How to read bytes of a file step by step in c# win. form

byte[] buffer = new buffer[blockSize];
FileStream fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read);
try
    {
    int count;
    while ((count = fileStream.Read(buffer, sum, blockSize)) > 0)
        {
        ... do something with the block.
        }
    }
finally
    {
    fileStream.Close();
    }