How do I use a generic interface with the factory pattern?

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This is one case where code really is worth a thousand words. The code that follows does not compile. I hope it is close enough to show what I am trying to do.

I don't see anything conceptually wrong with what I am trying to do but seem to be in a complete mix-up with syntax. Of course, if what I am trying to do is fundamentally a bad idea, I am always willing to learn from greater minds.

Hopefully, it goes without saying that this is a contrived example, lacking any error checking. I'm just trying to show the pattern I am trying to implement.

public interface IReturnsValue<T>
{
    void SetValues( List<T> listOfValues );
    T NextValue { get; }
}

public class IntegerReturner : IReturnsValue<int>
{
    public void SetValues( List<int> listOfValues ) { _values = listOfValues; }
    public int NextValue => _values[ _indexOfNextValue++ ];
    private int _indexOfNextValue = 0;
    private List<int> _values = null;
}

public class DoubleReturner : IReturnsValue<double>
{
    public void SetValues( List<double> listOfValues ) { _values = listOfValues; }
    public double NextValue => _values[ _indexOfNextValue++ ];
    private int _indexOfNextValue = 0;
    private List<double> _values = null;
}

public class ValueReturnerFactory
{
    public IReturnsValue<T> CreateValueReturner<T>( string name )
    {
        if( name == "IntegerReturner" )
            return new IntegerReturner();
        if( name == "DoubleReturner" )
            return new DoubleReturner();
        throw new ArgumentException();
    }
}

class Program
{
    static void Main( string[] args )
    {
        ValueReturnerFactory factory = new ValueReturnerFactory();
        IReturnsValue<T> firstValueReturner = factory.CreateValueReturner<int>( "IntegerReturner" );
        IReturnsValue<T> secondValueReturner = factory.CreateValueReturner<double>( "DoubleReturner" );
    }
}

What I have tried:

Every combination of generic code type parameter I can think of. :(

Posted 14-Nov-18 23:41pm

Patrick Skelton

Updated 14-Nov-18 23:59pm

v2

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lmoelleb · Accepted Answer · 2018-11-14T23:59:00

It doesn't work as the compiler can't check if T is always an int when using IntegerReturner etc. For example, the following call would be valid according to the method signature:

C#

factory.CreateValueReturner<bool>("IntegerReturner")

where it would have to return an IReturnsValue<int> as an IReturnsValue<bool>.

You can "take over responsibility" and cast in the factory. So:

C#

public IReturnsValue<T> CreateValueReturner<T>(string name)
{
    if (name == "IntegerReturner")
        return (IReturnsValue<T>)new IntegerReturner();
    if (name == "DoubleReturner")
        return (IReturnsValue<T>)new DoubleReturner();
    throw new ArgumentException();
}

you will then get a runtime exception if the types do not match.

You also have to specify the type in the Main method as T isn't defined here (or use var of course):

C#

IReturnsValue<int> firstValueReturner = factory.CreateValueReturner<int>("IntegerReturner");

IReturnsValue<double> secondValueReturner = factory.CreateValueReturner<double>("DoubleReturner");

It's hard to say if there are better ways to implement this, it depends on the rest of the code and personal preference. If it was me, I would consider if I could get rid of the name and select the class to instantiate from the type of T - then you can't have a situation where T and name do not match. The factory method can be something like this:

C#

public IReturnsValue<T> CreateValueReturner<T>()
{
    if (typeof(T) == typeof(int))
        return (IReturnsValue<T>)new IntegerReturner();
    if (typeof(T) == typeof(double))
        return (IReturnsValue<T>)new DoubleReturner();
    throw new ArgumentException();
}