从基础开始，从一个SQLHelper开始

最开始考虑的问题有这三点：

1.Access和SQLServer都要能用。

2.尽量简单，清晰。

3.性能不出大问题。

public class SQLHelp
{
#region 私有域

private static string ConnStr = ConfigurationManager.ConnectionStrings["SqlConnection"].ConnectionString;
private static string ProviderType = ConfigurationManager.AppSettings["ProviderType"].ToString();
private DbConnection _Conn;
private static DbProviderFactory DbFactory=DbProviderFactories.GetFactory(ProviderType);
private DbCommand _Cmd = DbFactory.CreateCommand();

private string _CmdText;
private CommandType _CmdType;
#endregion
#region 属性
private DbConnection Conn
{
get
{ return _Conn; }
set
{ _Conn = value; }
}

public DbCommand Cmd
{
get { return _Cmd; }
set { _Cmd = value; }
}
public string CmdText
{
get { return _CmdText; }
set
{
_CmdText = value;
_Cmd.CommandText = value;
}
}

public CommandType CmdType
{
get { return _CmdType; }
set
{
_CmdType = value;
_Cmd.CommandType = value;
}
}
#endregion
#region 私有方法
private void OpenCon()
{
try
{
if (Conn == null)
{
Conn = DbFactory.CreateConnection();
}
if (Conn.State == ConnectionState.Closed)
{
Conn.ConnectionString = ConnStr;
Conn.Open();
}
}
catch (Exception ex)
{

throw (new Exception("打开数据库出错，错误" + ex.Message));
}
}
private void CloseCon()
{
if (Conn.State == ConnectionState.Open)
{
Conn.Close();
}
}
#endregion
/// <summary>
/// 执行指定的SQL语句（如添加、更新、删除等操作）
/// </summary>
/// <returns>受影响的行数</returns>
public int ExecuteNonQuery()
{
OpenCon();
try
{
return Cmd.ExecuteNonQuery();
}
catch (Exception ex)
{
throw (new Exception("数据库执行错误: " + ex.Message));
}
finally
{

CloseCon();
}
}

/// <summary>
/// 使用 DataAdapter 提取数据返回为 DataTable，并为 DataTable 指定名称
/// </summary>
/// <returns>DataTable</returns>
public DataTable ExecuteDataTable()
{
OpenCon();
try
{
using (DbDataAdapter da =DbFactory.CreateDataAdapter())
{
da.SelectCommand = Cmd;
DataTable dt = new DataTable();
da.Fill(dt);
return dt;
}
}
catch (Exception ex)
{
throw (new Exception("数据库执行错误:" + ex.Message));
}
finally
{
CloseCon();
}
}
　　}
}

写的过程发现了很多问题：每个属性该不该静态，赋初值在哪里进行，属性的写法是否合适，需不需要构造函数。最主要的疑惑：SQL语句执行完了立即释放Cmd好，还是等多条都用同一个Cmd执行好。

然后发邮件给一位学长，学长回复：


这是改后的第二版：

public class SQLHelp
{
#region 私有域

private static string ConnStr = ConfigurationManager.ConnectionStrings["SqlConnection"].ConnectionString;
private static string ProviderType = ConfigurationManager.AppSettings["ProviderType"].ToString();
private DbConnection _Conn;
private static DbProviderFactory DbFactory=DbProviderFactories.GetFactory(ProviderType);
private DbCommand _Cmd;

private string _CmdText;
private CommandType _CmdType = CommandType.Text;
#endregion
#region 属性
private DbConnection Conn
{
get
{ return _Conn; }
set
{ _Conn = value; }
}

public DbCommand Cmd
{
get { return _Cmd; }
set { _Cmd = value; }
}
public string CmdText
{
get { return _CmdText; }
set { _CmdText = value;}
}

public CommandType CmdType
{
get { return _CmdType; }
set { _CmdType = value; }
}
#endregion
#region 私有方法
private void OpenCon()
{
try
{
if (Conn == null)
{
Conn = DbFactory.CreateConnection();
}
if (Conn.State == ConnectionState.Closed)
{
Conn.ConnectionString = ConnStr;
Conn.Open();
}
}
catch (Exception ex)
{

throw (new Exception("打开数据库出错，错误" + ex.Message));
}
}
private void CloseCon()
{
if (Conn.State == ConnectionState.Open)
{
Conn.Close();
}
if (Cmd != null)
{
Cmd.Dispose();
Cmd = null;
}
}
private void StartCommend()
{
OpenCon();
Cmd = Conn.CreateCommand();
Cmd.CommandText = CmdText;
Cmd.CommandType = CmdType;
}
#endregion
/// <summary>
/// 执行指定的SQL语句（如添加、更新、删除等操作）
/// </summary>
/// <returns>受影响的行数</returns>
public int ExecuteNonQuery()
{
StartCommend();
try
{
return Cmd.ExecuteNonQuery();
}
catch (Exception ex)
{
throw (new Exception("数据库执行错误: " + ex.Message));
}
finally
{
CloseCon();
}
}

/// <summary>
/// 使用 DataAdapter 提取数据返回为 DataTable，并为 DataTable 指定名称
/// </summary>
/// <returns>DataTable</returns>
public DataTable ExecuteDataTable()
{
StartCommend();
try
{
using (DbDataAdapter da =DbFactory.CreateDataAdapter())
{
da.SelectCommand = Cmd;
DataTable dt = new DataTable();
da.Fill(dt);
return dt;
}
}
catch (Exception ex)
{
throw (new Exception("数据库执行错误:" + ex.Message));
}
finally
{
CloseCon();
}
}
　　}
}

对有些概念依然模糊，需要每次执行完语句就直接断开链接吗？这里面的哪些写法不对？

求各位指教。