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experiment_software_csharp_12_net_8

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entry.State = EntityState.Detached;
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}
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throw;
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}
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}
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The preceding implementation just calls the SaveChangesAsync DbContext context method, which saves all changes to the database, but then it intercepts all concurrency exceptions and detaches all the entities involved in the concurrency error from the context. This way, the next time a command retries the whole failed operation, their updated versions will be reloaded from the database.
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In other words, when the update fails because of the interference of transaction B, we allow the interfering transaction B to complete its process. Then, EF automatically reloads all entities modified by B that contain the value of the concurrency check modified by B. This way, when the operation is retried, if no other transaction interferes, there will be no conflict on the concurrency check.
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The practical usage of concurrency checks is detailed in the A frontend microservice example of Chapter 21, Case Study.
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All repository implementations are defined in a Repositories folder to ensure better maintainability.
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Finally, all repositories are automatically discovered and added to the application DI engine, calling the AddAllRepositories method, which is defined in the DDD tools we added to the domain layer project. More details on how to ensure this method is called when the application starts are given in the example detailed description in the A frontend microservice section of Chapter 21, Case Study.
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Defining the application layer
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The application layer contains the definition of all business operations. These business operations use data provided by the user to modify domain layer abstraction aggregates, such as touristic packages. When all business operations involved in the current user request have been performed, an IUnitOfWork.SaveEntitiesAsync() operation is performed to save all changes to the database.
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As a first step, for simplicity, let’s freeze the application culture to en-US by adding the following code to the ASP.NET Core pipeline:
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app.UseAuthorization();
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// Code to add: configure the Localization middleware
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var ci = new CultureInfo(`en-US`);
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app.UseRequestLocalization(new RequestLocalizationOptions
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{
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DefaultRequestCulture = new RequestCulture(ci),
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SupportedCultures = new List<CultureInfo>
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{
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ci,
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},
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SupportedUICultures = new List<CultureInfo>
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{
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ci,
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}
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});
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As a second step, we can create a Tools folder to place the ApplicationLayer code, which you can find in the ch7 code of the GitHub repository associated with this book. With these tools in place, in Program.cs, we can add the code that automatically discovers and adds all queries, command handlers, and event handlers to the DI engine, as shown here:
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...
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...
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builder.Services.AddAllQueries(this.GetType().Assembly);
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builder.Services.AddAllCommandHandlers(this.GetType().Assembly);
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builder.Services.AddAllEventHandlers(this.GetType().Assembly);
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Then, we must add a Queries folder to place all queries and their associated interfaces. As an example, let’s have a look at the query that lists all packages:
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public class PackagesListQuery:IPackagesListQuery
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{
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private readonly MainDbContext ctx;
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public PackagesListQuery(MainDbContext ctx)
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{
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this.ctx = ctx;
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}
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public async Task<IReadOnlyCollection<PackageInfosViewModel>> GetAllPackages()
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{
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return await ctx.Packages.Select(m => new PackageInfosViewModel
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{
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StartValidityDate = m.StartValidityDate,
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...
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})
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.OrderByDescending(m=> m.EndValidityDate)
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.ToListAsync();
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}
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}
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The query object is automatically injected into the application DB context. The GetAllPackages method uses LINQ to project all of the required information into PackageInfosViewModel and sorts all results in descending order on the EndValidityDate property.
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The Commands folder contains all commands. As an example, let’s have a look at the command used to modify packages:
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public class UpdatePackageCommand: ICommand
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{
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public UpdatePackageCommand(IPackageFullEditDTO updates)
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{
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Updates = updates;
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}
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public IPackageFullEditDTO Updates { get; private set; }
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}
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Command handlers can be placed in the Handlers folder. It is worth analyzing the command that updates packages:
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private readonly IPackageRepository repo;
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private readonly IEventMediator mediator;
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public UpdatePackageCommandHandler(IPackageRepository repo, IEventMediator mediator)
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{
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this.repo = repo;
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this.mediator = mediator;
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}
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Its constructor has automatically injected the IPackageRepository repository and an IEventMediator instance needed to trigger event handlers. The following code also shows the implementation of the standard HandleAsync command handler method:
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public async Task HandleAsync(UpdatePackageCommand command)
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{
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bool done = false;
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IPackage model;
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while (!done)
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{
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try
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{
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model = await repo.Get(command.Updates.Id);
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if (model == null) return;
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model.FullUpdate(command.Updates);
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await mediator.TriggerEvents(model.DomainEvents);
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await repo.UnitOfWork.SaveEntitiesAsync();
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done = true;
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}
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catch (DbUpdateConcurrencyException)
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{
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// add some logging here
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}
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}
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}