Sub addref(ByRef one As Integer, Optional ByVal two As Integer = 18)
        one = one + two
        System.Console.WriteLine("one+two={0}", one)
     End Sub

    Sub Main()
        Dim a As Integer
        Dim b As Integer
        a = 10
        b = 20
        System.Console.WriteLine("调用add前，a={0}", a)
        addref(a, b)
        System.Console.WriteLine("调用add后，a={0},b={0}", a, b)
        System.Console.ReadLine()
    End Sub

为什么a和 b 都被 改变了呢，
 Sub addref(ByRef one As Integer, Optional ByVal two As Integer = 18)   two 已经 被我 用 byval 声明了，这是 为什么

哎呀  自己 弄通了

addref的参数two是可选参数，你在调用时赋值了

你把程序改成下面这样，测试你就知道问题是出在 System.Console.WriteLine("调用add后，a={0},b={0}", a, b)

Sub addref(ByRef one As Integer, Optional ByVal two As Integer = 18)
        one = one + two
        System.Console.WriteLine("one+two={0}", one)
     End Sub

    Sub Main()
        Dim a As Integer
        Dim b As Integer
        a = 10
        b = 20
        System.Console.WriteLine("调用add前，a={0}", a)
        addref(a, b)
        System.Console.WriteLine("调用add后，a={0},  a)   'a=30
        System.Console.WriteLine("调用add后，b={0}", b)   'b=20
        System.Console.ReadLine()
    End Sub