Question

I’m getting an error when trying to add generic object to a List<>.

Its probably related to Covariance and Contravariance but I not sure how to work around this. I’ve tried to restrict my generic types using where T: IRegister.

I have an interface to represent a register and then two classes which represent a ByteRegister and a DoubleWordRegister.

public interface IRegister
{
    string Name {get;set;}
}

 public class ByteRegister : IRegister
{
...
}

public class DoubleWordRegister : IRegister
{
...
}

I then have another class which represents a block of these registers all of the same type.

public class RegisterBlock<T> where T:IRegister
{
   private IList<T> _registers;

 ... constructors, properties etc

    public void AddRegister(T register)
    {
        _registers.Add(register);
    }
}

And finally I have a RegisterMap class which is used to define the list of register blocks and each register within the block.

public class RegisterMap
{
    private List<RegisterBlock<IRegister>> _blocks;

    public RegisterMap()
    {
        _blocks = new List<RegisterBlock<IRegister>>();

        RegisterBlock<ByteRegister> block1= new RegisterBlock<ByteRegister>("Block1", 0);
        block1.AddRegister(new ByteRegister("Reg1"));
        block1.AddRegister(new ByteRegister("Reg2"));
        _blocks.Add(block1);

        RegisterBlock<DoubleWordRegister> block2= new RegisterBlock<DoubleWordRegister>("Block2", 10);
        block2.AddRegister(new DoubleWordRegister("Reg3"));
        block2.AddRegister(new DoubleWordRegister("Reg4"));
        block2.AddRegister(new DoubleWordRegister("Reg5"));
         _blocks.Add(block2);
    }
}

However I’m getting the following error:

Error 20 Argument '1': cannot convert from 'RegisterBlock<ByteRegister>' to 'RegisterBlock<IRegister>' on the line _blocks.Add(block1) and similarly on _blocks.Add(block2);

Answer 1

This is indeed a **variance problem. You will need another interface for your RegisterBlock class, maybe IRegisterBlock:

public class RegisterBlock<T> : IRegisterBlock
    where T : IRegister

Then you can create a list of IRegisterBlock:

private List<IRegisterBlock> _blocks;

I actually had a similar situation in our code-base last week, and this is exactly how I resolved it.