使用SqlHelper的一个小技巧

在数据库设计时，也许会有一些约定，说一下我自己的三点基本硬性规定：

1、所有字段都为可空设定（主键、布尔类型字段除外）

2、字符串类型不允许有前后空格（可能特殊情况时除外）

3、如果是空字符串，则应存储Null（为了防止不同数据库类型对字符串为空和NULL的不同对待）

第一点是设计时的事情，在这里可以略过，而后两点则是归程序判断处理的，也许细心的你发现了，第二点和第三点是衔接的。

接下来我们来看一下后两点是如何实现的，以TextBox为例，我是这样进行的：

‘在验证数据时进行Me.txtRemark.Trim()

‘在保存时进行With info    If (Me.txtRemark.HasValue) Then        .Remark = Me.txtRemark.Text    Else        .Remark = Nothing    End IfEnd With

这段代码并不难理解，不过需要说明的是，必填的字段为了验证是否有值Trim是得有的，而不必填的字段实质上则只需要赋值的那一行，Trim和Null则可以交由底层SqlHelper里进行处理。

首先我们定义一个配置类来进行控制：

‘‘‘ <summary>‘‘‘ Database configuration‘‘‘ </summary>Friend NotInheritable Class Config

    ‘ Removes all occurrences of white space characters    Public Shared ReadOnly TrimString As Boolean = True    ‘ translate the empty string to null    Public Shared ReadOnly EmptyStringToNull As Boolean = True    ‘ translate the null boolean to false    Public Shared ReadOnly NullBooleanToFalse As Boolean = True    ‘ translate the null value to dbnull value    Public Shared ReadOnly NullValueToDBNull As Boolean = True

End Class

前三项正是我们要实现的功能的开关，而最后一项NullValueToDBNull则需要另外说明一下了：

在实体类中，值类型我都是用Nullable(Of T)来存储的，这当中就包含了Null的情况，而在传递至数据库时，Null是作为默认值还是DBNull呢？这是不确定的，所以这个开关就是用于约定Null作为DBNull处理。

接下来就是对SqlHelper的改造了，需要改动的只有一个方法：PrepareCommand

‘‘‘ <summary>‘‘‘ This method opens (if necessary) and assigns a connection, transaction, command type and parameters‘‘‘ to the provided command.‘‘‘ </summary>‘‘‘ <param name="command">the SqlCommand to be prepared</param>‘‘‘ <param name="connection">a valid SqlConnection, on which to execute this command</param>‘‘‘ <param name="transaction">a valid SqlTransaction, or ‘null‘</param>‘‘‘ <param name="commandType">the CommandType (stored procedure, text, etc.)</param>‘‘‘ <param name="commandText">the stored procedure name or T-SQL command</param>‘‘‘ <param name="commandParameters">an array of SqlParameters to be associated with the command or ‘null‘ if no parameters are required</param>Private Shared Sub PrepareCommand(ByVal command As SqlCommand, _                                    ByVal connection As SqlConnection, _                                    ByVal transaction As SqlTransaction, _                                    ByVal commandType As CommandType, _                                    ByVal commandText As String, _                                    ByVal commandParameters() As SqlParameter)

    ‘if the provided connection is not open, we will open it    If connection.State <> ConnectionState.Open Then        connection.Open()    End If

    ‘associate the connection with the command    command.Connection = connection

    ‘set the command text (stored procedure name or SQL statement)    command.CommandText = commandText

    ‘if we were provided a transaction, assign it.    If Not (transaction Is Nothing) Then        command.Transaction = transaction    End If

    ‘set the command type    command.CommandType = commandType

    ‘attach the command parameters if they are provided    If Not (commandParameters Is Nothing) Then        For Each p As SqlParameter In commandParameters            If (p.Direction <> ParameterDirection.Output) Then                Select Case p.DbType                    Case DbType.String, DbType.StringFixedLength, DbType.AnsiString, DbType.AnsiStringFixedLength                        If (Not p.Value Is Nothing AndAlso Not p.Value Is DBNull.Value) Then                            Dim str As String = p.Value.ToString()

                            If (Config.TrimString) Then                                str = str.Trim()                            End If

                            If (Config.EmptyStringToNull AndAlso str.Length = 0) Then                                str = Nothing                            End If

                            p.Value = str                        End If                    Case DbType.Boolean                        If (Config.NullBooleanToFalse AndAlso p.Value Is Nothing) Then                            p.Value = False                        End If                End Select

                If (Config.NullValueToDBNull AndAlso p.Value Is Nothing) Then                    p.Value = DBNull.Value                End If            End If

            command.Parameters.Add(p)        Next    End If

End Sub ‘PrepareCommand

可以看到根据Parameter的DbType作了相应的处理，这样处理后，非必填的字段，就只以只用一句赋值语句，剩下的去空白字符和Null判断就交由底层处理了，省心省力！~~~