HOW TO:初始化共享变量

“初始化共享变量”本不是一个HOW TO问题。这里提出来，主要的再次认识一下NEW。

在创建构造函数时，NEW是很有趣的，Sub New()有别于别的函数，因为它可以出现两次。

Public Class TestClass



    Shared Sub New()



        @#...



    End Sub



    Sub New()



        @#...



    End Sub




 


    @#...



End Class




 


当 Dim t As New TestClass时，它先执行Shared Sub New()然后再执行Sub New()。Shared Sub New()只执行一次，在实例类中，它在第一个实例产生时执行。




 


运用这个特性，Shared Sub New()在初始化类的共享变量时就会特别的方便。



下面作一个比较：




 


在静态类中，



Public Class MyCommand @#使用Shared Sub New()



    Private Shared cn As SqlClient.SqlConnection



    Shared Sub New()



        cn = New SqlClient.SqlConnection("Persist Security Info=False;Integrated Security=SSPI;database=northwind;server=(Local)")



    End Sub




 


    Public Shared Function Command(ByVal CommandText As String) As SqlClient.SqlCommand



        Return New SqlClient.SqlCommand(CommandText, cn)



    End Function



End Class




 


Public Class MyCommand0 @#不使用Shared Sub New()



    Private Shared cn As SqlClient.SqlConnection




 


    Public Shared Function Command(ByVal CommandText As String) As SqlClient.SqlCommand



        If cn Is Nothing Then



            cn = New SqlClient.SqlConnection("Persist Security Info=False;Integrated Security=SSPI;database=northwind;server=(Local)")



        End If




 


        Return New SqlClient.SqlCommand(CommandText, cn)



    End Function



End Class




 


在实例类中，



Public Class MyCommand1 @#使用Shared Sub New()




 


    Private Shared cn As SqlClient.SqlConnection



    Private _Command As SqlClient.SqlCommand




 


    Shared Sub New()



        cn = New SqlClient.SqlConnection("Persist Security Info=False;Integrated Security=SSPI;database=northwind;server=(Local)")



    End Sub




 


    Sub New()



        _Command = New SqlClient.SqlCommand("SELECT * FROM Orders", cn)



    End Sub




 


    Sub New(ByVal CommandText As String)



        _Command = New SqlClient.SqlCommand(CommandText, cn)



    End Sub




 


    Public ReadOnly Property Command() As SqlClient.SqlCommand



        Get



            Return _Command



        End Get



    End Property




 


    @#以下省略......



End Class




 


Public Class MyCommand2 @#不使用Shared Sub New()




 


    Private Shared cn As SqlClient.SqlConnection



    Private _Command As SqlClient.SqlCommand




 


    Sub New()



        If cn Is Nothing Then



            cn = New SqlClient.SqlConnection("Persist Security Info=False;Integrated Security=SSPI;database=northwind;server=(Local)")



        End If



        _Command = New SqlClient.SqlCommand("SELECT * FROM Orders", cn)



    End Sub




 


    Sub New(ByVal CommandText As String)




 


        If cn Is Nothing Then



            cn = New SqlClient.SqlConnection("ersist Security Info=False;Integrated Security=SSPI;database=northwind;server=(Local)")



        End If




 


        _Command = New SqlClient.SqlCommand(CommandText, cn)



    End Sub




 


    Public ReadOnly Property Command() As SqlClient.SqlCommand



        Get



            Return _Command



        End Get



    End Property




 


    @#以下省略......



End Class




 

原文转自：http://www.ltesting.net