How to read connection string inside .NET Standard Class library project from ASP.NET Core

Question

In my solution, I have a ASP.NET Core web project and a .NET Standard class library project. Class library project is the data access layer and I want to read the connection string from my appsettings.json (ASP.NET Core project) in my data access layer.

I have found few answers such as by Andrii Litvinov which looks like quite straight forward to implement but he also mentioned about implementing through Dependency Injection. I don't want to choose the easy shortcut way but looking for the dependency injection implementation?

I am not sure if having appsettings.json in my class library and then registering it through IConfigurationRoot is the better option (as explained here by JRB) but in my scenario, the connection string is in the appsettings.json file of the web project and I wan't constructor dependency injection implementation in my class library project to consume the connection string.

Answer 1

You can inject an instance of a class that implements IConfiguration See Here
Let's assume in your .net core app, you have a configuration file that looks something like this:

{
  "App": {
    "Connection": {
      "Value": "connectionstring"
    }
  }
}

In your data access layer (class library) you can take a dependency on IConfiguration

public class DataAccess : IDataAccess
{
    private IConfiguration _config;

    public DataAccess(IConfiguration config)
    {
        _config = config;
    }

    public void Method()
    {
        var connectionString = _config.GetValue<string>("App:Connection:Value"); //notice the structure of this string
        //do whatever with connection string
    }
}

Now, in your ASP.net Core web project, you need to 'wire up' your dependency. In Startup.cs, I'm using this (from the default boilerplate template)

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc();
        services.AddSingleton<IConfiguration>(Configuration); //add Configuration to our services collection
        services.AddTransient<IDataAccess, DataAccess>(); // register our IDataAccess class (from class library)
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseMvc();
    }
}

Now, when your code in your class library gets executed, the ctor gets handed the instance of IConfiguration you have set up in your web app

Note: You can create strongly typed settings class if you'd prefer, see here for more information

Answer 2

I would suggest Options pattern. You can create the class with configuration data, e.g.:

public class ConnectionStringConfig
{
    public string ConnectionString { get; set; }
}

Register it on Startup:

public void ConfigureServices(IServiceCollection services)
{
   ...    
   services.Configure<ConnectionStringConfig>(Configuration);
}

and inject in your data access layer

private readonly ConnectionStringConfig config;

public Repository(IOptions<ConnectionStringConfig> config) 
{
    this.config = config.Value;
}