找了很多，就这个管用，有时间好好研究一番

public static string GetMD5Hash(string fileName)
{
try
{
FileStream file = new FileStream(fileName, FileMode.Open);
System.Security.Cryptography.MD5 md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
byte[] retVal = md5.ComputeHash(file);
file.Close();

StringBuilder sb = new StringBuilder();
for (int i = 0; i < retVal.Length; i++)
{
sb.Append(retVal[i].ToString("x2"));
}
return sb.ToString();
}
catch (Exception ex)
{
throw new Exception("GetMD5Hash() fail,error:" + ex.Message);
}
}

public static string GetMD5Hash(byte[] bytedata)
{
try
{
System.Security.Cryptography.MD5 md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
byte[] retVal = md5.ComputeHash(bytedata);

StringBuilder sb = new StringBuilder();
for (int i = 0; i < retVal.Length; i++)
{
sb.Append(retVal[i].ToString("x2"));
}
return sb.ToString();
}
catch (Exception ex)
{
throw new Exception("GetMD5Hash() fail,error:" + ex.Message);
}
}

来源网址：

http://blog.csdn.net/snakorse/article/details/19578519

C#获取文件/字节数组MD5值方法