C# calculate MD5 for opened file?

how I can calculate MD5 hash for a file that is open or used by a process?

the files can be txt or and exe

my current code return error for an exe because it is running

here is my current code

public static string GetMd5HashFromFile(string fileName)
{
    FileStream file = new FileStream(fileName, FileMode.Open);
    MD5 md5 = new 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();
}

Cheers.