返回接口的通用方法

Question

I would like to write a generic method that return DAL interfaces but it doesn't works.

It's possible to make this :

public MyInterface GetDAL()
{
   return new DAL(); // DAL implements MyInterface
}

But not this :

public TInt GetDAL<TInt, TDAL>()
{
   return new TDAL();
}

or this

public TInt GetDAL<TInt, TDAL>()
{
   return (TInt)new TDAL();
}

I know I could return concrete class instead of interface but I don't understand why it doens't works, if TDAL implements TInt.

I have 10 DAL classes and I don't want to write 10 methods.

Thanks for your help

Answer 1

It does work if you tell the compiler the constraints for TDAL :

public TInt GetDAL<TInt, TDAL>() where TDAL : TInt, new()
{
   return new TDAL();
}

This tells the compiler that TDAL must implement TInt and have a parameterless constructor.
So now the compiler knows that for any type argument for TDAL the expression new TDAL() will work and that the result is assignable to TInt .

Answer 2

new TDAL() won't work because not all classes have a parameterless constructor, or inherit from TInt .

You need to add type constraints to your method eg

public TInt GetDAL<TInt, TDAL>() where TDAL : new(), TInt
{
   return new TDAL();
}

Then when you use your method, the compiler will enforce a compatible type to be used.

MyInterface dal = GetDAL<MyInterface, DAL>(); // Will compile

MyInterface dal = GetDAL<TMyInterface, string>(); // Won't compile