HOW TO:初始化共享变量

2022-07-15 80酷酷网 80kuku.com

　　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