BinaryWriter - Stream不可写？

Question

有人可以解释一下为什么在SaveBlock方法的第一行BinaryWriter工作室说我ArgumentException: Stream was not writable？

private FileStream _file;

....

public void OpenFile(string fileName)
    {
        this._file = new FileStream(fileName, FileMode.OpenOrCreate, FileAccess.ReadWrite);
    }

....

public void SaveBlock(int adress, int length, byte[] data)
    {
        using (var br = new BinaryWriter(_file))
        {
            for (int i = adress; i < (adress+length); i++)
            {
                br.BaseStream.Seek(adress, SeekOrigin.Current);
                foreach (var j in data)
                {
                    br.Write(data[j]);
                }
            }
        }
    }