Override One Field Only for One Test¶

Problem¶

A test needs one specific member of a composed type to hold a known value (e.g. asserting on it directly, or making a test's intent obvious), but every other member — and every other composed value of that member's own type elsewhere in the graph — should stay ordinarily composed.

Solution¶

public sealed record Customer(string Name, string Email);

[Fact]
public void ComposesACustomerWithAKnownEmail()
{
    var composer = Composer.Create(builder => builder
        .For<Customer>().Member(x => x.Email).Use("known@example.com"));

    var customer = composer.Create<Customer>();

    customer.Email.Should().Be("known@example.com");
    customer.Name.Should().NotBeNullOrWhiteSpace(); // still ordinarily composed
}

For<Customer>().Member(x => x.Email).Use(...) is scoped to this one Composer — nothing outside this test is affected, and no other member of Customer (or any other composed string) changes.

Discussion¶

Reach for .For<T>().Use(...) (without .Member(...)) instead when the override should apply to every composed value of a type, not just one member of one parent type — a member rule only applies inside the specific parent type it was declared against, by design.

Need the same override reused across many tests, not just one? Move it into a profile instead of repeating it per Composer.Create(...) call.

Override One Field Only for One Test¶

Problem¶

Solution¶

Discussion¶

See also¶