Question

For example, i have this:

class BasePacket
{
   int header;
   int type;
}

class ChildPacket1 : BasePacket
{
   //...
}

class ChildPacket2 : BasePacket
{
   //...
}

BasePacket bp;
Type t;
object obj = CreateNeededChildPacket(out t); //return one of childs as object and it's real type
bp = ...// anyway to cast obj to type represented by t? or by using something else?

Answer 1

In your example you are creating one of two derived classes and storing it in a base variable…this does not require any casting at all as it is perfectly fine to do this per the laws of inheritance. A base-type variable can always be assigned a more derived object since anything the base class would be able to do it is guaranteed the derived class can do as well, so it is safe. For example, a Math teacher can perform any action a regular Teacher can (such as GradePapers()). It would be safe to point a Math Teacher object with a Teacher variable.

BasePacket bp = (BasePacket) CreateNeededChildPacket();

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Edit:
In response to your comment, Ideally you would have the CreateNeededChildPacket() method return a BasePacket type (if the returned object is always dervied from BasePacket). Just as a base parameter can accept a derived object, a base return type can return a derived object, this is the beauty of inheritance. This would be much safer for someone down the road calling your method.

BasePacket CreateNeededChildPacket() 
{
    // do something
    return AnyObjectThatDerivesFromBasePacket;    
}

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If you cannot update the method then you could add some additional type-checking just to be safe.

object obj = CreateNeededChildPacket();
BasePacket bp = null;

if (obj is BasePacket) 
   bp = (BasePacket) obj;
else
   throw new Exception("Object was not a valid BasePacket type: " + obj.GetType.ToString());