How Would I Configure a DI container with a AWS Lambda Function using .NET Core?

Question

I would like to wire in a DI container to an AWS lambda function. It would be nice if there were a base class architecture to facilitate SOLID principles in AWS Lambda. Obviously there is no startup.cs class or other .NET Core initialization facilities available.

This approach would allow one to service out the testable pieces of a larger lambda function.

public class Function : Startup
{
    private IFooService _fooService;
    
    public Function(IFooService fooService)
    {
        _fooService = fooService;   
    }

    public SkillResponse FunctionHandler(SkillRequest input, ILambdaContext context)
    {
       FooBar fooBar = _fooService.GetFooBar();
    }
}

Kevin Smith · Accepted Answer · 2018-05-23 16:24:38Z

I've been doing this with AutoFac and on every function call creates a new scope:

public class Functions
{
    public static Lazy<ILifetimeScope> LifetimeScope { get; set; } = new Lazy<ILifetimeScope>(CreateContainer);

    private static ILifetimeScope CreateContainer()
    {
        var containerBuilder = new ContainerBuilder();
        containerBuilder.RegisterType<ServerAbc>()
            .AsImplementedInterfaces();

        return containerBuilder.Build();
    }

    /// <summary>
    /// A Lambda function
    /// </summary>
    public async Task Handle(ILambdaContext context)
    {
        using (var innerScope = LifetimeScope.Value.BeginLifetimeScope())
        {
            var service = innerScope.Resolve<IServerAbc>();

            await service.GoDoWork()
                .ConfigureAwait(false);
        }
    }
}

The public static Lazy<ILifetimeScope> is also so I can mock it out in my tests.

[Fact]
public async Task ShouldMostLikelyWork()
{
    var lifetimeScope = new Mock<ILifetimeScope>();
    lifetimeScope.Setup(x => x.Resolve<IServerAbc>()).Returns(new MockService());

    Functions.LifetimeScope = new Lazy<ILifetimeScope>(() => lifetimeScope.Object);
    var functions = new Functions();

    await functions.Handle(Mock.Of<ILambdaContext>())
        .ConfigureAwait(false);
}

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