VB.NET判断图片是否PS（修改）过

以下代码转自C# 不知道为什么 判断结果不正确

Public Sub IsImagePs(ByVal ImageFilePath As String)
Dim buffer(5210) As Byte
Dim fs As New FileStream(ImageFilePath, FileMode.Open)
fs.Read(buffer, 0, buffer.Length)
fs.Close()
Dim str = buffer(4869).ToString() 'A 65
str += buffer(4870).ToString() 'd 100
str += buffer(4871).ToString() 'o 111
str += buffer(4872).ToString() 'b 98
str += buffer(4873).ToString() 'e 101
str += buffer(4874).ToString() ' 32
str += buffer(4875).ToString() 'P 80
str += buffer(4876).ToString() 'h 104
str += buffer(4877).ToString() 'o 111
str += buffer(4878).ToString() 't 116
str += buffer(4879).ToString() 'o 111
str += buffer(4880).ToString() 's 115
str += buffer(4881).ToString() 'h 104
str += buffer(4882).ToString() 'o 111
str += buffer(4883).ToString() 'p 112
MsgBox(str)
If str = "65100111981013280104111116111115104111112" Then
MessageBox.Show("图片被PS修改过") ' PS处理过
Else
MessageBox.Show("没被PS修改")
End If
End Sub

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO;

namespace WindowsFormsApplication1
{
public partial classPsCheck : Form
{
public PsCheck()
{
InitializeComponent();
}

private void button1_Click(object sender, EventArgs e)
{
Byte[] buffer = new Byte[5120];

using (FileStream fs = new FileStream("E://photo.jpg", FileMode.Open))
{
fs.Read(buffer, 0, buffer.Length);
}

string str = buffer[4869].ToString(); //A 65
str += buffer[4870].ToString();//d 100
str += buffer[4871].ToString();//o 111
str += buffer[4872].ToString();//b 98
str += buffer[4873].ToString();//e 101
str += buffer[4874].ToString();// 32
str += buffer[4875].ToString();//P 80
str += buffer[4876].ToString();//h 104
str += buffer[4877].ToString();//o 111
str += buffer[4878].ToString();//t 116
str += buffer[4879].ToString();//o 111
str += buffer[4880].ToString();//s 115
str += buffer[4881].ToString();//h 104
str += buffer[4882].ToString();//o 111
str += buffer[4883].ToString();//p 112

if (str == "65100111981013280104111116111115104111112")
{
MessageBox.Show("图片被PS修改过");// PS处理过
}
else
{
MessageBox.Show("没被PS修改");
}
}
}
}