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The type 'T' must be a reference type in order to use it as parameter 'T' in the generic type or method 'QueryAPI.Query<T>()

I'm writing some code that should update some fields with a common logic on different remote objects. Therefore I use given API. The Test Class is my own implementation. The other two classes are given by the API.
When I write the following code i get the error
The type 'T' must be a reference type in order to use it as parameter 'T' in the generic type or method 'QueryAPI.Query<T>()
Code:
public class Test<T> where T : class, UnicontaBaseEntity
{
private async Task Foo<T>(QueryAPI queryAPI, CrudAPI crudAPI, SyncSettings syncSettings)
{
Task<T[]> result = await queryAPI.Query<T>();
}
}
public interface UnicontaBaseEntity : UnicontaStreamableEntity
{
int CompanyId { get; }
Type BaseEntityType();
}
public class QueryAPI : BaseAPI
{
...
public Task<T[]> Query<T>() where T : class, UnicontaBaseEntity, new();
...
}
Any ideas on this?
Thanks in advance.
KR
Mike

I would remove T from Foo() here since your parent class Test<T> is already generic.
You should also add a new() constraint otherwise there will be another error since QueryAPI expects a type with default constructor.
Also, some renames to include Async are in order.
public class Test<T> where T : class, UnicontaBaseEntity, new()
{
private async Task FooAsync(QueryAPI queryAPI, CrudAPI crudAPI, SyncSettings syncSettings)
{
Task<T[]> result = await queryAPI.QueryAsync<T>();
}
}
public interface UnicontaBaseEntity : UnicontaStreamableEntity
{
int CompanyId { get; }
Type BaseEntityType();
}
public class QueryAPI : BaseAPI
{
...
public Task<T[]> QueryAsync<T>() where T : class, UnicontaBaseEntity, new()
...
}

c# generic interfaces and inheritance

the chosen title for my question isn't quite precise, but I don't know the term I'm looking for or if it is even possible.
What I have in mind is a chain of consumer <- procucer <-product.
A consumer can "consume" producers and producers "produce" products of a certain type. Therefor I wrote:
public interface IProduct
{
string ProductName { get; }
}
public class Product : IProduct
{
public string ProductName { get { return "name of product"; } }
}
public interface IProducer<T>
{
T ProducerProperty { get; set; }
void ProducerMethod();
}
public class Producer<T> : IProducer<T> where T : IProduct
{
public Producer()
{
}
public T ProducerProperty { get; set; }
public void ProducerMethod()
{
}
}
public interface IConsumer<T>
{
T ConsumerProperty { get; set; }
void ConsumerMethod();
}
public class Consumer<T> : IConsumer<T>
{
private U producer; //U should be IProducer<IProduct>, doesen't work
public Consumer(U producer) //U should be IProducer<IProduct>, doesen't work
{
this.producer = producer;
}
public T ConsumerProperty { get; set; }
public void ConsumerMethod()
{
}
}
and the use case:
private IProducer<IProduct> producer; //DeviceManager
private IConsumer<IProducer<IProduct>> consumer; //DeviceViewManager
public MainPage()
{
this.InitializeComponent();
producer = new Producer<IProduct>();
consumer = new Consumer<IProducer<IProduct>>();
}
The consumer class uses the generic "U", which is imaginary at this point. I want the consumer class to use the type U. In the context of the given example you could think of a user who consumes different types of noodles from different manufacrures of noodles.
I want the generic classes be tied to the interfaces rather than to actual classes. But I culdn't manage to achieve this. I tried substituting the interfaces with base classes (e.g.: ProducerBase), but than the actual base classes were needed.

The Problem
If you define the Consumer class like this
public class Consumer<T, U> : IConsumer<T, U>
{
...
}
you get the errors
CS0314 The type 'T' cannot be used as type parameter 'T' in the generic type or method IConsumer<T, U>'. There is not boxing conversion or type parameter conversion from 'T' to ''IProducer'.
and
CS0314 The type 'U' cannot be used as type parameter 'U' in the generic type or method IConsumer<T, U>'. There is not boxing conversion or type parameter conversion from 'U' to ''IProducer'.
What this tells you is that you cannot derive from the IConsumer<T, U> interface, because it has constraints for its generic type parameters:
public interface IConsumer<T, U>
where T : IProducer<U> // T needs to "be" an IProducer<U>
where U : IProduct // U needs to "be" an IProduct
{
...
}
because the interface expects T and U to derive from IProducer<U> and IProduct respectively and the compiler doesn't know that the generic type parameters T and U of the Consumer class can actually be converted to an IProducer<U> and IProduct respectively.
The Solution
Therefore, if you define the Consumer like this and add constraints for T and U
public class Consumer<T, U> : IConsumer<T, U>
where T : IProducer<U>
where U : IProduct
{
...
}
it will work because now you specified that T and U will always be an IProducer<U> and IProduct respectively.
I hope above explanation was understandable; either way, you should also read this for a good understanding of constraints on generic types.

Generic Constraints C#

I need to make some of my classes inherit from Interface with generic field
like that
public Interface ICommon<Ttype>
{
Ttype Filed{get;set;}
}
public Class class1:Icommon<int>
{
int Filed{get;set;}
}
public Class class2:Icommon<double>
{
double Filed{get;set;}
}
I created a generic class with constraints that uses classes class1 and class2 to make some operations like that:
public Class GenericClass<Ttype,Tcommon> where Ttype:ICommon<Tcommon>
{
//forexample
public Ttype someOperation(Ttype x)
{
var a=x.Field;
//.............
}
}
every time I use the GenericClass I have to know the type of Field of the class I used say class1 or class2 to be able to pass it to match the generic constraint
Is there a way to write GenericClass like that:
public Class GenericClass<Ttype,Tcommon> where Ttype:**ICommon**
{
//forexample
public Ttype someOperation(Ttype x)
{
var a=x.Field;
//.............
}
}
by writting ICommon without <TCommon> ??
Update:
or how to edit ICommon interface to be like that
public Interface ICommon
{
Ttype Filed{get;set;}
}

I hope I understood what you intended to do:
public interface ICommon<T>
{
T Field { get; set; }
}
public class GenericClass<T>
{
public ICommon<T> SomeOperation(ICommon<T> x)
{
// do your stuff
}
}

Short answer is: no.
You need to tell the compiler type of generic argument.
In fact, GenericClass<int> and GenericClass<string> are two different classes in CLR.

C# The is no implicit reference conversion from 'B' to 'A' using Generics

I'm with problems to convert from the type derived to base type using Generics.
Class to manage the dictionary:
public class ManagerDictionary<TContext>
{
public ManagerDictionary()
{
this.Dictionary = new Dictionary<int, TContext>();
}
public IDictionary<int, TContext> Dictionary { get; private set; }
public void Register<TSubContext>(int context, TSubContext subContext) where TSubContext : TContext
{
this.Dictionary[context] = subContext;
}
}
Interface of the Process context:
public interface IProcessContext : IContext<ProcessViewModel>
{
}
My test class:
public class Foo<TViewModelContext> where TViewModelContext : ViewModeBase
{
public Foo(IProcessContext processContext)
{
// Create de Dictionary Manager.
this.ManagerDictionary = new ManagerDictionary<IContext<TViewModelContext>>();
// Register the process context on dictionary.
// The error is occurring here: The is no implicit reference conversion from 'IProcessContext' to 'IContext<TViewModelContext>'
this.ManagerDictionary.Register((int)ContextType.Process, processContext);
}
protected ManagerDictionary<IContext<TViewModelContext>> ManagerDictionary { get; set; }
}
When I try register the processContext, the problem occurs:
The is no implicit reference conversion from 'IProcessContext' to
IContext<TViewModelContext>
How can I resolve this problem?
Edit:
When I Create a inherited class of the Foo, I can register, but I need register on Foo class too.
public interface IAnotherProcessContext : IContext<ProcessTwoViewModel>
{
}
public class InheritedFoo : Foo<ProcessTwoViewModel>
{
public InheritedFoo(IAnotherProcessContext anotherProcessContext)
{
base.ManagerDictionary.Register((int)ContextType.InheritedProcess, anotherProcessContext);
}
}

You're trying to treat IContext<T> as if it's covariant with respect to T, but that interface isn't defined as being covariant.
Either make the interface be covariant, or alter your program such that you never expect an IContext<Child> to be implicitly convertible to an IContext<Parent>.

Generic method call cannot convert from class name to full class name

I have an interface as follows:
public interface ISelectEntity
{
List<T> GetFromDB<T, O>(O data);
}
I'm implementing it as follows within a StatusCodes class
List<StatusCodes> ISelectEntity.GetFromDB<StatusCodes, StatusCodesInputParameters>(StatusCodesInputParameters data)
{
return EntitiesClass.PopulateStatusCodes(EntitiesDAL.GetStatusCodes(data));
}
I'm receiving the following error, however:
Error 2 Argument '1': cannot convert from 'StatusCodesInputParameters' to 'Namespace.StatusCodesInputParameters'
Assuming that my namespace is called Namespace, that's the error I receive. All of this is within this one namespace. What am I doing wrong?

You cannot change method signatures in derived classes. You could define the generic argument on the interface though instead of doing it on the method:
public interface ISelectEntity<T, O>
{
List<T> GetFromDB(O data);
}
and a sample explicit implementation might look like this:
public class Foo : ISelectEntity<StatusCodes, StatusCodesInputParameters>
{
List<StatusCodes> ISelectEntity<StatusCodes, StatusCodesInputParameters>.GetFromDB(StatusCodesInputParameters data)
{
return EntitiesClass.PopulateStatusCodes(EntitiesDAL.GetStatusCodes(data));
}
}
and a sample implicit implementation:
public class Foo : ISelectEntity<StatusCodes, StatusCodesInputParameters>
{
public List<StatusCodes> GetFromDB(StatusCodesInputParameters data)
{
return EntitiesClass.PopulateStatusCodes(EntitiesDAL.GetStatusCodes(data));
}
}