.Net 下区别使用 ByRef/ByVal 的重要性

一直以来在 VB6 下，参数默认都是按照 ByRef  传送的，(即按地址传送)

而在 .Net(C# Ref,out ) 下， 参数默认是使用 ByVal （即按值传送)  传送的，变量的复本被传递.

按地址传送参数是否会比 按值传递 更快呢,我进行了以下测试,分别对 以下3种对象

DataTable ,Array ,String 进行了测试

测试中为了避免变量在循环中被缓存,每个循环传递的变量都是不同的

   Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim II As Int32
        Dim JJ As Int32
        Dim KK As Int32
        Dim t As Long
        Dim d1 As New DataTable
        Dim d2 As New DataTable
        Dim r As DataRow
        Dim a(4) As Int64

        d1.Columns.Add("col1")
        d1.Columns.Add("col2")
        d1.Columns.Add("col3")
        d1.Columns.Add("col4")
        d1.Columns.Add("col5")
        d1.Columns.Add("col6")

        d2.Columns.Add("col1")
        d2.Columns.Add("col2")
        d2.Columns.Add("col3")
        d2.Columns.Add("col4")
        d2.Columns.Add("col5")
        d2.Columns.Add("col6")

 

    
        t = Now.Ticks
        d1.Rows.Clear()
        For KK = 1 To 10000

            For II = 1 To 5
                r = d1.NewRow
                For JJ = 0 To 4

                    r(JJ) = CStr(II.ToString & JJ.ToString).ToString & KK.ToString
                Next

                d1.Rows.Add(r)

            Next

            TestDataTableByVal(d1)

        Next

        MsgBox("DataTable ByVal: " & CStr((Now.Ticks - t) / 10000)).ToString()

 

        t = Now.Ticks
        d2.Rows.Clear()
        For KK = 1 To 10000

            For II = 1 To 5
                r = d2.NewRow
                For JJ = 0 To 4

                    r(JJ) = CStr(II.ToString & JJ.ToString).ToString & KK.ToString
                Next

                d2.Rows.Add(r)

            Next

            TestDataTableByRef(d2)

        Next

        MsgBox("DataTable Byref : " & CStr((Now.Ticks - t) / 10000)).ToString()

 

        t = Now.Ticks

        For KK = 1 To 100000
            Dim s As String
            s = KK.ToString & "123456789012345678901234567890"
            TestStringByVal(s)

        Next

        MsgBox("String  byval:" & CStr((Now.Ticks - t) / 10000)).ToString()

 

        t = Now.Ticks

        For KK = 1 To 100000
            Dim s As String
            s = KK.ToString & "123456789012345678901234567890"
            TestStringByRef(s)

        Next

        MsgBox("String byref :" & CStr((Now.Ticks - t) / 10000)).ToString()

        t = Now.Ticks

        For KK = 1 To 100000
            For II = 0 To 4
                a(II) = Now.Ticks
            Next
            TestArrayByVal(a)

        Next

        MsgBox("Array byval :" & CStr((Now.Ticks - t) / 10000)).ToString()

 

        t = Now.Ticks

        For KK = 1 To 100000
            For II = 0 To 4
                a(II) = Now.Ticks
            Next
            TestArrayByRef(a)

        Next

        MsgBox("Array byref :" & CStr((Now.Ticks - t) / 10000)).ToString()
  
    End Sub
    Private Sub TestDataTableByVal(ByVal d As DataTable)
        Dim s As String
        Dim II As Int32
        With d.Rows(0)
            For II = 0 To 4
                s = .Item(II).ToString

            Next
        End With

    End Sub

    Private Sub TestDataTableByRef(ByRef d As DataTable)
        Dim s As String
        Dim II As Int32
        With d.Rows(0)
            For II = 0 To 4
                s = .Item(II).ToString

            Next
        End With
    End Sub

    Private Sub TestStringByVal(ByVal d As String)
        Dim s As String
        s = d
    End Sub

    Private Sub TestStringByRef(ByRef d As String)
        Dim s As String
        s = d
    End Sub

    Private Sub TestArrayByVal(ByVal a As Array)
        Dim d As Array
        d = a
    End Sub
    Private Sub TestArrayByRef(ByRef a As Array)
        Dim d As Array
        d = a
    End Sub

测试结果: (每次重新启动软件)

第1次
	DataTable	String	Array
ByVal	2687.5	125	140.625
ByRef	2906.25	93.75	125
第2次
	DataTable	String	Array
ByVal	2734.375	156.25	140.625
ByRef	2828.125	109.375	125
第3次
	DataTable	String	Array
ByVal	2687.5	171.875	125
ByRef	2687.5	93.75	125
第4次
	DataTable	String	Array
ByVal	2687.5	171.875	140.625
ByRef	2781.25	93.75	125
第5次
	DataTable	String	Array
ByVal	2703.125	156.25	140.625
ByRef	2828.125	109.375	125

String 在使用byref 传递时要比 Byval 快

而复杂的对象 即对象模型较多层的 则  byVal 比 Byref 快一点

数组 传递差别不大