C# 設計模式實戰：Abstract Factory 抽象工廠模式在 ADO.NET 中的應用（三）

.NET Framework 內建抽象工廠：DbProviderFactory

ADO.NET 2.0 完整實作抽象工廠模式，解決多資料庫存取的系列產品建立問題：

抽象工廠基類

public abstract class DbProviderFactory
{
    protected DbProviderFactory() { }

    public virtual DbCommand CreateCommand() => null;
    public virtual DbCommandBuilder CreateCommandBuilder() => null;
    public virtual DbConnection CreateConnection() => null;
    public virtual DbConnectionStringBuilder CreateConnectionStringBuilder() => null;
    public virtual DbDataAdapter CreateDataAdapter() => null;
    public virtual DbDataSourceEnumerator CreateDataSourceEnumerator() => null;
    public virtual DbParameter CreateParameter() => null;
    public virtual CodeAccessPermission CreatePermission(PermissionState state) => null;

    public virtual bool CanCreateDataSourceEnumerator => false;
}

具體工廠：OleDbFactory (Access)

public sealed class OleDbFactory : DbProviderFactory
{
    public static readonly OleDbFactory Instance = new OleDbFactory();
    private OleDbFactory() { }

    public override DbCommand CreateCommand() => new OleDbCommand();
    public override DbCommandBuilder CreateCommandBuilder() => new OleDbCommandBuilder();
    public override DbConnection CreateConnection() => new OleDbConnection();
    public override DbConnectionStringBuilder CreateConnectionStringBuilder() => new OleDbConnectionStringBuilder();
    public override DbDataAdapter CreateDataAdapter() => new OleDbDataAdapter();
    public override DbParameter CreateParameter() => new OleDbParameter();
    public override CodeAccessPermission CreatePermission(PermissionState state) => new OleDbPermission(state);
}

具體工廠：SqlClientFactory (SQL Server)

public sealed class SqlClientFactory : DbProviderFactory, IServiceProvider
{
    public static readonly SqlClientFactory Instance = new SqlClientFactory();
    private SqlClientFactory() { }

    public override DbCommand CreateCommand() => new SqlCommand();
    public override DbCommandBuilder CreateCommandBuilder() => new SqlCommandBuilder();
    public override DbConnection CreateConnection() => new SqlConnection();
    public override DbConnectionStringBuilder CreateConnectionStringBuilder() => new SqlConnectionStringBuilder();
    public override DbDataAdapter CreateDataAdapter() => new SqlDataAdapter();
    public override DbDataSourceEnumerator CreateDataSourceEnumerator() => SqlDataSourceEnumerator.Instance;
    public override DbParameter CreateParameter() => new SqlParameter();
    public override CodeAccessPermission CreatePermission(PermissionState state) => new SqlClientPermission(state);

    object IServiceProvider.GetService(Type serviceType) { /* 內部實作 */ }

    public override bool CanCreateDataSourceEnumerator => true;
}

實務應用優勢

• 客戶端代碼僅依賴 DbProviderFactory 抽象介面
• 切換資料庫只需更換工廠實例，零修改業務邏輯
• 系列產品（Connection、Command、DataAdapter 等）保證相容性
• 新增資料庫支援只需實作新的 Factory，符合開閉原則

使用範例

DbProviderFactory factory = SqlClientFactory.Instance; // 或 OleDbFactory.Instance
using (DbConnection conn = factory.CreateConnection())
{
    conn.ConnectionString = config.ConnectionString;
    using (DbCommand cmd = factory.CreateCommand())
    {
        cmd.Connection = conn;
        cmd.CommandText = "SELECT * FROM Users";
        using (DbDataAdapter adapter = factory.CreateDataAdapter())
        {
            adapter.SelectCommand = cmd;
            DataTable dt = new DataTable();
            adapter.Fill(dt);
        }
    }
}