Custom Configuration Providers in ASP.NET Core

Introduction

A friend of mine had a requirement in which he was interested to put the configuration settings of an ASP.NET Core application inside a SQL Server table and read them at startup of the application with a mechanism similar to what we already have for JSON files; also he wanted to have the ability that these variable being overridden or override other settings if applicable. In this article, I am going to describe how that could be achievable and how to implement such feature.

IConfigurationSource & ConfigurationProvider

Creating such a mechanism is twofold, first you need to create a configuration source and then a configuration provider. The former implements IConfigurationSource interface, and can hold the settings of your configuration, and it should implement the Build method, which returns an IConfigurationProvider instance that the IConfigurationBuilder can leverage to build the configurations of the application. Let’s see them in action:

public class SqlServerConfigurationSource : IConfigurationSource
{
    public string Server { get; set; } = "localhost";
    public string Database { get; set; } = "ConfigurationDb";
    public string User { get; set; } = "sa";
    public string Password { get; set; } = "yourStrong(!)Password";
    public string Table { get; set; } = "dbo.Configuration";
    public string KeyColumn { get; set; } = "Key";
    public string ValueColumn { get; set; } = "Value";

    public IConfigurationProvider Build(IConfigurationBuilder builder)
            => new SqlServerConfigurationProvider(this);
}

As you can see, it only has some properties which are holding the required values for the concrete IConfigurationProvider , and in the Build method it simply returns an instance of the concrete class that inherits from the ConfigurationProvider.

public class SqlServerConfigurationProvider : ConfigurationProvider , IDisposable
{
    private readonly SqlServerConfigurationSource _configurationSource;

    public SqlServerConfigurationProvider(SqlServerConfigurationSource configurationSource)
    {
        _configurationSource = configurationSource;
    }

    public override void Load()
    {
        // here you should connect to sql server and fetch the configurations
    }

    public void Dispose()
    {
        // cleanup all the resources
    }
}

In the concrete configuration provider you should override the Load method and fetch the configurations from the external storage, in this case, SQL Server; but as you see it can simply be any kind of storage.

Easy!? You’re right; the next step could be to add some extension methods to facilitate the usage of these two classes, so the developers could call AddSqlServerDb in the ConfigureAppConfiguration in the Program.cs, for instance, to be able to load the configuration from SQL Server.

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .ConfigureAppConfiguration(configure =>
        {
            configure.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true);
            configure.AddSqlServerDb();
        })
        .ConfigureWebHostDefaults(webBuilder => { webBuilder.UseStartup<Startup>(); });

With the above-mentioned configuration settings, if there is an intersection between keys defined in the appsettings.json file and the SqlServer those in the SQL Server will override it.

Another thing to mention is that you should create the database and tables manually for this sample:

CREATE  TABLE dbo.Configuration(

        [Key]   NVarChar(50)    NOT NULL PRIMARY KEY
    ,   [Value] NVarCHar(50)    NOT NULL
)
GO

Conclusion

That’s it. You could take advantage of these two primary classes to bring flexibility to your applications and read the required configuration from any external storage. Find the source code for this article on this GitHub repository.

If you are interested I wrote an open source library to ease this for everyone, please share and give a star if you liked it. Have a great day! And enjoy coding 😉