为何对同一个文件进行哈希,取得的哈希值却不同?

[booming]

FileStream filestream = new FileStream(".\\dw.rar",FileMode.Open,FileAccess.Read);
MessageBox.Show(filestream.Length.ToString());


long iLength = filestream.Length;
byte[] data = new byte[iLength];
byte[] result;

SHA256 shaM = new SHA256Managed();
result = shaM.ComputeHash(data);
MessageBox.Show(shaM.Hash.ToString());
MessageBox.Show(shaM.HashSize.ToString());

//-----------------------------------
FileStream filestream2 = new FileStream(".\\dw.rar",FileMode.Open,FileAccess.Read);
MessageBox.Show(filestream2.Length.ToString());


long iLength2 = filestream2.Length;
byte[] data2 = new byte[iLength2];
byte[] result2;
SHA256 shaM2 = new SHA256Managed();
result2 = shaM2.ComputeHash(data2);

MessageBox.Show(shaM2.Hash.ToString());
MessageBox.Show(shaM2.HashSize.ToString());
if(shaM2.Hash == shaM.Hash) MessageBox.Show("相等");
else MessageBox.Show("不相等");