CODEWITHSUNDEEP
c#generics
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Reputation: 1020

Generic type parameter error

I have these 3 abstract classes: 

public abstract class DataObject {}
public abstract class BusinessObject<T> where T : DataObject {}
public abstract class UIObject<T> where T : BusinessObject<DataObject> {}

I am able to create a CustomerDataObject which derives from DataObject, a CustomerBusinessObject which derives from BusinessObject<CustomerDataObject> but I can't create the CustomerUIObject class:

The type CustomerBusinessObject cannot be used as type parameter T in the generic type or method UIObject<T>. There is no implicit reference conversion from CustomerBusinessObject to BusinessObject<DataObject>.

What am I doing wrong?

Upvotes: 2

Views: 169

Answers (3)

Royi Namir
Royi Namir

Reputation: 148514

For Why - you already know

Now there is a way that you can do it - without changing logic :

You can add double constraint : 

public abstract class DataObject {}
public abstract class BusinessObject<T> where T : DataObject {}
public abstract class UIObject<T,U> where T : BusinessObject<U> where U:DataObject {}

This way you explicitly tell the compiler that U can be a type that derive from DataObject - which is exactly what your CustomerDataObject is.

But you will need to add : 

class CustomerDataObject:DataObject
{}
class CustomerBusinessObject :BusinessObject<CustomerDataObject>
{}
class CustomerUIObject :UIObject<CustomerBusinessObject,CustomerDataObject>
{}

This way you're specifying the CustomerDataObject which inherits from DataObject which is exactly what you meant at first place

Upvotes: 2

Viru
Viru

Reputation: 2246

you have to use Generics contraints to resolve this issue.

public abstract class UIObject<T,U> where T : BusinessObject<U>

Upvotes: 1

TomDoesCode
TomDoesCode

Reputation: 3681

You are explicitly stating that the T for UIObject inherits BusinessObject<DataObject> but your type CustomerBusinessObject actually inherits BusinessObject<CustomerDataObject> which is a different type.

Can you have CustomerBusinessObject inherit BusinessObject<DataObject> but then in the implementation always use CustomerDataObject? That would fix this error but might not be quite what you want.

public class CustomerBusinessObject : BusinessObject<DataObject> { }

Upvotes: 2

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