对csv文件读写时出现的奇怪的问题

小弟最近在写一个程序，从csv源文件中读取前两列数据到目标文件中，下面的是源代码。
运行并无任何报错，但是如果源文件中有1000行数据，运行后目标文件中只导入了934行。
500行导出479行，250行导出138行，150行导出138行，10行导出0行！
请各位大虾帮我看看哪里出错了！

Imports System.IO
Module Module1

Sub Main()
Dim Data_Item(1) As String
Dim SReadLine As Stream
Dim LineStr As String
Dim I As Integer
Dim sb As New FileStream("c:\MyFile.csv", FileMode.OpenOrCreate)
Dim sw As New StreamWriter(sb)

SReadLine = File.OpenRead("C:\aa.csv")

Dim SrReadLine As StreamReader = New StreamReader(SReadLine, _
System.Text.Encoding.ASCII)
SrReadLine.BaseStream.Seek(0, SeekOrigin.Begin)
sw.BaseStream.Seek(0, SeekOrigin.Begin)
While (SrReadLine.Peek() > -1)
LineStr = SrReadLine.ReadLine
Console.Write(LineStr)
Data_Item = LineStr.Split(","c)
sw.WriteLine(Data_Item(0) & "," & Data_Item(1))
Data_Item(0) = ""
Data_Item(1) = ""
End While



SrReadLine.Close()
Console.ReadLine()

End Sub
End Module1