Question

I have a few objects serialized [edit: using BinaryFormatter] this way

Obj_A 1    
Obj_A 2
Obj_A 3
Obj_B 1
Obj_B 2
Obj_B 3
Obj_B 4

How many A objects there are before B’s is decided by user on Runtime.

While deserializing, I have no way to guess when to switch form Obj_A deseriliazing to Obj_B

I welcome any relevant insight

EDIT : Serialization occurs on the flow and numbers of A’s and B’s is not known beforehand

ANSWER
Marc Gravell suggested something I was JUST NOT aware of :
Why bother at all ? Just let the formatter do the job and test types afterwards.
(I am surprised I was not massively aswered that by people here, so obvious…)

IFormatter formatter = new BinaryFormatter();
Stream stream = new FileStream("X:\\binary.dump", FileMode.Open);
object o = formatter.Deserialize(stream);

if (o.GetType() == typeof(Obj_A))
     ReadTheMSDN(Obj_A);

else if (o.GetType() == typeof(Obj_B))
     KnowObjectsYouHandle(Obj_B);

stream.Close();

Another less elegant solution was to force objects to Lists :

LIST Obj_A
    Obj_A 1    
    Obj_A 2
    Obj_A 3

LIST Obj_B
    Obj_B 1
    Obj_B 2
    Obj_B 3
    Obj_B 4

Answer 1

What serialisation are you using? Various options

• use a serialiser that handles this for you
• use something before each that tells you the type that follows (could be the full type, could be some marker code, or could be the element name (XML), member-name (JSON), etc)
• write the number if A before the As, the number of B before the Bs etc
• if using something like BinaryFormatter or NetDataContractSerializer (which both include type metadata), deserialize it as normal and use GetType() afterwards to find what you were given