Question

I have been getting a lot of traction from a builder pattern as a public class member of another class:

public class Part
{
    public class Builder
    {
        public string Name { get; set; }
        public int Type { get; set; }

        public Part Build()
        {
            return new Part(Name, Type);
        }
    }

    protected Part(string name, int type)
    {
        ...
    }
}

Note protected constructor – I like how I HAVE to use the builder to get a Part. Calls to

Part p = new Part.Builder() { Name = "one", Type = 1 }.Build();

work great. What I would like to do is use this builder to serve up a special kind of part based on the Type (for example):

public class SpecialPart : Part
{
    protected SpecialPart(string name, int type) : base(name, type) { }
}

And a slight change to the builder:

public Part Build()
{
    if (Type == _some_number_)
        return new SpecialPart(Name, Type);
    return new Part(Name, Type);
}

But this doesn’t work – Part.Builder can’t see SpecialPart’s protected constructor. How can I get Builder to work with descendents of Part and get the same must-have-a-builder semantics?

Answer 1

There are many ways to skin a cat, but the path of least resistance here is going to be making the constructors of your various part types public or internal.