I’ve been trying to figure out the best way to provide access to connections in my services via Dependency Injection (DI) for a while now. Typically I’ll pass the IConfiguration object through to the service using DI and then retrieve the connection string directly and create a new connection object from it like so.

using (SqlConnection sqlConnection = _config.GetConnectionString("DataConnection"))
{
	...Use the connection
}

However, in an effort to move away from just injecting the IConfiguration object and using it’s properties directly I figured I’d try and put together a connection factory that could be created at startup, injected via DI into services where it’s required and then used to generate DbConnection objects of the required type.

Happily Rick Strahl has already done something similar so most of my code is borrowed from him.

DbConnectionFactory.cs

Essentially, if you don’t want to have to load all of the possible data access libraries if you’re not going to be using them you need to use reflection to create an instance of only the specific ones that are being used.

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Common;
using System.Data.SqlClient;
using System.IO;
using System.Reflection;

namespace ProductDetails.Helpers
{
    //https://weblog.west-wind.com/posts/2017/nov/27/working-around-the-lack-of-dynamic-dbproviderfactory-loading-in-net-core

    internal enum DataAccessProviderTypes
    {
        SqlServer,
        SqLite,
        MySql,
        PostgreSql,
#if NETFULL
    OleDb,
    SqlServerCompact
#endif
    }

    public interface IDbConnectionFactory
    {
        IDbConnection CreateSqlConnection(string connectionName);

        IDbConnection CreateSqLiteConnection(string connectionName);

        IDbConnection CreateMySqlConnection(string connectionName);

        IDbConnection CreatePostgreSqlConnection(string connectionName);

#if NETFULL
        IDbConnection CreateOleDbConnection(string connectionName);

        IDbConnection CreateSqlServerCompactConnection(string connectionName);
#endif
    }

    public class DbConnectionFactory : IDbConnectionFactory
    {
        private readonly IDictionary<string, string> _connectionDictionary;

        public DbConnectionFactory(IDictionary<string, string> connectionDictionary)
        {
            _connectionDictionary = connectionDictionary;
        }

        private string GetConnectionString(string connectionName)
        {
            _connectionDictionary.TryGetValue(connectionName, out string connectionString);

            if (connectionString == null)
            {
                throw new Exception(string.Format("Connection string {0} was not found", connectionName));
            }

            return connectionString;
        }

        public IDbConnection CreateSqlConnection(string connectionName)
        {
            return CreateDbConnection(GetConnectionString(connectionName), DataAccessProviderTypes.SqlServer);
        }

        public IDbConnection CreateSqLiteConnection(string connectionName)
        {
            return CreateDbConnection(GetConnectionString(connectionName), DataAccessProviderTypes.SqLite);
        }

        public IDbConnection CreateMySqlConnection(string connectionName)
        {
            return CreateDbConnection(GetConnectionString(connectionName), DataAccessProviderTypes.MySql);
        }

        public IDbConnection CreatePostgreSqlConnection(string connectionName)
        {
            return CreateDbConnection(GetConnectionString(connectionName), DataAccessProviderTypes.PostgreSql);
        }

#if NETFULL
        public IDbConnection CreateOleDbConnection(string connectionName)
        {
            return CreateDbConnection(GetConnectionString(connectionName), DataAccessProviderTypes.OleDb);
        }

        public IDbConnection CreateSqlServerCompactConnection(string connectionName)
        {
            return CreateDbConnection(GetConnectionString(connectionName), DataAccessProviderTypes.SqlServerCompact);
        }
#endif

        private IDbConnection CreateDbConnection(string connectionString, DataAccessProviderTypes providerType)
        {
            // Assume failure.
            DbConnection connection = null;

            // Create the DbProviderFactory and DbConnection.
            if (connectionString != null)
            {
                DbProviderFactory factory = DbProviderFactoryUtils.GetDbProviderFactory(providerType);

                connection = factory.CreateConnection();
                connection.ConnectionString = connectionString;
            }
            // Return the connection.
            return connection;
        }
    }

    internal static class DbProviderFactoryUtils
    {
        public static DbProviderFactory GetDbProviderFactory(DataAccessProviderTypes type)
        {
            if (type == DataAccessProviderTypes.SqlServer)
                return SqlClientFactory.Instance; // this library has a ref to SqlClient so this works

            if (type == DataAccessProviderTypes.SqLite)
            {
#if NETFULL
        return GetDbProviderFactory("System.Data.SQLite.SQLiteFactory", "System.Data.SQLite");
#else
                return GetDbProviderFactory("Microsoft.Data.Sqlite.SqliteFactory", "Microsoft.Data.Sqlite");
#endif
            }
            if (type == DataAccessProviderTypes.MySql)
                return GetDbProviderFactory("MySql.Data.MySqlClient.MySqlClientFactory", "MySql.Data");
            if (type == DataAccessProviderTypes.PostgreSql)
                return GetDbProviderFactory("Npgsql.NpgsqlFactory", "Npgsql");
#if NETFULL
    if (type == DataAccessProviderTypes.OleDb)
        return System.Data.OleDb.OleDbFactory.Instance;
    if (type == DataAccessProviderTypes.SqlServerCompact)
        return DbProviderFactories.GetFactory("System.Data.SqlServerCe.4.0");
#endif

            throw new NotSupportedException(string.Format("Unsupported Provider Factory", type.ToString()));
        }

        public static DbProviderFactory GetDbProviderFactory(string providerName)
        {
#if NETFULL
    return DbProviderFactories.GetFactory(providerName);
#else
            var providername = providerName.ToLower();

            if (providerName == "system.data.sqlclient")
                return GetDbProviderFactory(DataAccessProviderTypes.SqlServer);
            if (providerName == "system.data.sqlite" || providerName == "microsoft.data.sqlite")
                return GetDbProviderFactory(DataAccessProviderTypes.SqLite);
            if (providerName == "mysql.data.mysqlclient" || providername == "mysql.data")
                return GetDbProviderFactory(DataAccessProviderTypes.MySql);
            if (providerName == "npgsql")
                return GetDbProviderFactory(DataAccessProviderTypes.PostgreSql);

            throw new NotSupportedException(string.Format("Unsupported Provider Factory", providerName));
#endif
        }

        public static DbProviderFactory GetDbProviderFactory(string dbProviderFactoryTypename, string assemblyName)
        {
            var instance = GetStaticProperty(dbProviderFactoryTypename, "Instance");
            if (instance == null)
            {
                var a = LoadAssembly(assemblyName);
                if (a != null)
                    instance = GetStaticProperty(dbProviderFactoryTypename, "Instance");
            }

            if (instance == null)
                throw new InvalidOperationException(string.Format("Unable to retrieve DbProvider Factory Form", dbProviderFactoryTypename));

            return instance as DbProviderFactory;
        }

        #region Reflection Utilities
        //https://github.com/RickStrahl/Westwind.Utilities/blob/master/Westwind.Utilities/Utilities/ReflectionUtils.cs

        /// <summary>
        /// Retrieves a value from  a static property by specifying a type full name and property
        /// </summary>
        /// <param name="typeName">Full type name (namespace.class)</param>
        /// <param name="property">Property to get value from</param>
        /// <returns></returns>
        public static object GetStaticProperty(string typeName, string property)
        {
            Type type = GetTypeFromName(typeName);
            if (type == null)
                return null;

            return GetStaticProperty(type, property);
        }

        /// <summary>
        /// Returns a static property from a given type
        /// </summary>
        /// <param name="type">Type instance for the static property</param>
        /// <param name="property">Property name as a string</param>
        /// <returns></returns>
        public static object GetStaticProperty(Type type, string property)
        {
            object result = null;
            try
            {
                result = type.InvokeMember(property, BindingFlags.Static | BindingFlags.Public | BindingFlags.GetField | BindingFlags.GetProperty, null, type, null);
            }
            catch
            {
                return null;
            }

            return result;
        }

        /// <summary>
        /// Helper routine that looks up a type name and tries to retrieve the
        /// full type reference using GetType() and if not found looking 
        /// in the actively executing assemblies and optionally loading
        /// the specified assembly name.
        /// </summary>
        /// <param name="typeName">type to load</param>
        /// <param name="assemblyName">
        /// Optional assembly name to load from if type cannot be loaded initially. 
        /// Use for lazy loading of assemblies without taking a type dependency.
        /// </param>
        /// <returns>null</returns>
        public static Type GetTypeFromName(string typeName, string assemblyName)
        {
            var type = Type.GetType(typeName, false);
            if (type != null)
                return type;

            var assemblies = AppDomain.CurrentDomain.GetAssemblies();
            // try to find manually
            foreach (Assembly asm in assemblies)
            {
                type = asm.GetType(typeName, false);

                if (type != null)
                    break;
            }
            if (type != null)
                return type;

            // see if we can load the assembly
            if (!string.IsNullOrEmpty(assemblyName))
            {
                var a = LoadAssembly(assemblyName);
                if (a != null)
                {
                    type = Type.GetType(typeName, false);
                    if (type != null)
                        return type;
                }
            }

            return null;
        }

        /// <summary>
        /// Overload for backwards compatibility which only tries to load
        /// assemblies that are already loaded in memory.
        /// </summary>
        /// <param name="typeName"></param>
        /// <returns></returns>        
        public static Type GetTypeFromName(string typeName)
        {
            return GetTypeFromName(typeName, null);
        }

        /// <summary>
        /// Try to load an assembly into the application's app domain.
        /// Loads by name first then checks for filename
        /// </summary>
        /// <param name="assemblyName">Assembly name or full path</param>
        /// <returns>null on failure</returns>
        public static Assembly LoadAssembly(string assemblyName)
        {
            Assembly assembly = null;
            try
            {
                assembly = Assembly.Load(assemblyName);
            }
            catch { }

            if (assembly != null)
                return assembly;

            if (File.Exists(assemblyName))
            {
                assembly = Assembly.LoadFrom(assemblyName);
                if (assembly != null)
                    return assembly;
            }
            return null;
        }
#endregion Reflection Utilities
    }
}

DbConnectionFactoryServiceCollectionExtensions.cs

To make it easier to use I then created a ServiceCollection extension. This passes through a Dictionary of connection names and connection strings to the DbConnectionFactory.

using Microsoft.Extensions.DependencyInjection;
using System;
using System.Collections.Generic;

namespace ProductDetails.Helpers
{
    public static class DbConnectionFactoryServiceCollectionExtensions
    {
        public static IServiceCollection AddDbConnectionFactory(this IServiceCollection collection, Dictionary<string, string> connections)
        {
            if (collection == null) throw new ArgumentNullException(nameof(collection));

            return collection.AddTransient<IDbConnectionFactory, DbConnectionFactory>(factory => new DbConnectionFactory(connections));
        }
    }
}

Startup.cs

The DbConnectionFactory can then be initialised in the ConfigureServices method of your Styartup file by passing though a dictionary of connection names and connection strings, in the below case coming from the ConnectionStrings section of appsettings.config.

private static void ConfigureServices(IServiceCollection serviceCollection)
{
	// Add logging
	serviceCollection.AddSingleton(LoggerFactory.Create(builder =>
	{
		builder
			.AddSerilog(dispose: true);
	}));

	serviceCollection.AddLogging();

	// Add access to generic IConfigurationRoot
	serviceCollection.AddSingleton<IConfigurationRoot>(configuration);

	// Add cache service
	serviceCollection.AddSingleton<ICacheService, InMemoryCache>();

	// Add API client
	serviceCollection.AddProductApi();

	// Add SQL client
	serviceCollection.AddDbConnectionFactory(configuration.GetSection("ConnectionStrings").Get<Dictionary<string, string>>());
	serviceCollection.AddProductSql();

	// Add app
	serviceCollection.AddTransient<App>();
}

ProductSqlClient.cs

I then created a helper method in my client to cast the returned DbConnection to the desired SqlConnection and to fetch only the particular connection required in the client.

private SqlConnection GetSqlConnection()
{
	return (SqlConnection)_dbConnectionFactory.CreateSqlConnection("DataConnection");
}

This can then be used like so.

public async Task<DataSet> GetDataSetAsync(List<string> tableNames, string schema = "dbo")
{
	DataSet dataSet = new DataSet();

	using (SqlConnection sqlConnection = GetSqlConnection())
	{
		...Use the connection
	}

	return dataSet;
}

This is admitadly much more convoluted than simply creating a SqlConnection from a connection string in the configuration file but should hopefully prove it’s worth in projects with more complicated data access requirements and ones which use multiple data providers.


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