DataAccess

Introduction

A simple data access base repository that can be used in other projects

Usage

Full example project can be found here https://github.com/Ian-Webster/sandbox/tree/main/nuget-samples/DataAccess.Sample

Install NuGet packages

Install the following NuGet packges into your project

1. DataAccess.Repository
2. Microsoft.EntityFrameworkCore
3. Microsoft.EntityFrameworkCore.Relational
4. Your choice of EF core database targeting package (for example https://www.nuget.org/packages/Microsoft.EntityFrameworkCore.SqlServer)

Create entites and entity type configurations

1. Create your database entity classes, as an example a movie entity;

   public class Movie
   {
   	public Guid MovieId { get; set; }
   	public string Name { get; set; }
   }

2. Create your entity type configuration classes, as an example an entity type configuration for the movie entity;

   public class MovieEntityTypeConfiguration: IEntityTypeConfiguration<Movie>
   {
   	public void Configure(EntityTypeBuilder<Movie> builder)
   	{
   		builder.HasKey(pk => pk.MovieId);
   		builder.ToTable(nameof(Movie), SchemaNames.Public);
   	}
   }

Create a database context class

You will need to create a database context class that inherits from DbContext

As an example here is the database context class for the Movie database;

public class MovieContext: DbContext // inherit from DbContext
{
	// inject DbContextOptions and base to the base class
    public MovieContext(DbContextOptions options): base(options)
    {        
    }

	// override OnModelCreating so we can apply our entity type configurations
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
		// get the the assembly your EntityTypeConfigurations are in
		// in this case they are in the same assembly as this context class
        var assembly = Assembly.GetAssembly(typeof(MovieContext));
        if (assembly == null)
        {
            throw new Exception($"Failed to get assembly for {nameof(MovieContext)}");
        }       
        modelBuilder.ApplyConfigurationsFromAssembly(assembly);
    }
}

Create repository classes

As an example the repository class for the movie entity;

using DataAccess.Repository;
using DataAccess.Sample.Data.DatabaseContexts;
using DataAccess.Sample.Domain.Entities;

public class MovieRepository: IMovieRepository
{
    private readonly IRepository<Movie> _movieRepository;
	// inject UnitOfWork for creating IRepository instances
    public MovieRepository(UnitOfWork<MovieContext> unitOfWork)
    {
		// create an instance of IRepository for our entity (Movie)
        _movieRepository = unitOfWork.Repository<Movie>();
    }

    public async Task<Movie?> GetMovieById(Guid movieId, CancellationToken token)
    {
		// perform a database read for the Movie entity
        return await _movieRepository.FirstOrDefault(m => m.MovieId == movieId, token);
    }
}

Configure IoC

In your IoC bootstrapping you need to;

1. Add your database context, as an example here is the set up for the MovieContext connecting to a postgres database;

   builder.Services.AddDbContext<MovieContext>(options =>
   {
   	options.UseNpgsql(builder.Configuration.GetConnectionString("PostgresConnection"));
   });

2. Set up your services;

   // add RepositoryFactory (this will be needed by your repository class)
   builder.Services.AddScoped<UnitOfWork<LibraryDatabaseContext>>();
   builder.Services.AddScoped<RepositoryFactory<MovieContext>>();
   // add your repositories
   builder.Services.AddScoped<IMovieRepository, MovieRepository>();

Thread safety

The DBContext class is not thread safe, the refactored version of this library does help improve thread safety but there are still potential issues, take the following code as an example;

public class BookController : ControllerBase
{
    private readonly IBookRepository _bookRepository;

    public BookController(IBookRepository bookRepository)
    {
        _bookRepository = bookRepository;
    }

    [HttpGet("theadTest")]
    public async Task<ActionResult> ThreadTest()
    {
        var task1 = Task.Run(() => _bookRepository.GetAllBooks(new CancellationToken()));
        var task2 = Task.Run(() => _bookRepository.GetAllBooks(new CancellationToken()));
		
        await Task.WhenAll(task1, task2);
        
        return Ok();
    }
}

the line await Task.WhenAll(task1, task2); will throw an exception "System.InvalidOperationException: A second operation was started on this context instance before a previous operation completed. This is usually caused by different threads concurrently using the same instance of DbContext. For more information on how to avoid threading issues with DbContext, see https://go.microsoft.com/fwlink/?linkid=2097913", the reason for this is that _bookRepository is scoped per request but we are making two separate calls to the database re-using the same DBContext leading to our thread error.

To fix the issue we must ensure that each of the tasks receives it's own instance of DBContext, we modify the ThreadTest method to look like this;

[HttpGet("theadTest")]
public async Task<ActionResult> ThreadTest()
{
    var task1 = Task.Run(async () =>
    {
        using var scope = _serviceScopeFactory.CreateScope();
        var repo = scope.ServiceProvider.GetRequiredService<IBookRepository>();
        await repo.GetAllBooks(Token);
    });

    var task2 = Task.Run(async () =>
    {
        using var scope = _serviceScopeFactory.CreateScope();
        var repo = scope.ServiceProvider.GetRequiredService<IBookRepository>();
        await repo.GetAllBooks(Token);
    });

    await Task.WhenAll(task1, task2);

    return Ok();
}

in addition we must in inject IServiceScopeFactory into our controller.

With the above change in place we ensure each of the two tasks receives their own instance of UnitOfWork and therefore their own instance of a DBContext.

Version history

• 3.0.1 - update documentation
• 3.0.0
  • refactored repository factory to follow unit of work pattern
  • added new functionality to IRepository for optional take on List, projection and paging
  • version bumps to NuGetPackages
• 2.0.0 - work in progress commit to support the DataAccess.Repository.HotChocolate package
• 1.0.4 - added usage instructions to readme file
• 1.0.3 - updated main readme file
• 1.0.2 - changes to main build action to attempt to fix NuGet publishing issue
• 1.0.1 - testing NuGet publish on merge to main post pull request
• 1.0.0 - initial test publication of the NuGet package