DependencyInjection.md

Dependency Injection

ASP.NET Core has a built in dependency injection framework to make it easy for your Controllers and Razor pages to get services they require. Services are registered with the dependency injection framework in the Startup class's ConfigureServices method.

The services for our DynamoDB data access layer have already been registered in the provided code but let's take look at how it was done.

Injecting AWS Services

The NuGet package AWSSDK.Extensions.NETCore.Setup adds new extension methods to the IServiceCollection object. We can add DynamoDB by adding the following line of code.

services.AddAWSService<Amazon.DynamoDBv2.IAmazonDynamoDB>();

The AddAWSService method uses the service interface, in this case IAmazonDynamoDB, to identify the AWS service. When an object is created that takes in an IAmazonDynamoDB as part of the constructor the service client will be create from the IServiceCollection. Since the IConfiguration object is also added to the IServiceCollection the AWSSDK.Extensions.NETCore.Setup package will look for the configuration values like region and profile that we recently set to construct the service client.

Adding our data access interface

In the DynamoDB section of this tutorial we showed ITODOListDataAccess interface and its implementation TODOListDataAccess which provides our APIs to access our TODO lists. The interface ITODOListDataAccess is also added to the IServiceCollection with the following line of code.

services.AddSingleton(typeof(ITODOListDataAccess), typeof(TODOListDataAccess));

How does it work

Now that we have both the AWS DynamoDB service client and our custom data access layer registered lets take a look at how the dependency injection work.

If we take a look at the file ServerlessTODOList.Frontend\Pages\MyLists.cshtml.cs which contains the class MyListsModel you can see it takes in an instance of ITODOListDataAccess as part of its constructor. When MyListsModel is created by the ASP.NET Core framework it looks to IServiceCollection for an instance of ITODOListDataAccess.

Since ITODOListDataAccess was registered as a singleton to the IServiceCollection it will be created the first time it is requested. TODOListDataAccess was specified to IServiceCollection as the implementer of ITODOListDataAccess but TODOListDataAccess takes in an instance of IAmazonDynamoDB. So now IServiceCollection will create an instance of IAmazonDynamoDB and pass that into the constructor of TODOListDataAccess which will then be passed in MyListsModel.

public class MyListsModel : PageModel
{
    ITODOListDataAccess DataAccess { get; set; }

    public IList<TODOList> TODOLists { get; set; }

    public MyListsModel(ITODOListDataAccess dataAccess)
    {
        this.DataAccess = dataAccess;
    ...

public class TODOListDataAccess : ITODOListDataAccess
{
    DynamoDBContext Context { get; set; }

    public TODOListDataAccess(IAmazonDynamoDB ddbClient)
    {
        var config = new DynamoDBContextConfig
        {
            Conversion = DynamoDBEntryConversion.V2,
            ConsistentRead = true
        };
        this.Context = new DynamoDBContext(ddbClient);
    ...

Full version of ConfigureServices

Here is the full version of ConfigureServices from the Startup class that handles our dependency injection. This method will have more changes in upcoming sections.

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        // This lambda determines whether user consent for non-essential cookies is needed for a given request.
        options.CheckConsentNeeded = context => true;
        options.MinimumSameSitePolicy = SameSiteMode.None;
    });

    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

    // AddAWSService is provided by the AWSSDK.Extensions.NETCore.Setup NuGet package.
    services.AddAWSService<Amazon.DynamoDBv2.IAmazonDynamoDB>();
    services.AddSingleton(typeof(ITODOListDataAccess), typeof(TODOListDataAccess));

    // Default SQL Server Entity Framework Identity setup
    services.AddDbContext<ApplicationDbContext>(options =>
        options.UseSqlServer(
            Configuration.GetConnectionString("DefaultConnection")));
    services.AddDefaultIdentity<IdentityUser>()
        .AddEntityFrameworkStores<ApplicationDbContext>();
}

---

• Getting Started
• What is a serverless application?
• Common AWS Serverless Services
• What are we going to build in this tutorial?
• TODO List AWS Services Used
• Using DynamoDB to store TODO Lists
• Handling service events with Lambda
• Getting ASP.NET Core ready for Serverless
  • Dependency Injection
  • Using Amazon Cognito for Identity
  • Persisting ASP.NET Core Data Protection Keys
  • AWS Systems Manager Parameter Store for Managing Configuration
  • ASP.NET Core wrap up
• Deploying ASP.NET Core as a Serverless Application
• Tear Down
• Final Wrap Up

Continue on to next page: Using Amazon Cognito for Identity