Abstract Data Access Layer Design

using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Data.SqlClient;

namespace XPeriment.DataAccessTasks.SqlServer
{
    public class SqlTaskPerformer:DataTaskPerformer
    {
        private SqlConnection currentconnection = null;

        public override ScriptBuilder ScriptBuilder
        {
            get { return new ScriptBuilder(); }
        }

        public override void Connect()
        {
            if (currentconnection == null)
                currentconnection = new SqlConnection(ConnectionString);
            if (currentconnection.State != ConnectionState.Open)
                currentconnection.Open();
        }

        private DataTable RunQuery(QueryTask query)
        {
            Connect();
            DataTable retval = new DataTable();
            SqlDataAdapter da = new SqlDataAdapter(ScriptBuilder.GetScript(query), currentconnection);
            da.Fill(retval);
            return retval;
        }

        private void RunUpdate(UpdateTask task)
        {
            Connect();
            string cmdstring = ScriptBuilder.GetScript(task);
            SqlCommand cmd = null;
            if (task.CurrentTransaction != null)
                cmd = new SqlCommand(cmdstring, currentconnection, (task.CurrentTransaction as SqlPerformerTransaction).SqlTransaction);
            else
                cmd = new SqlCommand(cmdstring, currentconnection);
            cmd.ExecuteNonQuery();
        }

        private void RunDelete(DeleteTask task)
        {
            Connect();
            string cmdstring = ScriptBuilder.GetScript(task);
            SqlCommand cmd = null;
            if (task.CurrentTransaction != null)
                cmd = new SqlCommand(cmdstring, currentconnection, (task.CurrentTransaction as SqlPerformerTransaction).SqlTransaction);
            else
                cmd = new SqlCommand(cmdstring, currentconnection);
            cmd.ExecuteNonQuery();
        }

        private void RunInsert(InsertTask task)
        {
            Connect();
            string cmdstring = ScriptBuilder.GetScript(task) + " SELECT SCOPE_IDENTITY() ";
            SqlCommand cmd = null;
            if (task.CurrentTransaction != null)
                cmd = new SqlCommand(cmdstring, currentconnection, (task.CurrentTransaction as SqlPerformerTransaction).SqlTransaction);
            else
                cmd = new SqlCommand(cmdstring, currentconnection);
            object identity = cmd.ExecuteScalar();
            task.ScopeIdentity = (identity is decimal)?(decimal)identity:0;
        }

        public override PerformerTransaction CreateTransaction()
        {
            Connect();
            return new SqlPerformerTransaction(currentconnection.BeginTransaction());
        }

        public override DataTable Run(BaseTask task)
        {
            if (task is QueryTask)
                return RunQuery(task as QueryTask);
            else if (task is InsertTask)
                RunInsert(task as InsertTask);
            else if (task is UpdateTask)
                RunUpdate(task as UpdateTask);
            else if (task is DeleteTask)
                RunDelete(task as DeleteTask);
            else
                throw new Exception("Data access task type not supported");
            return null;
        }

    }
}