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Reuse Configuration Through a Profile

Problem

Every test in a project needs the same baseline IConfiguration — copying the same ConfigurationBuilder/AddInMemoryCollection call into every test class is repetitive and drifts inconsistent over time.

Solution

using Microsoft.Extensions.Configuration;

public sealed class AppConfigurationProfile : ICompositionProfile
{
    public void Configure(CompositionBuilder builder) => builder
        .Register<IConfiguration>(() => new ConfigurationBuilder()
            .AddInMemoryCollection(new Dictionary<string, string?>
            {
                ["Retry:MaxAttempts"] = "3",
                ["Retry:DelayMilliseconds"] = "100",
            })
            .Build());
}

[Theory]
[Compose<AppConfigurationProfile>]
public void ReadsTheSharedBaselineConfiguration(IConfiguration configuration)
{
    configuration["Retry:MaxAttempts"].Should().Be("3");
}

Or applied to a hand-built Composer directly:

var composer = Composer.Create(builder => builder.AddProfile<AppConfigurationProfile>());

Discussion

ICompositionProfile.Configure is just an ordinary sequence of CompositionBuilder calls — Register<IConfiguration> needs no special profile-only mechanism to be reusable this way, the same as any other registration.

Registering the same exact type more than once is a build-time conflict in Compono, not last-write-wins or first-write-wins (docs/adr/0019-registrations-and-service-provider-injection.md) — so a test can't compose AppConfigurationProfile (which already calls Register<IConfiguration>) and then call Register<IConfiguration> again itself to layer an override; that collides and throws CompositionConfigurationException. A test that needs the shared baseline plus one local override instead passes the override into the profile's own constructor and applies it with the instance-based AddProfile(ICompositionProfile) overload, so the profile itself is the one and only place that calls Register<IConfiguration>:

public sealed class AppConfigurationProfile(IDictionary<string, string?>? overrides = null) : ICompositionProfile
{
    public void Configure(CompositionBuilder builder) => builder
        .Register<IConfiguration>(() => BuildConfiguration(overrides));
}

var composer = Composer.Create(builder => builder.AddProfile(
    new AppConfigurationProfile(new Dictionary<string, string?> { ["Retry:MaxAttempts"] = "0" })));

See Layer Configuration Overrides in a Test for BuildConfiguration's own layering (multiple AddInMemoryCollection calls inside one ConfigurationBuilder, not multiple Register<T> calls), and Composition Profiles for the full profile mechanics.

See also