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