Implicit Builder Conversion in C#

I recently attended a Leeds Sharp user group meeting on design patterns. Grant Crofton was presenting an example of using the builder pattern in tests and showed a neat trick that can be used in C# when using builders.

A builder class has a selection of “with” methods corresponding to different properties of the target object, where these are called in a chain. The final method call in this chain is then usually a Build() method that converts the builder object into the desired target object, i.e.

Order order = OrderBuilder.anOrder().withCustomer(customer).withAddress(address).withPostage(2.00).build();

I learned that by using implicit type conversion in C# with the “implicit operator” keywords, this final Build() call can be omitted. For example, here I have two NUnit test methods, the first of which uses a final Build() call for the builder object and the second which uses implicit type conversion:

[TestFixture]
class AddressTest
{
    [Test]
    public void AddressWithHouseNameAndNoNumberDisplaysCorrectly()
    {
        Address address = AddressBuilder.anAddress()
            .WithHouseName("Dunroamin")
            .WithStreet("Golden Meadow Lane")
            .WithTown("Otley St Catchpole")
            .Build();

        Assert.That(address.ToString(), Is.EqualTo("Dunroamin, Golden Meadow Lane, Otley St Catchpole"));
    }

    [Test]
    public void AddressWithNumberAndBuildingNameDisplaysCorrectly()
    {
        Address address = AddressBuilder.anAddress()
            .WithNumber("5")
            .WithBuildingName("Hipster Flats")
            .WithStreet("Ironic Street")
            .WithTown("Hipsterville");
        Assert.That(address.ToString(), Is.EqualTo("5 Hipster Flats, Ironic Street, Hipsterville"));
    }
}

The code to achieve this implicit type conversion in the AddressBuilder class is:

public static implicit operator Address(AddressBuilder builder)
{
    return new Address(builder.HouseName, builder.Number, builder.BuildingName,
        builder.Street, builder.Town);
}

The full code for this builder example is available on GitHub