what is wrong with this using generic implicit operator?

Question

if I use implicit operator in non generic builder class every thing is ok:

 public class ReligionBuilder
{
    private Religion _religion;

    public ReligionBuilder()
    {
        _religion = new Religion(){//some codes}

    }

    public ReligionBuilder AddToRepository()
    {
        Repository<Religion>.Add(_religion);
        return this;
    }
    public Religion Build()
    {
        return _religion;
    }

    public static implicit operator Religion(ReligionBuilder _builder)
    {
        return _builder.Build();

    }
}

I can use it :

 Religion religion=new ReligionBuilder().AddToRepository();

but if this operator is in generic class something is wrong:

 public abstract class DataTestBuilderBase<T> : IDataTestBuilder<T>
{
    protected T TestData { get; set; }

    public virtual T Build()
    {
        return TestData;
    }

    public abstract IDataTestBuilder<T> AddToRepository();
    public abstract IDataTestBuilder<T> WithDefault();

    public static implicit operator T(DataTestBuilderBase<T> builder)
    {
        return builder.Build();
    }
}


 public class PersonDataTestBuilder : DataTestBuilderBase<Person>
{
    private Person _person;

    public PersonDataTestBuilder()
    {
        //some codes
    }
    public override IDataTestBuilder<Person> AddToRepository()
    {
       //some codes
        return this;
    }
}

usage:

 PersonDataTestBuilder _testBuilder = new PersonDataTestBuilder();
        Person person = _testBuilder.AddToRepository();

the error is :cannot convert IDataTestBuilder to Person

what is the problem?

Answer 1

AddToRepository returns just IDataTestBuilder<Person> (in terms of the compile-time return type) - and there's no implicit conversion from that to Person. If you change the return type of the abstract method to DataTestBuilderBase<T> then it should work - although frankly I wouldn't want to use that implicit conversion anyway. I'm generally pretty cautious about providing implicit conversions - they can often make the code less clear, as I believe they do here.

Should you really be providing AddToRepository on a builder anyway? It feels like an inappropriate action for a builder - I would expect:

Person person = new PersonBuilder { /* properties */ }
                     .Build()
                     .AddToRepository();

EDIT: Just to show what I mean about changing the return type of AddToRepository, here's a short but complete program to demonstrate. It works fine.

using System;

public abstract class BuilderBase<T>
{
    public abstract T Build();
    public abstract BuilderBase<T> AddToRepository();

    public static implicit operator T(BuilderBase<T> builder)
    {
        return builder.Build();
    }
}

public class TestBuilder : BuilderBase<string>
{
    public override string Build()
    {
        return "Built by Build()";
    }

    public override BuilderBase<string> AddToRepository()
    {
        return this;
    }
}

class Program
{
    static void Main(string[] args)
    {
        string x = new TestBuilder().AddToRepository();
        Console.WriteLine(x);
    }
}