Abstract Data Access Layer Design

using System;
using System.Collections.Generic;
using System.Text;

namespace XPeriment.DataAccessTasks
{
    public class ScriptBuilder
    {
        public virtual string FormatValue(object obj)
        {
            if (obj is string)
                return "'" + obj.ToString() + "'";
            else if (obj is DateTime)
                return "'" + obj.ToString() + "'";
            else if (obj is Guid)
                return "'" + obj.ToString() + "'";
            else if (obj is bool)
                return (((bool)obj) == true)?"1":"0";
            else if (obj == null)
                return " NULL ";
            else if (obj == DBNull.Value)
                return " NULL ";
            else
                return obj.ToString();
        }
        
        public string GetScript(QueryTask query)
        {
            string retval = "SELECT ";
            if (query.Fields.Count == 0)
                retval += " * ";

            int count = 0;
            foreach(FieldData fd in query.Fields.Values)
            {
                retval += " " + fd.Name;
                if (count < query.Fields.Count - 1)
                    retval += " , ";
                count++;
            }

            retval += " FROM " + query.BaseTable;

            foreach (JoinData jd in query.Joins)
                retval += " " + jd.JoinType + " JOIN " + jd.TargetTable + " ON " + jd.Condition;

            if (query.Condition != string.Empty)
                retval += " WHERE " + query.Condition;

            return retval;
        }

        public string GetScript(InsertTask task)
        {
            string retval = "INSERT INTO " + task.BaseTable;
            retval += " (";
            int fieldcount = 0;
            foreach(string key in task.Fields.Keys)
            {
                retval += key;
                if (fieldcount < task.Fields.Count - 1)
                    retval += ",";
                fieldcount++;
            }
            retval += ") VALUES (";

            fieldcount = 0;
            foreach (string key in task.Fields.Keys)
            {
                retval += FormatValue((task.Fields[key] as FieldValueData).Value);
                if (fieldcount < task.Fields.Count - 1)
                    retval += ",";
                fieldcount++;
            }

            retval += ")";
            return retval;

        }

        public string GetScript(DeleteTask delete)
        {
            string retval = "DELETE " + delete.BaseTable + " WHERE " + delete.Condition ;
            return retval;
        }

        public string GetScript(UpdateTask update)
        {
            string retval = "UPDATE " + update.BaseTable;
            retval += " SET ";
            int count = 0;
            foreach (FieldData fd in update.Fields.Values)
            {
                FieldValueData fvd = fd as FieldValueData;
                retval += " " + fvd.Name + " = " + FormatValue(fvd.Value);
                if (count < update.Fields.Count - 1)
                    retval += ",";
                count++;
            }
            retval += " WHERE " + update.Condition;
            return retval;
        }
    }
}