简单工厂模式构建数据访问层以实现多种数据库之间的切换

1、新建一个 数据库访问基类DBHandler

///
/// 数据库访问基类
///
public abstract class DBHandler
{
    public DBHandler() { }

    #region 需要在子类中初始化的与数据库相关的特征类

    protected DbConnection dbConnection = null; //连接对象
    protected DbTransaction dbTransaction = null; //事务对象
    protected abstract DbCommand CreateCommand(); //从子类中构建DbCommand对象
    protected abstract DbDataAdapter CreateAdapter(); //从子类中构建DbDataAdapter对象
    protected abstract void BuilderCommand(DbDataAdapter adapter); //用于Update方法，构建DbDataAdapter中的UpdateCommand/InsertCommand/DeleteCommand
    protected abstract int GetTotalCount(); //用于分页查询，获取总记录数

    #endregion

    #region 子类中要用的数据或方法

    protected List parameters = new List();
    protected bool IsInTransaction = false; //是否处于事务当中
    //用于分页查询，检查当前SQL是否符合基本查询要求
    protected void CheckPageSQL()
    {
        this.CommandType = CommandType.Text;
        if (!this.CommandText.StartsWith("select", true, null))
        {
            throw new Exception("sql语句必须是select开头");
        }
        if (IsInTransaction)
        {
            throw new Exception("分页查询不能在事务中");
        }
    }

    #endregion

    #region  用于输入初始条件的属性和方法

    ///
    /// sql语句或存储过程名称
    ///
    public string CommandText { get; set; }

    ///
    /// 执行类型是SQL还是存储过程
    ///
    public CommandType CommandType { get; set; }

    ///
    /// 添加参数，将参数添加到List列表中保存起来
    ///
    /// 参数名
    /// 参数值
    public void AddParameter(string paraName, string paraValue)
    {
        this.parameters.Add(new Parameter(paraName,paraValue));
    }

    ///
    /// 清空当前参数列表
    ///
    public void ClearParameter()
    {
        this.parameters.Clear();
    }

    ///
    /// 用于分页查询，获取查询到的总记录数
    ///
    public int TotalCount
    {
        get
        {
            return this.GetTotalCount();
        }
    }

    #endregion

    #region 获取数据库的返回值

    ///
    /// 获取执行结果的第一行第一列的值
    ///
    ///
    public object ExecuteScalar()
    {
        try
        {
            if (dbConnection.State != ConnectionState.Open)
            {
                dbConnection.Open();
            }
            DbCommand cmd = this.CreateCommand();
            object r = cmd.ExecuteScalar();
            if (!this.IsInTransaction)
            {
                dbConnection.Close();
            }
            return r;
        }
        catch (Exception ex)
        {
            this.dbConnection.Close();
            throw new Exception(ex.Message);
        }
    }

    ///
    /// 执行没有数据集的命令 Update/Delete/Insert
    ///
    ///
    public int ExecuteNonQuery()
    {
        try
        {
            if (this.dbConnection.State != ConnectionState.Open)
            {
                this.dbConnection.Open();
            }
            DbCommand cmd = this.CreateCommand();
            int r = cmd.ExecuteNonQuery();
            if (!IsInTransaction)
            {
                dbConnection.Close();
            }
            return r;
        }
        catch (Exception ex)
        {
            dbConnection.Close();
            throw new Exception(ex.Message);
        }
    }

    ///
    /// 执行结果的DataTable集
    ///
    ///
    public DataTable ExecuteDataTable()
    {
        try
        {
            if (this.dbConnection.State != ConnectionState.Open)
            {
                this.dbConnection.Open();
            }
            DbDataAdapter adapter = this.CreateAdapter();
            DataTable dt = new DataTable();
            adapter.FillSchema(dt, SchemaType.Mapped);
            adapter.Fill(dt);
            if (!IsInTransaction)
            {
                dbConnection.Close();
            }
            return dt;
        }
        catch (Exception ex)
        {
            dbConnection.Close();
            throw new Exception(ex.Message);
        }
    }

    ///
    /// 用于分页查询，获取指定页码的数据集
    ///
    /// 每页记录数
    /// 当前页码
    /// 指定页的记录集
    public abstract DataTable ExecuteDataTable(int pageSize,int currentPageIndex);

    #endregion

    #region 将DataTable更新到数据库中

    public int UpdateData(DataTable dt)
    {
        try
        {
            if (this.dbConnection.State != ConnectionState.Open)
            {
                this.dbConnection.Open();
            }
            DbDataAdapter adapter = this.CreateAdapter();
            if (this.CommandType == CommandType.StoredProcedure)
            {
                this.CommandType = CommandType.Text;
            }
            this.BuilderCommand(adapter);
            int r = adapter.Update(dt);
            if (!IsInTransaction)
            {
                dbConnection.Close();
            }
            return r;
        }
        catch (Exception ex)
        {
            throw new Exception(ex.Message);
        }
    }

    #endregion

    #region 事务处理

    ///
    /// 开始一个事务
    ///
    public void BegionTransaction()
    {
        try
        {
            if (this.dbConnection.State != ConnectionState.Open)
            {
                this.dbConnection.Open();
            }
            this.dbConnection.BeginTransaction();
            this.IsInTransaction = true;
        }
        catch (Exception ex)
        {
            this.dbConnection.Close();
            this.IsInTransaction = false;
            throw ex;
        }
    }

    ///
    /// 回滚一个事务
    ///
    public void RollbackTransaction()
    {
        try
        {
            this.dbTransaction.Rollback();
            this.dbConnection.Close();
            this.IsInTransaction = false;
        }
        catch (Exception ex)
        {
            this.dbConnection.Close();
            this.IsInTransaction = false;
            throw ex;
        }
    }

    ///
    /// 提交一个事务
    ///
    public void CommitTransaction()
    {
        try
        {
            this.dbTransaction.Commit();
            this.dbConnection.Close();
            this.IsInTransaction = false;
        }
        catch (Exception ex)
        {
            this.dbConnection.Close();
            this.IsInTransaction = false;
            throw ex;
        }
    }

    #endregion

    #region 建立数据行对象


    #endregion

    #region 对序列的读取

    ///
    /// 获取序列的值，对于非oracle数据库，必须建立一个名为System_Sequence的表，表的字段为（Name(nvarchar,50),Value(int))
    ///
    ///
    ///
    public abstract int GetSequenceValue(string sequenceName);

    #endregion
}

2、新建一个参数类

public class Parameter
 {
    public string Name = string.Empty;
    public object Value = null;
    public Parameter(string name,string value)
    {
        this.Name = name;
        this.Value = value;
    }
 }

3、新建一个Sqlser数据库访问类

internal class DBhANDlerSQLServer:DBHandler
 {
     public DBhANDlerSQLServer(string connectionString)
         : base()
     {
         this.dbConnection = new SqlConnection(connectionString);
     }
     protected override DbCommand CreateCommand()
     {
         SqlCommand cmd = new SqlCommand();
         cmd.Connection = (SqlConnection)this.dbConnection;
         if (this.IsInTransaction)
         {
             cmd.Transaction = (SqlTransaction)this.dbTransaction;
         }
         if (this.CommandType == CommandType.TableDirect)
         {
             cmd.CommandType = CommandType.Text;
             cmd.CommandText = string.Format("select * from {0}", this.CommandText);
         }
         else
         {
             cmd.CommandType=CommandType;
             cmd.CommandText = CommandText;
         }
         if (this.parameters.Count > 0)
         {
             foreach (Parameter p in parameters)
             {
                 cmd.Parameters.AddWithValue(p.Name, p.Value);
             }
         }
         return cmd;
     }

     protected override DbDataAdapter CreateAdapter()
     {
         SqlCommand cmd = (SqlCommand)this.CreateCommand();
         SqlDataAdapter adapter = new SqlDataAdapter(cmd);
         return adapter;
     }

     protected override void BuilderCommand(DbDataAdapter adapter)
     {
         new SqlCommandBuilder((SqlDataAdapter)adapter);
     }

     protected override int GetTotalCount()
     {
         this.CheckPageSQL();
         string sql = this.CommandText; //保留原始SQL
         string sqlWithOutOrderField = string.Empty; //将原始SQL语句去掉 order by 后的部分，用于查询总记录数
         int startIndex = sql.LastIndexOf("order by");
         if (startIndex >= 0)
         {
             sqlWithOutOrderField = sql.Substring(0, startIndex);
         }
         else
         {
             sqlWithOutOrderField = sql;
         }
         this.CommandText = string.Format("select count(*）from ({0}) t1",sqlWithOutOrderField);
         int r = int.Parse(this.ExecuteScalar().ToString());
         this.CommandText = sql;
         return r;
     }

     public override DataTable ExecuteDataTable(int pageSize, int currentPageIndex)
     {
         this.CheckPageSQL();
         string sql = this.CommandText; //保留原始SQL
         string orderBy = string.Empty; //将order by字句保留下来
         string sqlWithSelectAndOrder = sql.Substring(6); //去掉select以及整个order by
         int startIndex = sqlWithSelectAndOrder.ToLower().LastIndexOf("order by");
         if (startIndex > 0)
         {
             orderBy = sqlWithSelectAndOrder.Substring(startIndex);
             sqlWithSelectAndOrder = sqlWithSelectAndOrder.Substring(0, startIndex);
         }
         else
         {
             throw new Exception("sql的分页查询必须有order by");
         }
         if (pageSize == 0) //返回所有数据
         {
             this.CommandText = sql;
             this.ExecuteDataTable();
         }
         DataTable dt = new DataTable();
         if (currentPageIndex == 1) //如果当前页为1
         {
             this.CommandText = string.Format("select top {0} {1} {2}", pageSize, sqlWithSelectAndOrder, orderBy);
             dt = this.ExecuteDataTable();
         }
         else  //适合sqlserver2005及以上的版本，但必须带 order by子句
         {
             StringBuilder sb = new StringBuilder();
             sb.Append("select * from ");
             sb.AppendFormat("(select Row_Number() over ({0}) as RowNum,{1})t1",orderBy,sqlWithSelectAndOrder);
             sb.AppendFormat(" where RowNum between {0} and {1}",pageSize*(currentPageIndex-1),pageSize*currentPageIndex-1);
             this.CommandText = sb.ToString();
             dt = this.ExecuteDataTable();
         }
         this.CommandText = sql;
         return dt;
     }

     public override int GetSequenceValue(string sequenceName)
     {
         //sqlserver 先要建立配套的系列表 System_Sequence
         /*建表语句：if not exists(select * from sysobjects where Name='System_Sequence')
         create table System_Sequence
         {
         Name varchar(50),
         Value int,
         constraint "PK_SsystemSequence" primary key(Name)
         };
         insert into System_Sequence(Name,Value) values('Sequence_<表名>',0); */

         this.CommandType = CommandType.Text;
         this.BegionTransaction();
         this.CommandText = string.Format("Update System_Sequence set Value=Value+1 where Name='{0}'", sequenceName);
         this.ExecuteNonQuery();
         this.CommandText = string.Format("select Value from System_Sequence where Name='{0}'", sequenceName);
         int r = this.ExecuteNonQuery();
         this.CommitTransaction();
         return r;
     }
 }

4、新建一个工厂类

public enum DatabaseType
{
    SqlServer = 1,
    Oracle = 2,
    ODBC = 3,
    OLEDB = 4
}

ublic class DBHandlerFactory
{
   //禁止产生类的实例
   private DBHandlerFactory() { }

   ///
   /// 读取webconfig中的ConnectionString配置节点构造实例
   ///
   ///
   ///
   public static DBHandler GetHandler(string connStr)
   {
      ConnectionStringSettings  ccs = ConfigurationManager.ConnectionStrings[connStr];
      string providerName = ccs.ProviderName.ToLower();
      DatabaseType dbType = DatabaseType.SqlServer;
      switch (providerName)
      {
          case "":
          case "sqlserver":
          case "system.data.sqlclient":
              dbType = DatabaseType.SqlServer;
              break;
          case "oracle":
          case"system.data.oracleclient":
              dbType = DatabaseType.Oracle;
              break;
          case "odbc":
          case "system.data.odbc":
              dbType = DatabaseType.ODBC;
              break;
          case "oledb":
          case "system.data.oledb":
              dbType = DatabaseType.OLEDB;
              break;
          default:
              throw new Exception("请按照格式定义ProviderName属性");
      }
      switch (dbType)
      {
          case DatabaseType.SqlServer:
              return new DBhANDlerSQLServer(ccs.ConnectionString);
          case DatabaseType.Oracle:
              return null;
          case DatabaseType.ODBC:
              return null;
          case DatabaseType.OLEDB:
              return null;
          default:
              return null;
      }
   }

   ///
   /// 直接以连接字符串和数据库类型构造
   ///
   ///
   ///
   ///
   public static DBHandler GetHandler(string connStr,DatabaseType dbType)
   {
       switch (dbType)
       {
           case DatabaseType.SqlServer:
               return new DBhANDlerSQLServer(connStr);
           case DatabaseType.Oracle:
               return null;
           case DatabaseType.ODBC:
               return null;
           case DatabaseType.OLEDB:
               return null;
           default:
               return null;
       }
   }
}

5.....其它的数据库访问类，可以参照sqlserver数据库访问类创建。