I am having issue with the code, I have my page layout as below.
I am communicating to database to get data for Main Content. It is List<SomeClass> that I am getting from database. Now I want same List<SomeClass> to be available for RightContent. Both components are custom and have different layout but can share same List rather than making same call twice. (Sequence is MainContent Initialized() method gets called first)
I created a service class AppDataService with below property. Also added to IServiceCollection services in startup.
public List<SomeClass> sharedListOfSomeClass = new List<SomeClass>();
In MainContent I am injecting AppDataService and assigning sharedListOfSomeClass with database values.
Now if I am injecting AppDataService in Right Content and and trying to access sharedListOfSomeClass I am getting it as null.
I know I am missing binding here because both the components are different in terms of html and can't bind it to any html tags.
Could any one please help me out to achieve this. I want to make single call to database for both the components.
If you want to have some global state of the app and share it between different components, the most reasonable way to do it is to create a State class that will contain the global state data
public class State
{
public List<SomeClass> SomeClassObjectsCollection { get; set; } = new List<SomeClass>();
}
In your Startup (or Program if you use Blazor wasm) you should add a State object as a singleton
services.AddSingleton<State>()
and on every page, where you need an access to state (or even in _Imports if you want to access it often) add
#inject State State
After that on any page you can refer to State.SomeClassObjectsCollection and get the same data.
The key point is adding a state as a singleton. If you will add is as transient or even scoped, the dependency container will create new instances of State.
One option is to pass the list to the components as parameter. Define a parameter in the component's code.
[Parameter] public List<SomeClass> sharedListOfSomeClass { get; set; }
In the parent pass the set the parameter:
<MyCustomComponent sharedListOfSomeClass="#MyVariableHoldingTheListValues" />
Other way I can think of is to make a static list and reference the static list in the components.
The scenario of the injection gives you null because the service could be registered as transient or scoped servervice. Not as singleton.
Related
I have a Dictionary that will be populated with data from the database at startup, with a method that takes the key as a parameter, and returns the value. How to make the dictionary publicly accessible to all controllers? After searching, I learned that I would need to use Dependency Injection, but I'm failing at implementing it. Any resource that can get me on track is highly appreciated.
There are many ways to implement your question with/without DI. One of which is to write a static class that will be filled upon app startup.
No dependency injection:
Declare a static class that contains your dictionary. By being static there would only be 1 instance on app start.
public static class StaticDictionary {
public Dictionary<string,int> MyDictionary {get;set;}
}
In your Startup.cs - Configure method, append your db context in the parameters.
public void Configure(..., YourDbContext dbContext)
In the Configure method again, append your code that fills the dictionary.
public void Configure(..., YourDbContext dbContext){
...
// no need to modify the code above this, just append the fill dictionary code
foreach(var item in dbContext.TableName.ToList()){
StaticDictionary.MyDictionary.Add(...);
}
}
In your controllers, you could access StaticDictionary without DI.
public IActionResult Index{
var something = StaticDictionary.MyDictionary["Something"];
return View();
}
I had seen some books(e.g programming entity framework code first Julia Lerman) define their domain classes (POCO) with no initialization of the navigation properties like:
public class User
{
public int Id { get; set; }
public string UserName { get; set; }
public virtual ICollection<Address> Address { get; set; }
public virtual License License { get; set; }
}
some other books or tools (e.g Entity Framework Power Tools) when generates POCOs initializes the navigation properties of the the class, like:
public class User
{
public User()
{
this.Addresses = new IList<Address>();
this.License = new License();
}
public int Id { get; set; }
public string UserName { get; set; }
public virtual ICollection<Address> Addresses { get; set; }
public virtual License License { get; set; }
}
Q1: Which one is better? why? Pros and Cons?
Edit:
public class License
{
public License()
{
this.User = new User();
}
public int Id { get; set; }
public string Key { get; set; }
public DateTime Expirtion { get; set; }
public virtual User User { get; set; }
}
Q2: In second approach there would be stack overflow if the `License` class has a reference to `User` class too. It means we should have one-way reference.(?) How we should decide which one of the navigation properties should be removed?
Collections: It doesn't matter.
There is a distinct difference between collections and references as navigation properties. A reference is an entity. A collections contains entities. This means that initializing a collection is meaningless in terms of business logic: it does not define an association between entities. Setting a reference does.
So it's purely a matter of preference whether or not, or how, you initialize embedded lists.
As for the "how", some people prefer lazy initialization:
private ICollection<Address> _addresses;
public virtual ICollection<Address> Addresses
{
get { return this._addresses ?? (this._addresses = new HashSet<Address>());
}
It prevents null reference exceptions, so it facilitates unit testing and manipulating the collection, but it also prevents unnecessary initialization. The latter may make a difference when a class has relatively many collections. The downside is that it takes relatively much plumbing, esp. when compared to auto properties without initialization. Also, the advent of the null-propagation operator in C# has made it less urgent to initialize collection properties.
...unless explicit loading is applied
The only thing is that initializing collections makes it hard to check whether or not a collection was loaded by Entity Framework. If a collection is initialized, a statement like...
var users = context.Users.ToList();
...will create User objects having empty, not-null Addresses collections (lazy loading aside). Checking whether the collection is loaded requires code like...
var user = users.First();
var isLoaded = context.Entry(user).Collection(c => c.Addresses).IsLoaded;
If the collection is not initialized a simple null check will do. So when selective explicit loading is an important part of your coding practice, i.e. ...
if (/*check collection isn't loaded*/)
context.Entry(user).Collection(c => c.Addresses).Load();
...it may be more convenient not to initialize collection properties.
Reference properties: Don't
Reference properties are entities, so assigning an empty object to them is meaningful.
Worse, if you initiate them in the constructor, EF won't overwrite them when materializing your object or by lazy loading. They will always have their initial values until you actively replace them. Worse still, you may even end up saving empty entities in the database!
And there's another effect: relationship fixup won't occcur. Relationship fixup is the process by which EF connects all entities in the context by their navigation properties. When a User and a Licence are loaded separately, still User.License will be populated and vice versa. Unless of course, if License was initialized in the constructor. This is also true for 1:n associations. If Address would initialize a User in its constructor, User.Addresses would not be populated!
Entity Framework core
Relationship fixup in Entity Framework core (2.1 at the time of writing) isn't affected by initialized reference navigation properties in constructors. That is, when users and addresses are pulled from the database separately, the navigation properties are populated.
However, lazy loading does not overwrite initialized reference navigation properties.
In EF-core 3, initializing a reference navigation property prevents Include from working properly.
So, in conclusion, also in EF-core, initializing reference navigation properties in constructors may cause trouble. Don't do it. It doesn't make sense anyway.
In all my projects I follow the rule - "Collections should not be null. They are either empty or have values."
First example is possible to have when creation of these entities is responsibility of third-part code (e.g. ORM) and you are working on a short-time project.
Second example is better, since
you are sure that entity has all properties set
you avoid silly NullReferenceException
you make consumers of your code happier
People, who practice Domain-Driven Design, expose collections as read-only and avoid setters on them. (see What is the best practice for readonly lists in NHibernate)
Q1: Which one is better? why? Pros and Cons?
It is better to expose not-null colections since you avoid additional checks in your code (e.g. Addresses). It is a good contract to have in your codebase. But it os OK for me to expose nullable reference to single entity (e.g. License)
Q2: In second approach there would be stack overflow if the License class has a reference to User class too. It means we should have one-way reference.(?) How we should decide which one of the navigation properties should be removed?
When I developed data mapper pattern by myself I tryed to avoid bidirectional references and had reference from child to parent very rarely.
When I use ORMs it is easy to have bidirectional references.
When it is needed to build test-entity for my unit-tests with bidirectional reference set I follow the following steps:
I build parent entity with emty children collection.
Then I add evey child with reference to parent entity into children collection.
Insted of having parameterless constructor in License type I would make user property required.
public class License
{
public License(User user)
{
this.User = user;
}
public int Id { get; set; }
public string Key { get; set; }
public DateTime Expirtion { get; set; }
public virtual User User { get; set; }
}
It's redundant to new the list, since your POCO is depending on Lazy Loading.
Lazy loading is the process whereby an entity or collection of entities is automatically loaded from the database the first time that a property referring to the entity/entities is accessed. When using POCO entity types, lazy loading is achieved by creating instances of derived proxy types and then overriding virtual properties to add the loading hook.
If you would remove the virtual modifier, then you would turn off lazy loading, and in that case your code no longer would work (because nothing would initialize the list).
Note that Lazy Loading is a feature supported by entity framework, if you create the class outside the context of a DbContext, then the depending code would obviously suffer from a NullReferenceException
HTH
The other answers fully answer the question, but I'd like to add something since this question is still relevant and comes up in google searches.
When you use the "code first model from database" wizard in Visual Studio all collections are initialized like so:
public partial class SomeEntity
{
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
public SomeEntity()
{
OtherEntities = new HashSet<OtherEntity>();
}
public int Id { get; set; }
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
public virtual ICollection<OtherEntity> OtherEntities { get; set; }
}
I tend to take wizard output as basically being an official recommendation from Microsoft, hence why I'm adding to this five-year-old question. Therefore, I'd initialize all collections as HashSets.
And personally, I think it'd be pretty slick to tweak the above to take advantage of C# 6.0's auto-property initializers:
public virtual ICollection<OtherEntity> OtherEntities { get; set; } = new HashSet<OtherEntity>();
Q1: Which one is better? why? Pros and Cons?
The second variant when virtual properties are set inside an entity constructor has a definite problem which is called "Virtual member call in a constructor".
As for the first variant with no initialization of navigation properties, there are 2 situations depending on who / what creates an object:
Entity framework creates an object
Code consumer creates an object
The first variant is perfectly valid when Entity Framework creates a object,
but can fail when a code consumer creates an object.
The solution to ensure a code consumer always creates a valid object is to use a static factory method:
Make default constructor protected. Entity Framework is fine to work with protected constructors.
Add a static factory method that creates an empty object, e.g. a User object, sets all properties, e.g. Addresses and License, after creation and returns a fully constructed User object
This way Entity Framework uses a protected default constructor to create a valid object from data obtained from some data source and code consumer uses a static factory method to create a valid object.
I use the answer from this Why is my Entity Framework Code First proxy collection null and why can't I set it?
Had problems with constructor initilization. Only reason I do this is to make test code easier. Making sure collection is never null saves me constantly initialising in tests etc
Hi I have a problem with the structure of my code, it somehow goes into Circular Dependency. Here is an explanation of how my code looks like:
I have a ProjectA contains BaseProcessor and BaseProcessor has a reference to a class called Structure in ProjectB. Inside BaseProcessor, there is an instance of Structure as a variable.
In projectB there are someother classes such as Pricing, Transaction etc.
Every class in ProjectB has a base class called BaseStructure i.e. Structure, Pricing and Transaction classes all inherited from BaseStructure.
Now in Pricing and Transaction classes, I want to call a method in BaseProcessor class from BaseStructure class which causing Circular Dependency.
What I have tried is:
Using Unity, but I didn't figure out how to make it work because I try to use function like:
unityContainer.ReferenceType(IBaseProcessor, BaseProcessor)
in BaseStructure then it will need a reference of BaseProcessor which also cause Circular Dependency.
And I've also tried creating an interface of IBaseProcessor and create a function(the function I want to call) declaration in this interface. And let both BaseProcessor and BaseStructure inherit this interface. But how can I call the function in Pricing and Transaction class without create an instance of BaseProcessor?
Can anyone please tell me how to resolve this problem other than using reflection?
Any help will be much appreciated. Thanks :)
You could use the lazy resolution:
public class Pricing {
private Lazy<BaseProcessor> proc;
public Pricing(Lazy<BaseProcessor> proc) {
this.proc = proc;
}
void Foo() {
this.proc.Value.DoSomethin();
}
}
Note that you haven't to register the Lazy because Unity will resolve it by BaseProcessor registration.
Your DI container can't help solving the circular reference, since it is the dependency structure of the application that prevents objects from being created. Even without a DI container, you can't construct your object graphs without some special 'tricks'.
Do note that in most cases cyclic dependency graphs are a sign of a design flaw in your application, so you might want to consider taking a very close look at your design and see if this can't be solved by extracting logic into separate classes.
But if this is not an option, there are basically two ways of resolving this cyclic dependency graph. Either you need to fallback to property injection, or need to postpone resolving the component with a factory, Func<T>, or like #onof proposed with a Lazy<T>.
Within these two flavors, there are a lot of possible ways to do this, for instance by falling back to property injection into your application (excuse my C#):
public class BaseStructure {
public BaseStructure(IDependency d1) { ... }
// Break the dependency cycle using a property
public IBaseProcessor Processor { get; set; }
}
This moves the IBaseProcessor dependency from the constructor to a property and allows you to set it after the graph is constructed. Here's an example of an object graph that is built manually:
var structure = new Structure(new SomeDependency());
var processor = new BaseProcessor(structure);
// Set the property after the graph has been constructed.
structure.Processor = processor;
A better option is to hide the property inside your Composition Root. This makes your application design cleaner, since you can keep using constructor injection. Example:
public class BaseStructure {
// vanilla constructor injection here
public BaseStructure(IDependency d1, IBaseProcessor processor) { ... }
}
// Defined inside your Composition Root.
private class CyclicDependencyBreakingProcessor : IBaseProcessor {
public IBaseProcessor WrappedProcessor { get; set; }
void IBaseProcessor.TheMethod() {
// forward the call to the real processor.
this.WrappedProcessor.TheMethod();
}
}
Now instead of injecting the BaseProcessor into your Structure, you inject the CyclicDependencyBreakingProcessor, which will be further initialized after the construction of the graph:
var cyclicBreaker = new CyclicDependencyBreakingProcessor();
var processor = new BaseProcessor(new Structure(new SomeDependency(), cyclicBreaker));
// Set the property after the graph has been constructed.
cyclicBreaker.WrappedProcessor = processor;
This is basically the same as before, but now the application stays oblivious from the fact that there is a cyclic dependency that needed to be broken.
Instead of using property injection, you can also use Lazy<T>, but just as with the property injection, it is best to hide this implementation detail inside your Composition Root, and don't let Lazy<T> values leak into your application, since this just adds noise to your application, which makes your code more complex and harder to test. Besides, the application shouldn't care that the dependency injection is delayed. Just as with Func<T> (and IEnumerable<T>), when injecting a Lazy<T> the dependency is defined with a particular implementation in mind and we're leaking implementation details. So it's better to do the following:
public class BaseStructure {
// vanilla constructor injection here
public BaseStructure(IDependency d1, IBaseProcessor processor) { ... }
}
// Defined inside your Composition Root.
public class CyclicDependencyBreakingProcessor : IBaseProcessor {
public CyclicDependencyBreakingBaseProcessor(Lazy<IBaseProcessor> processor) {...}
void IBaseProcessor.TheMethod() {
this.processor.Value.TheMethod();
}
}
With the following wiring:
IBaseProcessor value = null;
var cyclicBreaker = new CyclicDependencyBreakingProcessor(
new Lazy<IBaseProcessor>(() => value));
var processor = new BaseProcessor(new Structure(new SomeDependency(), cyclicBreaker));
// Set the value after the graph has been constructed.
value = processor;
Up until now I only showed how to build up the object graph manually. When doing this using a DI container, you usually want to let the DI container build up the complete graph for you, since this yields a more maintainable Composition Root. But this can make it a bit more tricky to break the cyclic dependencies. In most cases the trick is to register the component that you want to break with a caching lifestyle (basically anything else than transient). Per Web Request Lifestyle for instance. This allows you to get the same instance in a lazy fashion.
Using the last CyclicDependencyBreakingProcessor example, we can create the following Unity registration:
container.Register<BaseProcessor>(new PerRequestLifetimeManager());
container.RegisterType<IStructure, Structure>();
container.RegisterType<IDependency, SomeDependenc>();
container.Register<IBaseProcessor>(new InjectionFactory(c =>
new CyclicDependencyBreakingProcessor(
new Lazy<IBaseProcessor>(() => c.GetInstance<BaseProcessor>())));
I've been modeling a domain for a couple of days now and not been thinking at all at persistance but instead focusing on domain logic. Now I'm ready to persist my domain objects, some of which contains IEnumerable of child entities. Using RavenDb, the persistance is 'easy', but when loading my objects back again, all of the IEnumerables are empty.
I've realized this is because they don't have any property setters at all, but instead uses a list as a backing field. The user of the domain aggregate root can add child entities through a public method and not directly on the collection.
private readonly List<VeryImportantPart> _veryImportantParts;
public IEnumerable<VeryImportantPart> VeryImportantParts { get { return _veryImportantParts; } }
And the method for adding, nothing fancy...
public void AddVeryImportantPart(VeryImportantPart part)
{
// some logic...
_veryImportantParts.Add(part);
}
I can fix this by adding a private/protected setter on all my IEnumerables with backing fields but it looks... well... not super sexy.
private List<VeryImportantPart> _veryImportantParts;
public IEnumerable<VeryImportantPart> VeryImportantParts
{
get { return _veryImportantParts; }
protected set { _veryImportantParts = value.ToList(); }
}
Now the RavenDb json serializer will populate my objects on load again, but I'm curious if there isn't a cleaner way of doing this?
I've been fiddeling with the JsonContractResolver but haven't found a solution yet...
I think I've found the root cause of this issue and it's probably due to the fact that many of my entities were created using:
protected MyClass(Guid id, string name, string description) : this()
{ .... }
public static MyClass Create(string name, string description)
{
return new MyClass(Guid.NewGuid(), name, description);
}
When deserializing, RavenDb/Json.net couldn't rebuild my entities in a proper way...
Changing to using a public constructor made all the difference.
Do you need to keep a private backing field? Often an automatic property will do.
public IList<VeryImportantPart> VeryImportantParts { get; protected set; }
When doing so, you may want to initialize your list in the constructor:
VeryImportantParts = new List<VeryImportantPart>();
This is optional, of course, but it allows you to create a new class and start adding to the list right away, before it is persisted. When Raven deserializes a class, it will use the setter to overwrite the default blank list, so this just helps with the first store.
You certainly won't be able to use a readonly field, as it couldn't be replaced during deserialization. It might be possible to write a contract resolver or converter that fills an existing list rather than creating a new one, but that seems like a rather complex solution.
Using an automatic property can add clarity to your code anyway - as it is less confusing whether to use the field or the property.
I am developing a Windows Phone 7 app and am using the MVVM pattern. I have a need to pass a parameter to the contructor of the ViewModel for a page. All my datacontexts and binding are done in XAML. Through my research I've seen that I need to do so using a dependency injector such as NInject.
Here's a little detail on whats going on:
I have a page with a ListPicker that lists various tasks. Each task has a unique TaskID. When an item is selected I need to open another page that will show the selected Tasks detail. My ViewModel and binding is all done and works if I use a static TaskID in the ViewModel but of course I need to use a variable.
I've setup NInject in the project and the various classes needed such as ViewModelLocator and my NInjectModule as shown here:
public class LighthouseNInjectModule : NinjectModule
{
public override void Load()
{
this.Bind<TaskViewModel>().ToSelf().WithConstructorArgument("TaskID", 2690);
}
}
Note that I have hardcoded a TaskID here and using this code this value properly gets injected into my constructor. Of course, this is hardcoded and I need to get the TaskID for the selected ListPicker item. I know how to get the selected ID from the ListPicker but how do I make NInject aware of it so when my class constructor is run it will have the correct value?
Here is the basic definition of my ViewModel class showing use of the Injector attribute.
public class TaskViewModel : INotifyPropertyChanged
{
[Inject]
public TaskViewModel(int TaskID)
{
//run function to get data using TaskID
}
}
WithConstructorArgument has another oveload that accepts a lazy evaluated Func<Context, object>.