HOW TO:初始化共享变量

作者： unknow

“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

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