For specific context, I develop in C#, but I suppose this could be applied to other languages/frameworks.
I understand that creating a property with a private getter and a public setter is perfectly legal:
public int MyInt { private get; set; }
I'm having trouble figuring out why one would want to allow a client to set a property without being able get its current value. Under what circumstances might this type of behavior desired?
It's not the 'client' which changes the value, it's other parts of the program.
It's possible that you want other objects have the ability to change the state of an object, via that property.
The other parts of the object don't care for the state of that object however, it's kept for internal logic only.
It all depends on the case and the structure of your application.
Related
A similar question has been answered here:
How can I pass a runtime parameter as part of the dependency resolution?
However, I was wondering how this can be done when registering a generic class?
Normally, I would register it as following:
services.AddScoped(typeof(ITest<>), typeof(Test<>));
But what if I want to pass a runtime parameter to constructor? Without using DI, it would be something like:
new Test<MyClass>(string mystring, int myInt)
In the linked answer it's suggests using a factory method but this is giving me an error if I don't pass it the exact type.
The alternative would be to get an instance without passing a runtime parameter in the constructor and instead using a setter method after getting exact instance. I would like to avoid this however because every time after getting instance you must remember to call setter method.
Is there some way around it? I guess I could use some factory class instead of registering it in startup class...
EDIT:
After reading Steven's answer which was very useful, I updated question with more concrete example:
Following example is inside some method:
//instance of repository are passed inside constructor of class
//calling some to update/insert
//IMPORTANT - calling external service I want save parameters to db no matter what
using(var ctx=new DbContext())
{
//create log object
ctx.logs.add(Obj)
ctx.save()
}
//some code after
Let's say I want to be consistent and call method of my loggingrepository and there add logging object and save everything to database
However, every repository in constructor accepts DbContext, which is registered as scoped (durig one request).
If it's inside transaction, saving depends about code after calling external service and it can throw exception and save nothing.
So yeah, I could create new dbContext and pass it in logging method or call some private logging function and save inside it,
but point is that if I would ask for instance of loggingRepository I would want DI to pass this localy created dbContext variable to constructor
and not one registered as scoped inside startup method, so that addind and saving log happens no matter what external service or code after calling it does.
My situation in something similar, but it's going for some data in db based on current user and I don't wanna pass same parameter to numerous method, but only inside class constructor.
The general solution in injecting primitive configuration values into your application components, is to extract them into a Parameter Object. This gives those values a new, unambiguous type, which can be registered into your container:
// Parameter Object
public TestConfiguration
{
public string Mystring;
public int MyInt;
}
// (Generic) class using the Parameter Object
public class Test<T>
{
public Test(TestConfiguration config) { ... }
}
// Registering both
services.AddScoped(typeof(ITest<>), typeof(Test<>));
services.AddSingleton(new TestConfiguration { Mystring = ..., Myint = ... });
Configuration values are not considered to be runtime data as their values are known at startup and constant for the duration of the application. That's why you can supply them to the constructors of your application components.
Real runtime data, however, should not be passed on to a component during construction. Runtime data are values that are not known at startup and typically passed along by the user through a web request, are retrieved from the database, session, or anything that can change during the lifetime of the application.
Instead of passing runtime data in through the constructor, you should either:
Pass runtime data through method calls of the API or
Retrieve runtime data from specific abstractions that allow resolving runtime data.
You can find more information about passing runtime data here.
Following advices from people on the internet about service references, I got rid of them now and split the service/data contracts into a common assembly accesible by both the server and the client. Overall this seems to work really well.
However I’m running into problems when trying to use custom objects, or rather custom subtypes, in the service. Initially I wanted to define only interfaces in the common assembly as the contract for the data. I quickly learned that this won’t work though because the client needs a concrete class to instantiate objects when receiving objects from the service. So instead I used a simple class instead, basically like this:
// (defined in the common assembly)
public class TestObject
{
public string Value { get; set; }
}
Then in the service contract (interface), I have a method that returns such an object.
Now if I simply create such an object in the service implementation and return it, it works just fine. However I want to define a subtype of it in the service (or the underlying business logic), that defines a few more things (for example methods for database access, or just some methods that work on the objects).
So for simplicity, the subtype looks like this:
// (defined on the server)
public class DbTestObject : TestObject
{
public string Value { get; set; }
public DbTestObject(string val)
{
Value = val;
}
}
And in the service, instead of creating a TestObject, I create the subtype and return it:
public TestObject GetTestObject()
{
return new DbTestObject("foobar");
}
If I run this now, and make the client call GetTestObject, then I immediately get a CommunicationException with the following error text: “The socket connection was aborted. This could be caused by an error processing your message or a receive timeout being exceeded by the remote host, or an underlying network resource issue. Local socket timeout was '00:09:59.9380000'.”
I already found out, that the reason for this is that the client does not know how to deserialize the DbTestObject. One solution would be to declare the base type with the KnownTypeAttribute to make it know about the subtype. But that would require the subtype to be moved into the common assembly, which is of course something I want to avoid, as I want the logic separated from the client.
Is there a way to tell the client to only use the TestObject type for deserialization; or would the solution for this be to use data transfer objects anyway?
As #Sixto Saez has pointed out, inheritance and WCF don't tend to go together very well. The reason is that inheritance belongs very much to the OO world and not the messaging passing world.
Having said that, if you are in control of both ends of the service, KnownType permits you to escape the constraints of message passing and leverage the benefits of inheritance. To avoid taking the dependency you can utilise the ability of the KnownTypeAttribute to take a method name, rather than a type parameter. This allows you to dynamically specify the known types at run time.
E.g.
[KnownType("GetKnownTestObjects")]
[DataContract]
public class TestObject
{
[DataMember]
public string Value { get; set; }
public static IEnumerable<Type> GetKnownTestObjects()
{
return Registry.GetKnown<TestObject>();
}
}
Using this technique, you can effectively invert the dependency.
Registry is a simple class that allows other assemblies to register types at run-time as being subtypes of the specified base class. This task can be performed when the application bootstraps itself and if you wish can be done, for instance, by reflecting across the types in the assembly(ies) containing your subtypes.
This achieves your goal of allowing subtypes to be handled correctly without the TestObject assembly needing to take a reference on the subtype assembly(ies).
I have used this technique successfully in 'closed loop' applications where both the client and server are controlled. You should note that this technique is a little slower because calls to your GetKnownTestObjects method have to be made repeatedly at both ends while serialising/deserialising. However, if you're prepared to live with this slight downside it is a fairly clean way of providing generic web services using WCF. It also eliminates the need for all those 'KnownTypeAttributes' specifying actual types.
Im am not new to WCF web services but there has been a couple of years since the last time I used one. I am certain that last time I used a WCF service you could determine the type of object returned from a service call when developing the code. EG;
MyService.Models.ServiceSideObjects.User user = myServiceClient.GetUser();
You were then free to use the 'user' object client-side. However now it seems as if the WCF service will not return anything more than objects containing basic value types (string, int ect). So far I have remedied this by defining transfer objects which contain only these basic value types and having the service map the complex 'User' objects properties to simple strings and int's in the transfer object.
This becomes a real pain when, for example you have custom type objects containing more complex objects such as my Ticket object.
public class Ticket
{
public Agent TicketAgent {get;set;}
public Client Owner {get;set;}
public PendingReason TicketPendingReason {get;set;}
}
As simply mapping this object graph to a single transfer class with a huge list of inter-related system-typed properties gives a very 'dirty' client-side business model. Am I wrong in thinking that I SHOULD be able to just receive my Ticket object from a service method call and deal with it client side in the same state it was server-side ?
I realise this is probably a violation of some SoA principal or similar but my desktop app currently consuming this service is the ONLY thing that will consume ever consume it. So i do not care if many other clients will be able to manage the data types coming back from the service and therefore require some hugely normalised return object. I just want my service to get an object of type Ticket from its repository, return this object to the client with all its properties intact. Currently all I get is an object with a single property 'ExtentionData' which is unusable client-side.
Hope this makes sense, thank you for your time.
I might've missed a memo, but I think you need to decorate your model classes with DataContractAttribute and your properties with DataMemberAttribute, like so:
[DataContract( Namespace = "http://example.com" )]
public class Ticket
{
[DataMember]
public Agent TicketAgent { get; set; }
[DataMember]
public Client Owner { get; set; }
[DataMember]
public PendingReason TicketPendingReason { get; set; }
}
This is why you probably want to set up a DTO layer, to avoid polluting your model classes.
As for ExtensionData, it's used for forward-compatibility: http://msdn.microsoft.com/en-us/library/ms731083.aspx
I have marked Niklas's response as an answer as it has solved my issue.
While it seems you do not NEED to use [DataContract] and [DataMember], in some cases, I believe it could cause the issues I was experiencing. When simply transferring custom typed objects which, in themselves, only have simply typed properties, no attributes needed. However, when I attempted to transfer a custom typed object which itself had collections / fields of more custom typed objects there attributes were needed.
Thank you for your time.
NHibernate requires not only settable properties of your domain to be virtual but also get-only properties and methods. Does anyone know what the reason for this is?
I cannot imagine a possible use.
The reason is lazy loading. In order to make lazy loading possible, a proxy class is created. It must intercept every call from "outside" in order to load your entity before actual method/property is executed. If some methods/properties were not virtual it would be impossible to intercept these calls and entity wouldn't be loaded.
I'm not an NHibernate expert, but from reading Oren's blogs, and from what I've learned of NH, the basic pattern of use is to proxy the objects for the ORM. This means, among other things, that the only things you'll be able to map are going to be things that are made virtual, otherwise NH would have to use a different strategy to redefine the implementations under the hood.
Because you may want to access your settable properties from there, and maybe in some fancy indirect or reflection way. So it's to be 100% sure that when your entity is used, it's initialized.
Example:
public string GetSmth
{
get
{
// NHibernate will not know that you access this field.
return _name;
}
}
private string _name;
public virtual string Name { get { return _name; } set { _name = value; } }
Here's Ayende explaing this in relation to Entity Framework:
http://ayende.com/Blog/archive/2009/05/29/why-defer-loading-in-entity-framework-isnrsquot-going-to-work.aspx
AddProduct is a non virtual method
call, so it cannot be intercepted.
Accessing the _products field also
cannot be intercepted.
The only reason I see why one would want method execution without messing with NH proxy (i.e. loading data) is when you have method that do not access your class' data. But in this case, if this method does not use your class' data, what does it belong to that class at all?
I have two .NET 3.5 WCF services build with VS2008.
I have two WCF clients in Silverlight to consume these services. The clients are generated with the 'Add Service Reference'. I am using Silverlight 4.
ONE of the proxies is generated with Specified properties for each property. This is a 'message-in' class for my service method :
// properties are generated for each of these fields
private long customerProfileIdField;
private bool customerProfileIdFieldSpecified;
private bool testEnvField;
private bool testEnvFieldSpecified;
Now my other service (still with a Silverlight client) does NOT generate Specified properties.
Now I don't care about 'tenets of good SOA'. I just want to get rid of these damn properties because in the context of what I'm doing I absolutely hate them.
There has to be some difference between the two services - but I don't want to have to completely rip them apart to find out the difference.
A similar question before had the answer 'you cant do it' - which is definitely not true because I have it - I just don't know what I did differently.
Edit: I am now in a situation where I regenerate my Silverlight 4 proxy to my 3.5 WCF service (all on the same localhost machine) that sometimes I get 'Specified' properties and sometimes I don't. I no longer think (as I suspected originally) that this is due solely to some endpoint configuration or service level [attribute]. Theres certain triggers in the message itself that cause Specified to be generated (or not). There may be many factors involved or it may be something very simple.
try this in your WCF service where the property is declared
[DataMember(IsRequired=true)]
public bool testEnvField { get; set; }
IsRequired=true will negate the need for the testEnvFieldSpecified property
These extra Specified properties are generated for value types which are being specified as optional in either the contract or the attribute markup.
As value types have a value by default, the extra Specified flags are being added for these properties, to allow the client (and server) to distinguish between something explicitly not specified or explicitly specified - which may well be set to the default value. Without it, integers would always end up being 0 (and being serialized) even if you don't set them (because of the mapping to int) in your client code. So when you do, you need to also make sure that you set the Specified flag to true, otherwise these properties will not get serialized.
So to prevent these flags being generated for value types, you would have to change the contract to make these value type properties mandatory, instead of optional.
Hope that makes sense.
OK I've found one thing so far that will cause Specified properties to be generated:
The presence of an XTypedElement in the message.
These are used by Linq2XSD. I was returning an element from a Linq2XSD model.
This triggered Specified properties to be generated EVERYTHING in all my classes :
public XTypedElement Foo { get; set; }
This however didn't :
public XElement Foo { get; set; }
Still curious as to why this is, and if there are any other things that trigger this.
NOTE: I realize this is an old question. I'm adding this here because this question comes up as a top result on Google, and it's helpful information for whoever comes looking.
Try adding this line into your operation contract declaration:
[XmlSerializerFormat]
It should look something like this:
namespace WebServiceContract
{
[ServiceContract(Namespace = "http://namespace")]
[XmlSerializerFormat] //This line here will cause it to serialize the "optional" parameters correctly, and not generate the extra
interface InterfaceName
{
/*...Your web service stuff here...*/
}
}
I found that if I put a DataTable in a service DataContract then the generated client will use xml serializer and thus generate the *IsSpecified members.