Question

I want to know why we must set the serializable attribute to save an object in view state.

Also, which type of objects can we store in view state?

Answer 1

ViewState is serialized using binary serialization using ObjectStateFormatter. Quote from the documentation:

The ObjectStateFormatter class is
optimized to serialize and format many
common .NET Framework reference types,
as well as constants. The following
table lists the types that are
optimized.

Array, DateTime, Int16, String, ArrayList,
Double, Int32, String [], Boolean, Enum,
null (Nothing), String.Empty, Byte,
Hashtable, Pair, Triplet, Char,
HybridDictionary, Single, Type, Color,
IDictionary,

Additionally, while
conventional string types and string
arrays are written to and from a
serialized binary writer unaltered,
some strings are optimized by creating
internal string tables. Strings are
optimized using these tables if the
string has an associated TypeConverter
object or if the string is actually an
instance of the IndexedString class.

Other types not listed above are
binary-serialized using a
BinaryFormatter object if they
implement the ISerializable interface
or are decorated with the
SerializableAttribute attribute. The
ObjectStateFormatter class is not
optimized for any of these
serializable types.

If the ObjectStateFormatter class
encounters a type that is not
serializable, an ArgumentException
exception is thrown.

For an object to be binary serializable in the ViewState it needs to be decorated with the [Serializable] attribute. So you can put in ViewState any object that has this attribute. Note that simple types like string, int, float, … can also be placed in ViewState.