Compose a Substitute With One Method Stubbed¶

Problem¶

A composed interface dependency is fine as an ordinary substitute for most of the test — every call it doesn't need to care about can behave however NSubstitute's own defaults behave — except for one specific call, whose return value the test needs to control.

Solution¶

public interface IOrderRepository
{
    Task<Order> SaveAsync(Order order, CancellationToken cancellationToken);
}

[Theory]
[Compose<ApplicationTestProfile>] // a profile that calls builder.UseNSubstitute()
public async Task Handle_ReturnsTheOrderTheRepositoryReturned(
    [Shared] IOrderRepository repository, OrderHandler handler, PlaceOrder command, Order savedOrder)
{
    repository.SaveAsync(Arg.Any<Order>(), Arg.Any<CancellationToken>()).Returns(savedOrder);

    var result = await handler.Handle(command);

    result.Should().Be(savedOrder);
}

repository is a real, composed NSubstitute substitute — [Shared] makes it the exact instance OrderHandler's own constructor received, so stubbing SaveAsync here affects the call handler.Handle(...) actually makes. Every other member of IOrderRepository (if there were more) stays an ordinary, unconfigured substitute call.

Discussion¶

Configure the stub before the composed value is used, not after — NSubstitute setup only affects calls made after Returns(...) runs, the same as configuring any other NSubstitute substitute.

Want to verify the call happened, not control its return value? Use Received(...) the same way, on the same [Shared] instance — the two aren't mutually exclusive; stub the call's return value and verify it was called exactly once in the same test if both matter.

Compose a Substitute With One Method Stubbed¶

Problem¶

Solution¶

Discussion¶

See also¶