self referencing loop detected for type
They suggest three solutions, none of which work:
1) Ignore the circular reference:
public async Task<HttpResponseMessage> GetProjects()
{
var projects = _projectService.GetProjects().ToList();
JsonSerializerSettings settings = new JsonSerializerSettings()
{
ReferenceLoopHandling = ReferenceLoopHandling.Ignore
};
string jsonString = JsonConvert.SerializeObject(projects, settings);
return Request.CreateResponse(HttpStatusCode.OK, jsonString);
}
This resulted in the call to SerializeObject(...) hanging for a bit then throwing a System.OutOfMemoryException (which tells me the circular references were NOT being ignored).
Mind you, the author of this proposed solution at stackoverflow says to set the ignore setting in WebApiConfig.cs but I tried that and it has no effect.
He also says:
"If you want to use this fix in a non-api ASP.NET project, you can add the above line to Global.asax.cs, but first add: var config = GlobalConfiguration.Configuration;"
Mine's a web API with no global file so I shouldn't have to do this.
I also don't want to ignore circular references because I don't want to lose data.
2) Preserve the circular reference:
public async Task<HttpResponseMessage> GetProjects()
{
var projects = _projectService.GetProjects().ToList();
JsonSerializerSettings settings = new JsonSerializerSettings()
{
ReferenceLoopHandling = ReferenceLoopHandling.Serialize,
PreserveReferencesHandling = PreserveReferencesHandling.Objects
};
string jsonString = JsonConvert.SerializeObject(projects, settings);
return Request.CreateResponse(HttpStatusCode.OK, jsonString);
}
This just resulted in the request timing out because it would just hang.
Again, the author says to put this in WebApiConfig.cs, but again this had no effect.
3) Add ignore/preserve reference attributes to the objects and properties:
Ignoring Categories:
public partial class Project
{
...
[JsonIgnore]
public virtual ICollection<Category> Categories { get; set; }
...
}
This has no effect. I hover over the project list and see that it still has categories, and each category still has an instance of the project. I still get the same exception.
Again, even if this worked, I don't want to ignore the categories.
Preserve Categories:
[JsonObject(IsReference = true)]
public partial class Project
{
...
public virtual ICollection<Category> Categories { get; set; }
...
}
Again, same results.
Even if this method worked, the attributes wouldn't be preserved. I'd be doing it on Entity Framework classes which are re-generated automatically every time I recompile. (Is there a way to tell it to set these attributes in the model? Can I set them on the other half of the partial class?)
Alternatively, I'm open to suggestions other than converting to json and sending back in the response. Is there another way to get the data back to the client?
What would be the fix to this problem? Thanks.
Briefly
The best way to fix this problem is to create completely brand-new Models (xxxModel, xxxViewModel, xxxResponse, etc..) on Presentation layer which will be returned to end-users. Than just cast one object to another using AutoMapper or your own custom methods.
Keep your database entities separate from real world!
In detail
There are so many problems that you will encounter:
Disclosure of sensitive data. Your database entity could/will contain sensitive data which end-users shouldn't receive;
Performance issues and waste of RAM and CPU. It would be better to load only those properties that end-users is required, instead all;
Serialization problems. EF entities almost always contain Navigation properties which will be serialized together in case lazy-loading enabled. Imagine dozens related entities, which will be lazy-loaded when your composite root is being serialized. It will cause dozens unexpected requests to database;
Fragility. Any changes related your EF entities will affect on Presentation Layer and on end-users. For instance, in case with API, new added property just extend response, but deleted or renamed will break logic in your customers' application.
There are a lot of other problems, just be careful.
I would recommend not Serializing Entity Framework classes and creating a specific class that only inherits from Object and has only the data you need
I have a simple hierarchy
public abstract class CommunicationSupport
{
public SupportTypeEnum Type { get; set; }
public Country Origin { get; set; } // National or Foreign support
}
public class TelecomSupport : CommunicationSupport
{
public string Number { get; set; }
}
public class PostalSupport : CommunicationSupport
{
public Address Address { get; set; }
}
I plan to use the Table-per-type hierarchy for my DB. So 3 tables will be created, one base and two child using the same PK as the base.
My problem is that I want to be able to update a CommunicationSupport by changing it's type.
Let's say that I create a TelecomSupport, save it and then change it's type to a PostalSupport and save it again (update). The result I expect is for EF to keep the same base record (CommunicationSupport Id) but delete the record in the TelecomSupport table and create a new one in the PostalSupport.
So TelecomSupport and PostalSupport are exclusive and cannot share the same base CommunicationSupport.
How can I do that using EntityFramework 5?
Thanks for your help!
I don't have a good answer, but I can think of four "solutions" that are really workarounds:
Don't use DBMS-computed values for your primary keys (if you already use natural keys, it's fine).
Use DBMS-computed surrogate keys.
Follow something like the state pattern.
Do some evil voodoo with the object state manager.
Update: There seems to be a popular consensus that trying isn't even worth it; most people thus simply use stored procedures instead to work around the problem.
Changing Inherited Types in Entity Framework
Entity Framework: Inheritance, change object type
Changing the type of an (Entity Framework) entity that is part of an inheritance hierarchy
Changing the type of an entity that is part of an inheritance hierarchy
Using natural keys
First, remember that the objects tracked by the EF are part of your DAL, not your domain model (regardless of whether you use POCOs or not). Some people don't need a domain model, but keep it in mind, as we can now think of these objects as representations of table records we manipulate in ways we wouldn't with domain objects.
Here, we use IDbSet.Remove to delete the records of the entity, then add new ones with the same primary key using IDbSet.Add, all in a single transaction. See the ChangeType method in the sample code below.
In theory, integrity is OK, and in theory, EF could detect what you're trying to do and optimize things. In practice, it currently doesn't (I profiled the SQL interface to verify this). The result is that it looks ugly (DELETE+INSERT instead of UPDATE), so if system beauty and performance are issues, it's probably a no-go. If you can take it, it's relatively straightforward.
Here is some sample code I used to test this (if you want to experiment, simply create a new console application, add a reference to the EntityFramework assembly, and paste the code).
A is the base class, X and Y are subclasses. We consider Id to be a natural key, so we can copy it in the subclasses copy constructors (here only implemented for Y). The code creates a database and seeds it with a record of type X. Then, it runs and changes its type to Y, obviously losing X-specific data in the process. The copy constructor is where you would transform data, or archive it if data loss is not part of the business process. The only piece of "interesting" code is the ChangeType method, the rest is boilerplate.
using System;
using System.ComponentModel.DataAnnotations.Schema;
using System.Data.Entity;
using System.Linq;
namespace EntitySubTypeChange {
abstract class A {
[DatabaseGenerated(DatabaseGeneratedOption.None)]
public int Id { get; set; }
public string Foo { get; set; }
public override string ToString() {
return string.Format("Type:\t{0}{3}Id:\t{1}{3}Foo:\t{2}{3}",
this.GetType(), Id, Foo, Environment.NewLine);
}
}
[Table("X")]
class X : A {
public string Bar { get; set; }
public override string ToString() {
return string.Format("{0}Bar:\t{1}{2}", base.ToString(), Bar, Environment.NewLine);
}
}
[Table("Y")]
class Y : A {
public Y() {}
public Y(A a) {
this.Id = a.Id;
this.Foo = a.Foo;
}
public string Baz { get; set; }
public override string ToString() {
return string.Format("{0}Baz:\t{1}{2}", base.ToString(), Baz, Environment.NewLine);
}
}
class Program {
static void Main(string[] args) {
Display();
ChangeType();
Display();
}
static void Display() {
using (var context = new Container())
Console.WriteLine(context.A.First());
Console.ReadKey();
}
static void ChangeType()
{
using (var context = new Container()) {
context.A.Add(new Y(context.A.Remove(context.X.First())));
context.SaveChanges();
}
}
class Container : DbContext {
public IDbSet<A> A { get; set; }
public IDbSet<X> X { get; set; }
public IDbSet<Y> Y { get; set; }
}
static Program() {
Database.SetInitializer<Container>(new ContainerInitializer());
}
class ContainerInitializer : DropCreateDatabaseAlways<Container> {
protected override void Seed(Container context) {
context.A.Add(new X { Foo = "Base Value", Bar = "SubType X Value" });
context.SaveChanges();
}
}
}
}
Output:
Type: EntitySubTypeChange.X
Id: 0
Foo: Base Value
Bar: SubType X Value
Type: EntitySubTypeChange.Y
Id: 0
Foo: Base Value
Baz:
Note: If you want an auto-generated natural key, you can't let EF ask the DBMS to compute it, or EF will prevent you from manipulating it the way you want (see below). In effect, EF treats all keys with computed values as surrogate keys, even though it still happily leaks them (the bad of both worlds).
Note: I annotate the subclasses with Table because you mentioned a TPT setup, but the problem is not actually related to TPT.
Using surrogate keys
If you consider a surrogate key to be truly internal, then it doesn't matter if it changes under your nose as long as you can still access your data the same way (using a secondary index for example).
Note: In practice, many people leak surrogate keys all around (domain model, service interface, ...). Don't do it.
If you take the previous sample, simply remove the DatabaseGenerated attribute and the assignment of the Id in the copy constructor of the subtypes.
Note: With its value generated by the DBMS, the Id property is completely ignored by EF and doesn't serve any real purpose other than being analyzed by the model builder to generate the Id column in the SQL schema. That and being leaked by bad programmers.
Output:
Type: EntitySubTypeChange.X
Id: 1
Foo: Base Value
Bar: SubType X Value
Type: EntitySubTypeChange.Y
Id: 2
Foo: Base Value
Baz:
Using the state pattern (or similar)
This solution is probably what most people would consider the "proper solution", since you can't change the intrinsic type of an object in most object-oriented languages. This is the case for CTS-compliant languages, which includes C#.
The problem is that this pattern is properly used in a domain model, not in a DAL like one implemented with EF. I'm not saying it's impossible, you may be able to hack things up with complex types or TPH constructs to avoid the creation of an intermediary table, but most likely you'll be swimming up the river until you give up. Hopefully someone can prove me wrong though.
Note: You can decide that you want your relational model to look different, in which case you may bypass this problem altogether. It wouldn't be an answer to your question though.
Using internal EF voodoo
I've rather quickly looked around the reference documentation for DbContext, ObjectContext and ObjectStateManager, and I can't immediately find any way to change the type of an entity. If you have better luck than me, you may be able to use DTOs and DbPropertyValues to do your conversion.
Important note
With the first two workarounds, you'll likely hit a bunch of problems with navigational properties and foreign keys (because of the DELETE+INSERT operation). This would be a separate question.
Conclusion
EF is not that flexible when you do anything non-trivial, but it keeps improving. Hopefully this answer won't be relevant in the future. It's also possible that I'm not aware of an existing killer-feature that would make what you want possible, so don't make any decisions based on this answer.
Using MVC4 with Entoty Framework CodeFirst I am having problems with the following scenario:
public class Survey
{
public QuestionCollection Questions {get;set;}
}
public class QuestionCollection : List<IQuestion> //Just for MVC
{ }
public class QuestionType1 : IQuestion { ... }
public class QuestionType2 : IQuestion { ... }
Seems straightforward. Now in my controller I want to get the Survey so i have:
DataContext context = new DataContext ();
var survey = context.Surveys.Include(s => s.Questions).SingleOrDefault(s => s.Id == id);
It does compile but runtime it gives me the error:
A specified Include path is not valid. The EntityType 'Survey' does not declare a navigation property with the name 'Questions'.
What am I doing wrong here?
Is there any good tutorial on this topic?
Entity Framework Code First requires that navigation collections be declared as an ICollection<T>. Also, to enable lazy loading of the associations, it should be virtual. This is because, unless otherwise specified, EF will return a proxy object wrapping your declared class. Since your original QuestionCollection property is a concrete implementation, it can't override that in the proxy and enable the navigation. Questions has to be declared as the interface.
Your concerns and requirements in EF are different than in MVC, and they aren't always compatible. If you really, really wanted QuestionCollection, you'll have to map that.
Your Surveys entity should look like this:
public class Survey
{
public virtual ICollection<Question> Questions { get; set; }
}
Also, EF can't implement entity types declared as an interface. This won't work: public virtual ICollection<IQuestion> - your individual question types must all inheirit from a common abstract or concrete instance. They can still implement the interface, but your entity type properties cannot be declared that way.
I would highly recommend going through this series of blog posts on inheritance structures in
EF.
The way you would set up your questions entities would look like this:
// You can still keep the IQuestion interface around for MVC
public abstract class Question : IQuestion
{
// ... properties ...
}
public class QuestionType1 : Question
{
// ... properties ...
}
public class QuestionType2 : Question
{
// ... properties ...
}
public class Survey
{
// Note, collection of Question, not the interface.
public virtual ICollection<Question> Questions { get; set; }
}
Depending on how exactly you want your table structure to look, the base Question class may or may not be abstract. Refer to the above blog posts to see the various options - Table per Type, Table per Hierarchy, Table per Concrete.
Have a look at this link,I think you need to set some auto policies http://forums.asp.net/t/1816051.aspx/1
I have a base class and two derived classes.
Each of the derived classes implements the same type as a property - the only difference is the property name.
Sadly I don't have much influence on the class design -> they have been generated from a wsdl file.
I then have a property on the BaseType to encapsulate the common property. The plan was to use this property in my web views etc.
I have used the famous "Fruit-Example" to demonstrate the problem:
public class FruitBase
{
public virtual int ID { get; set; }
//
// The plan is to use this property in mvc view
//
[NotMapped]
public virtual FruitnessFactor Fruitness
{
get
{
if (this.GetType().BaseType == typeof(Apple))
return ((Apple)this).AppleFruitness;
else if (this.GetType().BaseType == typeof(Orange))
return ((Orange)this).OrangeFruitness;
else
return null;
}
}
}
public class FruitnessFactor { }
In my MVC controller, the following query works absolutely fine:
return View(context.FruitEntities
.OfType<Apple>().Include(a =>a.AppleFruitness)
.ToList());
But this one doesn't:
return View(context.FruitEntities
.OfType<Apple>().Include(a =>a.AppleFruitness)
.OfType<Orange>().Include(o => o.OrangeFruitness)
.ToList());
The error message I get is:
DbOfTypeExpression requires an expression argument with a polymorphic result type that is compatible with the type argument.
I am using EF 5.0 RC and the Code First approach.
Any help is much appreciated!
As far as I can tell you can't apply Include on multiple subtypes in a single database query. You can query one type (OfType<Apple>().Include(a => a.AppelFruitness)) and the same for another subtype. The problem is that you can't concat the results in the same query because the result collections have different generic types (apples and oranges).
One option would be to run two queries and copy the result collection into a new collection of the base type - as you already indicated in the comment section under your question.
The other option (which would only need a single query) is a projection. You would have to define a projection type (you could also project into an anonymous type)...
public class FruitViewModel
{
public FruitBase Fruit { get; set; }
public FruitnessFactor Factor { get; set; }
}
...and then can use the query:
List<FruitViewModel> fruitViewModels = context.FruitEntities
.OfType<Apple>()
.Select(a => new FruitViewModel
{
Fruit = a,
Factor = a.AppleFruitness
})
.Concat(context.FruitEntities
.OfType<Orange>()
.Select(o => new FruitViewModel
{
Fruit = o,
Factor = o.OrangeFruitness
}))
.ToList();
If you don't disable change tracking (by using AsNoTracking) the navigation properties get populated automatically when the entities get attached to the context ("Relationship fixup") which means that you can extract the fruits from the viewModel collection...
IEnumerable<FruitBase> fruits = fruitViewModels.Select(fv => fv.Fruit);
...and you'll get the fruits including the FruitnessFactor properties.
This code is pretty awkward but a direct approach without using a projection has been asked for several times without success:
bottleneck using entity framework inheritance
Entity Framework - Eager loading of subclass related objects
How do I deeply eager load an entity with a reference to an instance of a persistent base type (Entity Framework 4)
I am using DBContext and have two classes whose properties are all virtual. I can see in the debugger that I am getting a proxy object when I query the context. However, a collection property is still null when I try to add to it. I thought that the proxy would ensure that collection is initialized.
Because my Poco object can be used outside of its data context, I added a check for the collection being null in the constructor and create it if necessary:
public class DanceStyle
{
public DanceStyle()
{
if (DanceEvents == null)
{
DanceEvents = new Collection<DanceEvent>();
}
}
...
public virtual ICollection<DanceEvent> DanceEvents { get; set; }
}
That works outside the data context but if I retrieve an object using a query, although the test is true, when I try to set it, I get following exception: 'The property 'DanceEvents' on type 'DanceStyle_B6089AE40D178593955F1328A70EAA3D8F0F01DDE9F9FBD615F60A34F9178B94' cannot be set because the collection is already set to an EntityCollection.'
I can see it is null and I cannot add to it, but neither can I set it to a collection because the proxy says it is already set. Therefore I cannot use it. I'm confused.
Here is the DanceEvent class:
public class DanceEvent
{
public DanceEvent()
{
if (DanceStyles == null)
{
DanceStyles = new Collection<DanceStyle>();
}
}
...
public virtual ICollection<DanceStyle> DanceStyles { get; set; }
}
I have omitted the other value-type properties from the code above. I have no other mappings for those classes in the context class.
As you correctly observed in the answer to your own question, removing the "virtual" keyword from the collection properties works around the problem, by preventing the Entity Framework from creating a change tracking proxy. However, this is not a solution for many people, because change tracking proxies can be really convenient and can help prevent issues when you forget to detect changes at the right places in your code.
A better approach would be to modify your POCO classes, so that they instantiate the collection properties in their get accessor, rather than in the constructor. Here's your POCO class, modified to allow change tracking proxy creation:
public class DanceEvent
{
private ICollection<DanceStyle> _danceStyles;
public virtual ICollection<DanceStyle> DanceStyles
{
get { return _danceStyles ?? (_danceStyles = new Collection<DanceStyle>()); }
protected set { _danceStyles = value; }
}
}
In the above code the collection property is no longer automatic, but rather has a backing field. It's better if you leave the setter protected, preventing any code (other than the proxy) from subsequently modifying these properties. You will notice that the constructor was no longer necessary and was removed.
I found the solution to this problem here: Code First adding to collections? How to use Code First with repositories?
I removed 'virtual' from all properties except collections and lazy loaded objects, that is, all native types.
But I still don't understand how you can end up with the situation where you have a null collection that you cannot use and have no way to set it to a valid collection.
I also found this answer from Rowan Miller on an MSDN forum
Hi,
If you make all your properties virtual then EF will generate proxy classes at runtime that derives from your POCO classed, these proxies allow EF to find out about changes in real time rather than having to capture the original values of your object and then scan for changes when you save (this is obviously has performance and memory usage benefits but the difference will be negligible unless you have a large number of entities loaded into memory). These are known as 'change tracking proxies', if you make your navigation properties virtual then a proxy is still generated but it is much simpler and just includes some logic to perform lazy loading when you access a navigation property.
Because your original code was generating change tracking proxies, EF was replacing your collection property with a special collection type to help it find out about changes. Because you try and set the collection back to a simple list in the constructor you are getting the exception.
Unless you are seeing performance issues I would follow Terrence's suggestion and just remove 'virtual' from your non-navigation properties.
~Rowan
So it appears that I only have the problem with a full 'change tracking proxy' if all my properties are virtual. But given that, why can I still not use the virtual property on the change tracking proxy? This code blows up on line three because ds2.DanceEvents is null and cannot be set in the constructor:
DanceStyle ds2 = ctx.DanceStyles.Where(ds => ds.DanceStyleId == 1).Single();
DanceEvent evt = CreateDanceEvent();
ds2.DanceEvents.Add(evt);
I'm still confused, even though my code is now working because of the fix above.
Old question...
Poco class:
public partial class MyPOCO
{
public MyPOCO()
{
this.MyPocoSub = new HashSet<MyPocoSub>();
}
//VIRTUAL
public virtual ICollection<MyPocoSub> MyPocoSub { get; set; }
}
and proxy code:
public override ICollection<MyPocoSubSet> MyPocoSubSets
{
get
{
ICollection<MyPocoSubSet> myPocoSubSets = base.MyPocoSubSets;
if (!this.ef_proxy_interceptorForMyPocoSubSets(this, myPocoSubSets))
{
return base.MyPocoSubSets;
}
return myPocoSubSets;
}
set
{
if (value != this.RelationshipManager.GetRelatedEnd("WindowsFormsApplication.Models.MyPocoSubSet_MyPOCO", "MyPocoSubSet_MyPOCO_Source"))
{
// EXCEPTION
throw new InvalidOperationException("The property 'MyPocoSubSets' on type 'MyPOCO_A78FCE6C6A890855C68B368B750864E3136B589F9023C7B1D90BF7C83FD291AC' cannot be set because the collection is already set to an EntityCollection.");
}
base.MyPocoSubSets = value;
}
}
As you can see that exception raised in proxy class in ExtityFramework 5. This means that behavior still exist.