For one week I'm getting the below type missmatch error. I search through the internet, looked at how to use generics, but I couldn't find what I'm doing wrong. could anyone please tell me how to fix this problem
static void Main(string[] args) {
JSonHttpClient httpClient;
// ....
public ListAlertsResponse ListAlerts(ListAlertsRequest listAlertsRequest) {
//HERE COUSES THE ERROR !!!
return (ListAlertsResponse)httpClient.DoGetRequest(listAlertsRequest);
}
}
error:
Error 5 Argument 1:
cannot convert from 'ListAlertsRequest' to BaseRequest<BaseResponse>'
My classes and interfaces
public class JsonHttpClient {
public BaseResponse DoGetRequest(BaseRequest<BaseResponse> request) {
return new BaseResponse(...) }
}
public interface Request {}
public interface Response {}
public abstract class BaseResponse : Response {}
public abstract class BaseRequest<T> : Request where T : BaseResponse {}
public class ListAlertsResponse : BaseResponse {}
public class ListAlertsRequest : BaseRequest<ListAlertsResponse> {}
You're sending a type ListAlertsRequest to DoGetRequest which needs a parameter of type BaseRequest<BaseResponse>. ListAlertsRequest is not of type BaseRequest<BaseResponse>
listAlertsRequest is not a subclass of BaseRequest
public class JsonHttpClient
{
public BaseResponse DoGetRequest<T>(BaseRequest<T> request) where T : BaseResponse
{
return new BaseResponse(...)
}
}
DoGetReq returns BaseResponse:
public BaseResponse DoGetRequest(BaseRequest request)
so either return the actual type you're hoping for (ListAlertsResponse), or write a routine to convert it, or (best imo), have ListAlertsResponse take BaseResponse in its constructor and grab whatever info is needed at that point.
A little improvement of T McKeown's answer:
public TResponse DoGetRequest<TResponse>(BaseRequest<TResponse> request)
where TResponse : BaseResponse
{
return default(TResponse);
}
Since BaseRequest is constrained by parameter T, you should put constraint on generic method too. Also, you can omit casting from caller code, using generic return type:
// response is already ListAlertsResponse
var response = httpClient.DoGetRequest(listAlertsRequest);
DoGetRequest returns BaseResponse which you can't cast to a type of a derived class ListAlertsResponse. Think about the following, what if:
public class BaseResponse
{
public int NumberA { get; set; }
}
And the derived class had
public class ListAlertsResponse : BaseResponse
{
public string StringB { get; set; }
}
What do you think this cast will set test.StringB to?
var test = (ListAlertsResponse)httpClient.DoGetRequest(listAlertsRequest);
The runtime doesn't know how to properly cast this for you, if you want to do this, you need to create a converter yourself, or better yet, not do this, you can cast into a base class, but not the other way around.
Related
I have a service that returns the base class response. For sure it has derived responses.
public class ResponseBase
{
public string Response { get; set; }
}
public class DerivedResponse : ResponseBase
{
public string AdditionalResponse { get; set; }
}
Here is the service code:
public class SomeService
{
public ResponseBase GetResponse() => new DerivedResponse();
}
Then I need to handle different responses in different ways. Obviously try to get appropriate behavior via 100500 if\elses is not a good solution. And I decide to have One special response handler for each concrete response.
public interface IHandlerBase<T>
where T : ResponseBase
{
void Handle(T response);
}
public class DerivedResponseHandler : IHandlerBase<DerivedResponse>
{
public void Handle(DerivedResponse response)
{
}
}
Also, we need to encapsulate behavior which will decide what handler to get in order to handle the concrete response and I think not a bad solution for that will be factory. And I got a problem with that because I don't know what to return since I don't know compile-time derived type:
public class HandlerFactory {
public IHandlerBase<> /*WHAT RETURN HERE*/ CreateHandler(ResponseBase response)
{
//decide the derived type of the 'response' and return appropriate handler
}
}
For sure we can remove generic parameter, accept base class in all handlers, then convert to special in the concrete handlers, but I don't think it is a good solution. So could you please advise how to do that in a clean way, maybe some patterns or solutions?
We can do that by creating new NotGeneric interface:
public interface IHandlerBase
{
void Handle(ResponseBase response);
}
public interface IHandlerBase<T> : IHandlerBase
where T : ResponseBase
{
void Handle(T response);
}
public class DerivedResponseHandler : IHandlerBase<DerivedResponse>
{
public void Handle(DerivedResponse response)
{
}
public void Handle(ResponseBase response)
{
var actualParameter = (DerivedResponse) response;
Handle(actualParameter);
}
}
In that case, we have 2 methods in the derived handler: one for actual handling, and another for adapting (something like adapter pattern). The question is it okay, and maybe somebody has a better solution.
I've got an interface for requests and an interface for responses. I want to call something like this
public IResponse Get(IRequest req)
The request object is serialized and sent out on a bus.
I get a response back but I need to deserialize the response into the IResponse class that corresponds with the specific IRequest class. What's the right way to tie a response to a request so that whenever someone implements one of these request/response pairs they have to constrain it to a certain type?
Here's what I've tried:
public interface IRequest<T> where T : IResponse
{
Type GetResponseType(T t);
}
public interface IResponse { }
public class Res : IResponse
{
public string response { get; set; }
}
public class Req : IRequest<Res>
{
public string request { get; set; }
public Type GetResponseType(Res t)
{
return t.GetType();
}
}
The problem is, I don't know how to pass in an IRequest now:
public IResponse Get(IRequest req)
I get the error:
Using the generic type 'IRequest' requires 1 types argument
I tried:
public IResponse Get(IRequest<IResponse> req)
But when I try to pass in my class I get:
Argument 1: cannot convert from 'Req' to 'IRequest<(IResponse)>'
Does anyone know the proper way to do something like this, or at least let me know how to get my class to work as a parameter?
You could construct your method like that:
public IResponse Get<T>(IRequest<T> req) where T : IResponse
Regarding the signature, I'd use what Pawel suggests:
public IResponse Get<T>(IRequest<T> req) where T : IResponse
Regarding the implementation - do you really need a parameter for the GetResponseType) method? I ask because it seems like GetResponseType is tied to the generic type T and can always be inferred from it. The code below illustrates the point I am trying to make.
public interface IRequest<T> where T : IResponse
{
Type GetResponseType();
}
public interface IResponse { }
public class Res : IResponse
{
public string response { get; set; }
}
public class Req : IRequest<Res>
{
public string request { get; set; }
public Type GetResponseType()
{
return typeof(Res);
}
}
I have written some code and I'm curious as to whether or not there is a danger in what I'm doing that I'm unaware of.
I have tried searching and most of the questions I found dealt with how to make things generic which isn't my issue. I also looked in the C# spec for .net 4.5 under section 13.4.3 - Generic Methods and 7.5.2 in regards to Type inference and finally 7.5.2.12 Inferred return type and they don't really cover what I'm trying to do.
Basically I have a hierarchy of classes
public class SomeBaseClass { }
public class SomeClass : SomeBaseClass { }
public class AnotherClass : SomeBaseClass { }
public class BaseData
{
public SomeBaseClass mMember;
public BaseData() { }
public TType GetMember<TType>()
where TType : SomeBaseClass
{
return (TType)mMember;
}
}
public class Data : BaseData
{
public Data()
{
mMember = new SomeClass();
}
//Is this bad
public SomeClass GetMember()
{
return base.GetMember<SomeClass>();
}
}
The compiler doesn't complain because I'm not hiding the base class method. This is shown that intellisense lists them as two separate methods. I've written several tests which all behave the way I would expect them to and when looking at things like List<> there are instances of methods that have both a generic and nongeneric implementation (for example AsParallel from ParallelEnumerable) but the difference is that in this case both methods exist in the same class and take in a generic and nongeneric parameter respectively.
The tests I ran and showed work the way I would expect are listed below.
class Program
{
static void Main(string[] args)
{
BaseData lData = new Data();
Data lData2 = new Data();
//Call base method with type
SomeBaseClass lTest = lData.GetMember<SomeClass>();
//Cast and call derived method
SomeClass lTest2 = ((Data)lData).GetMember();
//Call base method with type and then cast
SomeClass lTest3 = (SomeClass)lData.GetMember<SomeBaseClass>();
//Call derived method directly
SomeClass lTest4 = lData2.GetMember();
//Throw InvalidCastException
SomeBaseClass lTest5 = lData2.GetMember<AnotherClass>();
}
}
The main reason for this is that I would like that any caller code doesn't have to know the generic type when the class itself already has this information. It's to avoid having to write
lData.GetMemberType<...>();
all over the place.
I apologize if this question is too broad or opinionated. Mostly I'm just wondering if there is anything in this scenario that wouldn't work the way I would think or have a hidden bug etc.
Your question is a little too vague to give a very good answer (what are you using this for? what is the purpose of this design?).
I don't think the name overlap is all that problematic, but it does seem like a symptom of a problematic design and a misuse of generics (all that casting should clue you in on that).
Ideally, your class itself should be generic, and you should use the generic type parameter throughout. That will save you from all the casting you are doing:
public class SomeBaseClass { }
public class SomeClass : SomeBaseClass { }
public class AnotherClass : SomeBaseClass { }
public class BaseData<TType> where TType : SomeBaseClass
{
protected TType mMember;
public BaseData() { }
public BaseData(TType member)
: this()
{
mMember = member;
}
public TType GetMember()
{
return mMember;
}
}
public class Data : BaseData<SomeClass>
{
public Data()
: base(new SomeClass())
{
}
// no need to implement GetMember(); base class has it covered
}
I'm currently have an implementation of Chain of Responsibility which return objects that implement IResponse.
public interface IRequest
{
}
public interface IResponse
{
}
public interface IFactory
{
bool CanHandle(IRequest request);
IResponse HandleRequest(IRequest request);
}
public class Foo : IResponse
{
public void SpecificMethod()
{
Console.WriteLine("SpecificMethod() only belongs to Foo");
}
}
public class FooRequest : IRequest
{
}
public class FooFactory : IFactory
{
public bool CanHandle(IRequest request)
{
return request is FooRequest;
}
public IResponse HandleRequest(IRequest request)
{
return new Foo();
}
}
public class FactoryManager
{
private readonly List<IFactory> _factoryImplementations = new List<IFactory>();
public void Register(IFactory factory)
{
_factoryImplementations.Add(factory);
}
public IResponse HandleRequest(IRequest request)
{
foreach (var factory in _factoryImplementations)
{
if (factory.CanHandle(request))
{
return factory.HandleRequest(request);
}
}
return null;
}
}
class Program
{
static void Main()
{
var manager = new FactoryManager();
manager.Register(new FooFactory());
var foo = (Foo) manager.HandleRequest(new FooRequest()); // How can I remove this cast?
foo.SpecificMethod();
Console.ReadLine();
}
}
The purpose of this implementation is to make it easy to replace implementations whenever I need. The problem is that I have to explicitly cast the type which I made the request for if I want to do anything specific with the object, like accessing foo.SpecificMethod().
Is there any way to have this (Foo) cast gone?
Edit: It's possible to solve this issue with a dynamic variable, but a statically typed way of solving it would be preferrable.
If you want to be able to call a unique function that isn't on the main interface, you will have to cast it (or request a more specific interface with that method on it).
Using an interface means "This method will have these available public methods". You can inherit from multiple interfaces (public interface IMoreSpeific : IGeneric) but you can't make calls to specific class implementations that have other methods without casting it.
You can make something generic like a DoWork() method on your interface, but the purpose of the interface is to have something reusable and generic.
Basically, I'm trying to do something like this:
SomeRequest request = new SomeRequest();
SomeResponse response = request.GetResponse();
List<Stuff> stuff = response.GetData();
SomeRequest and SomeResponse are classes that both implement the IRequest and IResponse interfaces respectively:
public class SomeRequest : IRequest<SomeResponse>
{
public SomeResponse GetResponse() { ... }
}
public class SomeResponse : IResponse<List<Stuff>>
{
public List<Stuff> GetData() { ... }
}
My IResponse interface looks like this:
public interface IResponse<T>
{
T GetData();
}
The issue I'm running into is with my IRequest interface. I want my IRequest interface's generic (T) to be of type IResponse< T >.
public interface IRequest<T> where T : ?????
{
T GetResponse();
}
I can't figure out what I'm supposed to put after the "where T".
I found two solutions here: C# generic "where constraint" with "any generic type" definition?
The first solution is to specify IResponse< T> generic's type in IRequest like so:
public interface IRequest<T, U> where T : IResponse<U>
but that seems weird because the Request should only have knowledge of the Response and not the type that Response is supposed to return on GetData().
The second option is to create a non-generic interface IResponse and use that in IRequest's generic type constraint, which would look something like this:
public interface IResponse { }
public interface IResponse<T> { ... }
public interface IRequest<T> where T : IResponse
{
BaseResponse GetResponse();
}
This solution however caused a compile error in my SomeRequest class:
public class SomeRequest : IRequest<SomeResponse>
{
public SomeResponse GetResponse() { ... }
}
Error CS0738: SomeRequest does not implement interface member IRequest<SomeResponse>.GetResponse() and the best implementing candidate SomeRequest.GetResponse() return type SomeResponse does not match interface member return type IResponse
So now I'm out of ideas. Any help would be greatly appreciated!
How about:
public interface IRequest<T>
{
IResponse<T> GetResponse();
}
This way you can say things like: new MyRequest().GetResponse().GetData() without having to worry about the exact intermediate response type.
Is it possible you're overcomplicating it? Here's how I implemented what I think you want to do:
public interface IRequest<T>
{
T GetResponse();
}
public interface IResponse<T>
{
T GetData();
}
public class MyRequest : IRequest<MyResponse>
{
public MyResponse GetResponse()
{
return new MyResponse();
}
}
public class MyResponse : IResponse<MyData>
{
public MyData GetData()
{
return new MyData() { Name = "Test" };
}
}
public class MyData
{
public string Name { get; set; }
}
I have my two interfaces, my two implementations of those interfaces, and I can consume them like the following:
MyRequest request = new MyRequest();
MyResponse response = request.GetResponse();
MyData data = response.GetData();