为什么读取的txt文件是乱码呢，

Public Class Form1 Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
OpenFileDialog1.ShowDialog()
TextBox1.Text = OpenFileDialog1.FileName()
End Sub Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
OpenFileDialog2.ShowDialog()
TextBox2.Text = OpenFileDialog2.FileName()
End Sub Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
Dim R1 As New System.IO.StreamReader(OpenFileDialog1.FileName)
Dim R2 As New System.IO.StreamReader(OpenFileDialog2.FileName)
Dim W As New System.IO.StreamWriter(OpenFileDialog3.FileName)
Dim str1 As String
Dim str2 As String
Dim str3 As String = "aaaaaaaaaaa"
Dim str4 As String
str1 = R1.ReadLine()
str2 = R2.ReadLine()
Do While str1 <> Nothing And str2 <> Nothing str4 = str1 + str3 + str2
W.WriteLine(str4)
str1 = R1.ReadLine()
str2 = R2.ReadLine()
Loop
R1.Close()
R2.Close()
W.Close()
End Sub Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click
OpenFileDialog3.ShowDialog()
TextBox3.Text = OpenFileDialog3.FileName()
End Sub Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load End Sub
End Class