class FileBase
{
    private List<byte> header;
    private List<byte> data = new List<byte>();

    public FileBase(int headerlength){ header = new List<byte>(new byte[headerlength]); }

    protected void AddData(byte[] d){ data.Add(d); }
    protected void AddData(string d){ data.AddRange( Encoding.Default.GetBytes(d) ); }
    /* ...more overloads... */

    public void Save(BinaryWriter bw)
    {
        bw.Write(header.ToArray());
        bw.Write(data.ToArray());
    }
}

class BmpFile : FileBase
{
    public FileBase():base(2){}

    public int Width
    {
        get{ return BitConverter.ToInt32(header,0); }
        set
        { 
            header.RemoveRange(0,2); 
            header.InsertRange(0,BitConverter.GetBytes(value));
        }
    }
}