如何在不修改原有模型的前提下实现嵌套对象的Builder构建器

完全不需要修改原有实体类，只需要为每个层级的实体独立实现无侵入的Builder类即可，核心逻辑是父层级Builder的嵌套配置方法接收子Builder的配置委托，内部完成子实体的构建和赋值。

完整实现代码 #

// 最底层Contact对应的Builder
public class ContactBuilder
{
    private readonly Contact _contact = new Contact();

    public ContactBuilder WithPhoneNo(string phoneNo)
    {
        _contact.PhoneNo = phoneNo;
        return this;
    }

    public Contact Build() => _contact;
}

// Address对应的Builder
public class AddressBuilder
{
    private readonly Address _address = new Address();

    public AddressBuilder WithStreet(string street)
    {
        _address.Street = street;
        return this;
    }

    public AddressBuilder WithContact(Action<ContactBuilder> configure)
    {
        var contactBuilder = new ContactBuilder();
        configure(contactBuilder);
        _address.Contact = contactBuilder.Build();
        return this;
    }

    public Address Build() => _address;
}

// Owner对应的Builder
public class OwnerBuilder
{
    private readonly Owner _owner = new Owner();

    public OwnerBuilder WithName(string name)
    {
        _owner.Name = name;
        return this;
    }

    public OwnerBuilder WithAddress(Action<AddressBuilder> configure)
    {
        var addressBuilder = new AddressBuilder();
        configure(addressBuilder);
        _owner.Address = addressBuilder.Build();
        return this;
    }

    public Owner Build() => _owner;
}

// 最顶层Advertisment对应的Builder
public class AdBuilder
{
    private readonly Advertisment _ad = new Advertisment();

    public AdBuilder WithId(string id)
    {
        _ad.AdvertismentId = id;
        return this;
    }

    public AdBuilder WithOwner(Action<OwnerBuilder> configure)
    {
        var ownerBuilder = new OwnerBuilder();
        configure(ownerBuilder);
        _ad.Owner = ownerBuilder.Build();
        return this;
    }

    public Advertisment Build() => _ad;
}

使用方式 #

注意你示例中把WithContact放在WithOwner下的写法不符合实体层级关系，正确写法应该把Contact配置放在Address的配置委托内，和实体的嵌套结构保持一致，最终调用效果如下：

var ad = new AdBuilder()
            .WithId("123")
            .WithOwner(ownerBuilder =>
                ownerBuilder.WithName("NameOfOwner")
                .WithAddress(addressBuilder => addressBuilder.WithStreet("SomeStreet")
                    .WithContact(contactBuilder => contactBuilder.WithPhoneNo("123-345-789"))))
            .Build();

优化建议 #

如果实体字段多、层级复杂，手写Builder重复工作量大，可以考虑用C#源生成器自动生成所有实体对应的Builder代码，因为你的实体是由JSON Schema统一生成的，每次更新Schema后只需要重新运行一次源生成器即可同步更新Builder，不需要手动维护代码。

内容的提问来源于stack exchange，提问作者tylkonachwile