Append to file using BinaryFormatter

Question

I'm using this code

    for (int i = 0; i < 3; ++i)
    {
        List<int> tl = new List<int>();
        tl.Add(5);
        tl.Add(4);
        using (var fileStream = new FileStream(@"C:\file.dat", FileMode.Append))
        {
            var bFormatter = new BinaryFormatter();
            bFormatter.Serialize(fileStream, tl);
            //fileStream.Close();
        }

        var list = new List<int>();

        using (var fileStream = new FileStream(@"C:\file.dat", FileMode.Open))
        {
            var bFormatter = new BinaryFormatter();
            //while (fileStream.Position != fileStream.Length)
            //{
            //     list.Add((int)bFormatter.Deserialize(fileStream));
            //}
            list = (List<int>)bFormatter.Deserialize(fileStream);
            //fileStream.Close();
        }
    }

I expect .dat file will be

5 4 5 4 5 4

but it's only

5 4

also this code return also

5 4

        List<int> tl = new List<int>();
        tl.Add(5);
        tl.Add(4);
        using (var fileStream = new FileStream(@"C:\file.dat", FileMode.Append))
        {
            var bFormatter = new BinaryFormatter();
            bFormatter.Serialize(fileStream, tl);
        }

        tl.Clear();
        tl.Add(3);
        tl.Add(2);
        using (var fileStream = new FileStream(@"C:\file.dat", FileMode.Append))
        {
            var bFormatter = new BinaryFormatter();
            bFormatter.Serialize(fileStream, tl);
        }

        var list = new List<int>();

        using (var fileStream = new FileStream(@"C:\file.dat", FileMode.Open))
        {
            var bFormatter = new BinaryFormatter();
            list = (List<int>)bFormatter.Deserialize(fileStream);
        }

it looks like it de-serialize only first portion that was appended.

why data don't append?

UPDATE: So the solution is:

        var list = new List<int>();

        using (var fileStream = new FileStream(@"C:\file.dat", FileMode.Open))
        {
            var bFormatter = new BinaryFormatter();
            while (fileStream.Position != fileStream.Length)
            {
                var t = (List<int>)(bFormatter.Deserialize(fileStream));
                list.AddRange(t);
            }
        }

Answer 1

You are adding three lists of ints, one after the other, and only reading the first one back. I think what your intent may be is to append to a (single) existing list, in which case you'd have to

1. Read your list back into memory
2. Add your new elements
3. Write the list back out to the file in overwrite (not append) mode

Answer 2

BinaryFormatter is not listed as being appendable. As it happens, you can usually get away with deserializing multiple times until you get to the EOF (and merging manually), but: there are other serializers which are explicitly designed to be appendable. For example, protocol buffers is an appendable format: concatenation is identical to merge. Further: if the outer element is a list, appending to the file is identical to adding to the composes list.

With protobuf-net, this is just:

for (int i = 0; i < 3; ++i)
{
    List<int> tl = new List<int>();
    tl.Add(5);
    tl.Add(4);
    using (var fileStream = new FileStream(@"C:\file.dat", FileMode.Append))
    {
        Serializer.Serialize(fileStream, tl);
    }

    using (var fileStream = new FileStream(@"C:\file.dat", FileMode.Open))
    {
        list = Serializer.Deserialize<List<int>>(fileStream);
    }
}

At the end of each loop iteration, list (i.e. after deserialization) has 2, then 4, then 6 elements.