Writing Config Files with Aspire

ConfigFiles.cs

public record class ConfigFileContext(
    DistributedApplicationModel AppModel,
    Stream Stream,
    CancellationToken CancellationToken
);

[DebuggerDisplay($"{{{nameof(Path)},nq}}")]
public class ConfigFileAnnotation(string path, Func<ConfigFileContext, Task> generator) : IResourceAnnotation
{
    public FileInfo Path { get; } = new FileInfo(path);
    public Func<ConfigFileContext, Task> Generator => generator;
}

public class ConfigFileWriterHook(DistributedApplicationExecutionContext _executionContext, ResourceLoggerService _resourceLoggerService) : IDistributedApplicationLifecycleHook
{
    public async Task AfterEndpointsAllocatedAsync(DistributedApplicationModel appModel, CancellationToken cancellationToken = default)
    {
        if (_executionContext.IsPublishMode)
        {
            return;
        }

        foreach (var resource in appModel.Resources)
        {
            foreach (var annotation in resource.Annotations.OfType<ConfigFileAnnotation>())
            {
                var logger = _resourceLoggerService.GetLogger(resource.Name);
                logger.LogInformation("Building Config File {File}", annotation.Path);

                using var stream = annotation.Path.Open(FileMode.Create, FileAccess.Write, FileShare.None);
                var context = new ConfigFileContext(appModel, stream, cancellationToken);
                await annotation.Generator(context);
            }
        }
    }
}

public static class ConfigFileExtensions
{
    public static IResourceBuilder<T> WithConfigurationFile<T>(this IResourceBuilder<T> builder, string path, in ReferenceExpression.ExpressionInterpolatedStringHandler value)
        where T : IResource
    {
        var expression = ReferenceExpression.Create(value);
        return builder.WithConfigurationFile(path, async context =>
        {
            var stringValue = await expression.GetValueAsync(default);
            using var writer = new StreamWriter(context.Stream);
            await writer.WriteAsync(stringValue);
        });
    }

    public static IResourceBuilder<T> WithConfigurationFile<T>(this IResourceBuilder<T> builder, string path, Func<ConfigFileContext, Task> generator)
        where T : IResource
    {
        builder.ApplicationBuilder.Services.TryAddLifecycleHook<ConfigFileWriterHook>();
        return builder.WithAnnotation(new ConfigFileAnnotation(path, generator));
    }
}

Examples.cs

// Building an xml file for an executable
builder
    .AddExecutable("test", ...)
    .WithConfigurationFile("c:\\temp\\foo.xml", $"""
        <?xml version="1.0" encoding="utf-8"?>
        <root>
            <dbConnectionString>{database.Resource.ConnectionStringExpression}</dbConnectionString>
        </root>
        """);

//Adding a config file for use by a container, and mount it into the container.
// This example is for https://opentelemetry.io/docs/collector/configuration/
builder
    .AddContainer(ResourceName, "otel/opentelemetry-collector-contrib", "0.91.0")
    .WithBindMount("c:\\temp\\otel-config.yaml", $"{configContainerDir}/config.yaml")
    .WithConfigurationFile("c:\\temp\\otel-config.yaml", async context =>
    {
        var config = BuildOtelConfig();

        using var writer = new StreamWriter(context.Stream);
        new YamlDotNet.Serialization.Serializer().Serialize(writer, config);
    });