I see a pervasive belief (2009 article) throughout the internet that the Hashtable class is not serializable; however, I cannot find any modern documentation that supports this notion.
The belief stems from another ill-documented belief that the IDictionary interface prevents serialization; however, I cannot find anything in MSDN that supports this claim, today.
Further, Hashtable implements ISerializable and contains extension methods that accept serialization information.
So, what's the deal? Is Hashtable serializable? Where is the documentation that supports this notion surrounding IDictionary?
Further Clarification (please read):
The statement that IDictionary is not serializable is supported by plenty of documentation; however, this focuses on the use of XML-based serialization interactions with a class. ISerializable as mentioned both in the comments, below, and through MSDN indicates that a class is serializable. It also means the class the must be responsible for its own serialization.
I think this negates the statement that a Hashtable is not serializable. That is perhaps the genesis of my question.
The pervasive belief is so pervasive because it's true:
var t = new Hashtable();
t.Add("Hi!", "I'm here");
t.Add("Hm", "Yup");
var serializer = new XmlSerializer(typeof(Hashtable));
using (var sw = new StringWriter())
{
serializer.Serialize(sw, t);
Console.WriteLine(sw.ToString());
}
throws
NotSupportedException: The type System.Collections.Hashtable is not supported because it implements IDictionary.
That doesn't mean that it's literally impossible to serialize a hash table. Of course I can just iterate over all the keys and values, write them to a string and then reconstruct the hashtable from that. It's just that I can't use the serialization infrastructure fully.
What's the reasoning here? It's actually quite simple - XmlSerializer is designed to produce good XML, in the spirit of the interchange format XML was designed to be. And XML doesn't have any kind of dictionary or "key-value" mechanism that would fit. So to support hashtable serialization, they'd have to make their own "sub-format" with its own rules. And back when .NET was being designed, this was a huge no-no - XML was an interchange format. Any extension (hah) to the format means you're no longer compatible, no matter how good of an idea you have.
Of course, nowadays, everyone and their grandmother are producing XML data that isn't used for interchange purposes. And it's not entirely a bad thing (after all, .NET config files are also a XML format). But it's also kind of wrong.
In contrast, take something like BinaryFormatter. That's a class where the .NET team designed the whole format, and isn't limited by a standard. And lo and behold - BinaryFormatter can serialize and deserialize a Hashtable just fine.
So the slightly more correct belief would be "Hashtable cannot be serialized to valid standard XML. The XmlSerializer class in particular will throw an error when you attempt to serialize a Hashtable."
Does Hashtable implement ISerializable? Absolutely:
public class Hashtable : IDictionary, ICollection, IEnumerable, ISerializable, IDeserializationCallback, ICloneable
Can we serializae a Hashtable to XML? Let's try it:
var hash = new System.Collections.Hashtable();
hash[7] = "7";
hash[8] = "8";
var serializer = new System.Xml.Serialization.XmlSerializer(typeof(System.Collections.Hashtable));
TextWriter writer = new System.IO.StreamWriter(#"C:\SomeFile.xml");
serializer.Serialize(writer, hash);
Result... Error as you expected
An exception of type 'System.NotSupportedException' occurred in System.Xml.dll but was not handled in user code
Additional information: The type System.Collections.Hashtable is not supported because it implements IDictionary.
So, it would appear that indeed, it's still the case in .Net 4.5+
But lets try one more time with a binary serialization...
var hash = new System.Collections.Hashtable();
hash[7] = "7";
hash[8] = "8";
var formatter = new System.Runtime.Serialization.Formatters.Binary.BinaryFormatter();
Stream stream = new FileStream(#"C:\SomeFolder\SomeFile.bin", FileMode.Create, FileAccess.Write, FileShare.None);
formatter.Serialize(stream, hash);
stream.Close();
Result... No Errors thrown... So the issue appears to be related to IDictionary and XmlSerialization, but not all Serialization
If you really need to do this to XML, ManoDestra had a nice link https://blogs.msdn.microsoft.com/adam/2010/09/10/how-to-serialize-a-dictionary-or-hashtable-in-c/
Also, interestingly, XML Serialization mentions that you can't serialize unsigned longs or collections there-of.
Microsoft XML Serialization (MSDN)
Microsoft seems to say that it is certainly possible to do this.
https://msdn.microsoft.com/en-us/library/b85344hz(v=vs.110).aspx
using System;
using System.IO;
using System.Collections;
using System.Runtime.Serialization.Formatters.Binary;
using System.Runtime.Serialization;
public class App
{
[STAThread]
static void Main()
{
Serialize();
Deserialize();
}
static void Serialize()
{
// Create a hashtable of values that will eventually be serialized.
Hashtable addresses = new Hashtable();
addresses.Add("Jeff", "123 Main Street, Redmond, WA 98052");
addresses.Add("Fred", "987 Pine Road, Phila., PA 19116");
addresses.Add("Mary", "PO Box 112233, Palo Alto, CA 94301");
// To serialize the hashtable and its key/value pairs,
// you must first open a stream for writing.
// In this case, use a file stream.
FileStream fs = new FileStream("DataFile.dat", FileMode.Create);
// Construct a BinaryFormatter and use it to serialize the data to the stream.
BinaryFormatter formatter = new BinaryFormatter();
try
{
formatter.Serialize(fs, addresses);
}
catch (SerializationException e)
{
Console.WriteLine("Failed to serialize. Reason: " + e.Message);
throw;
}
finally
{
fs.Close();
}
}
static void Deserialize()
{
// Declare the hashtable reference.
Hashtable addresses = null;
// Open the file containing the data that you want to deserialize.
FileStream fs = new FileStream("DataFile.dat", FileMode.Open);
try
{
BinaryFormatter formatter = new BinaryFormatter();
// Deserialize the hashtable from the file and
// assign the reference to the local variable.
addresses = (Hashtable) formatter.Deserialize(fs);
}
catch (SerializationException e)
{
Console.WriteLine("Failed to deserialize. Reason: " + e.Message);
throw;
}
finally
{
fs.Close();
}
// To prove that the table deserialized correctly,
// display the key/value pairs.
foreach (DictionaryEntry de in addresses)
{
Console.WriteLine("{0} lives at {1}.", de.Key, de.Value);
}
}
}
Related
I'm working with some highly custom objects from a nuget package that I need to store in the local SQLite db using Xamarin. I tried using this code
public static byte[] Serialize(ComplexType c)
{
byte[] arrayData;
using(MemoryStream stream = new MemoryStream())
{
BinaryFormatter formatter = new BinaryFormatter();
formatter.Serialize( stream, c );
arrayData = stream.ToArray();
stream.Close();
}
return arrayData;
}
public static ComplexType Deserialize(byte[] arrayData)
{
ComplexType c;
using(MemoryStream stream = new MemoryStream())
{
stream.Write( arrayData, 0, arrayData.Length );
stream.Seek( 0, SeekOrigin.Begin );
BinaryFormatter formatter = new BinaryFormatter();
c = (ComplexType) formatter.Deserialize( stream );
}
return c;
}
to serialize and store the object then retrieve it and deserialize it. It seems like it worked fine to do the serialization but now it turns out that the object is not marked serializable. I can't add the SerializableAttribute property to it because it's from a nuget. I tried editing the object definition in the nuget but of course it's locked.
Is there any easy way to pull off serializing an object not marked serializable or perhaps some other simple method of storing a non-serializable custom object in a SQLite database? Or should I just byte the bullet and write really complicated setters and getters to store all of the custom object's individual custom properties broken down into their individual lists of standard properties and basically deconstruct and reconstruct the object every time it goes to and from the database?
I'm seeing mixed reports on ISerializable and derived classes. Will implementing ISerializable on a derived class from the custom object allow the derived class to be serialized? What about IXMLSerializable? Will either of these work? I just want to get some clarification if possible before I go way down the wrong rabbit hole.
Let's say I have
List<object> mainList = new List<object>();
And it contains
List<string> stringList = new List<string();
List<CustomClass> custList = new List<CustomClass>();
mainList.Add(stringList);
mainList.Add(custList);
To serialize
Stream stream;
BinaryFormatter formatter = new BinaryFormatter();
formatter.Serialize(stream, mainList);
To deserialize
Stream stream = (Stream)o;
BinaryFormatter formatter = new BinaryFormatter();
List<object> retrievedList = (List<object>)formatter.Deserialize(stream);
At this point, I receive an error that the stream read (deserialization) reached the end of the stream without retrieving a value.
Do I need to specify something besides...
[Serializable]
public class CustomClass { .... }
in the custom class to make this work? Can I not deserialize a List> that contains different type of object every time?
I tried
IList list = (IList)Activator.CreateInstance(typeof(custClassList[0]))
and tried to send and receive this, but got the same issue.
I can however serialize and deserialize a specified type or List, but I really need it to be dynamic.
Basically, BinaryFormatter is a joke. It works in some cases, but will fail in almost identical scenarios for unknown reasons.
The best and superior alternative to BinaryFormatter is the third-party library protobuf-net (https://github.com/mgravell/protobuf-net), developed by Marc Gravel.
This beauty solved all the problems I was having in one pass. It's much easier to set up and reacts more perfectly to complex, custom classes.
I should also mention that it's faster, in the terms of de/serialization.
In order to fix the issue that causes the error "stream read (deserialization) reached the end of the stream ", the stream position needs to reset to 0 as follows...
stream.Position = 0;
Do I need to specify something besides...
[Serializable] public class CustomClass { .... }
Nope...That should be good for what you are doing.
in the custom class to make this work? Can I not deserialize a List>
that contains different type of object every time?
You should be able to serialize any object.
I created a binary serialized file of my object, so that I could use it as a database file for many other projects. It works quite fine with the same project, but when I try to deserialize that file from other projects, it won't. The error that appears says, "xxxx.xxxx assembly not found". So, how should I serialize the object in order to make it assembly independent???
Here's my code :
// Binary formatter
IFormatter formatter = new BinaryFormatter();
Stream stream = new FileStream("BinarySerialization.bin",
FileMode.Create,
FileAccess.Write, FileShare.None);
formatter.Serialize(stream, words);
stream.Close();
What amendments do I need to do??? Please provide me with a working code example/sample.
Thanks.
BinaryFormatter is a type-based serialized; it deeply embeds type metadata into the output. I would wager that you have copied the class definition between the projects - that isn't enough for BinaryFormatter, since that is no longer the same Type (types are bound to their assembly).
In your scenario, it sounds to me that the correct thing to do here would be to use a contract-based serializer; for example:
xml (XmlSerializer, DataContractSerializer, etc)
json (JavascriptSerializer, JSON.net, etc)
binary (protobuf-net, etc)
There would would entirely in your scenario, and would also have much better version tolerance (BinaryFormatter is very brittle with versions)
You mention "XMLs aren't safe here, as I don't want the users to know the contents of the main database file." - in that case protobuf-net has the "advantage" of being not human-readable, but note: none of these, nor BinaryFormatter are encrypted; if I wanted to, I could obtain the contents if I really, really wanted to. If you need strong security, use proper encryption. In which case your code becomes (in addition to maybe a few marker attributes):
using(var stream = new FileStream("BinarySerialization.bin", FileMode.Create,
FileAccess.Write, FileShare.None))
{
Serializer.Serialize(stream, words);
}
Edit to show (per comments) how to serialize Dictionary<string, List<Word>> where Word is a class with 2 string members (most of the code here is just plumbing to show a complete example):
using System;
using System.Collections.Generic;
using System.IO;
using ProtoBuf;
[ProtoContract]
public class Word {
[ProtoMember(1)]
public string Foo { get; set; }
[ProtoMember(2)]
public string Bar { get; set; }
}
static class Program {
public static void Main() {
var data = new Dictionary<string, List<Word>>{
{"abc", new List<Word> {
new Word { Foo = "def", Bar = "ghi"},
new Word { Foo = "jkl", Bar = "mno"}
}},
{"pqr", new List<Word> {
new Word {Foo = "stu", Bar = "vwx"}
}}
};
using(var file = File.Create("my.bin")) {
Serializer.Serialize(file, data);
}
Dictionary<string, List<Word>> clone;
using(var file = File.OpenRead("my.bin")) {
clone = Serializer.Deserialize<
Dictionary<string, List<Word>>>(file);
}
foreach(var pair in clone) {
Console.WriteLine(pair.Key);
foreach(var word in pair.Value){
Console.WriteLine("\t{0} | {1}", word.Foo, word.Bar);
}
}
}
}
I would put all my models that have to be serialized into a seperate assembly.
Then you reference this assembly everywhere you need to deserialize the Models.
If not you need some kind of generator, that recreates the models based on some schema (same as WCF does with its utilities) or use plain formats like XML to persist your data.
The serialization mechanism in .NET creates a helper dll out of the type to serialize and deserialize your data in runtime. First it will spit out a code file that gets compiled and then the helper dll is loaded to do the serialization and deserialization.
If for some reason something happens while the helper .dll gets created - let's say a compilation error- then the runtime will not found this dll.
If the dll name in your case is some random character then I would say you are facing the problem described above. You can troubleshoot this by switching on an undocument switch. See the following article:
HOW TO: Debug into a .NET XmlSerializer Generated Assembly
I have a collection of objects at runtime, which is already serializable, I need to persist the state of the object to a file. I did a quick coding using BinaryFormatter and saved A serialized object to a file.
I was thinking that I can save object per line. but when i open the file in a notepad, it was longer than a line. It wasnt scrolling. How can i store an binary serialized object per line?
I am aware that i can use a separator after each object so while reading them back to the application, i can know the end of the object. Well, according to information theory, this increases the size of the data(Sipser book).
What s the best algorithm to come up with a separator that woudldnt break the information?
Instead of binary serialization? Do you think JSon format is more feasible? can i store the entity in a json format, line by line?
Also, serialization/deserialization introduces overhead, hits the performance. Would Json be faster?
ideas?
Thanks.
Thanks.
Serialization functions like a FIFO queue, you dont have to read parts of the file because the formatter does it for you you just have to know the order you pushed objects inside.
public class Test
{
public void testSerialize()
{
TestObj obj = new TestObj();
obj.str = "Some String";
IFormatter formatter = new BinaryFormatter();
Stream stream = new FileStream("MyFile.bin", FileMode.Create, FileAccess.Write, FileShare.None);
formatter.Serialize(stream, obj);
formatter.Serialize(stream, 1);
formatter.Serialize(stream, DateTime.Now);
stream.Close();
}
public void TestDeserialize()
{
Stream stream = new FileStream("MyFile.bin", FileMode.Open, FileAccess.Read, FileShare.None);
IFormatter formatter = new BinaryFormatter();
TestObj obj = (TestObj)formatter.Deserialize(stream);
int obj2 = (int)formatter.Deserialize(stream);
DateTime dt = (DateTime)formatter.Deserialize(stream);
stream.Close();
}
}
[Serializable]
class TestObj
{
public string str = "1";
int i = 2;
}
Well,
Serialization/deserialization introduces overhead, would Json be faster?
JSON is still a form of serialisation, and no it probably wouldn't be faster than binary serialisation - binary serialisation is intended to be compact and quick, wheras JSON serialisation puts more emphasis on readability and so many be slower as is very likely to be less compact.
You could serialise each object individually and emit some separator between each object (e.g. a newline character), but I don't know what separator you could use that is guarenteed to not appear in the serialised data (what happens if you serialise a string containing a newline character?).
If you use a separator that the .Net serialisation framework emits then obviously you will make it difficult (if not impossible) to correctly determine where the breaks between objects are leading to deserialisation failures.
Why exactly do you want to put each object on its own line?
Binary serialization saves the data to arbitrary bytes; these bytes can include newline characters.
You're asking to use newlines as separators. Newlines are no different from other separators; they will also increase the size of the data.
You could also create a ArrayList and add the objects to it and then serialize it ;)
ArrayList list = new ArrayList();
list.Add(1);
list.Add("Hello World");
list.Add(DateTime.Now);
BinaryFormatter bf = new BinaryFormatter();
FileStream fsout = new FileStream("file.dat", FileMode.Create);
bf.Serialize(fsout, list);
fsout.Close();
FileStream fsin = new FileStream("file.dat", FileMode.Open);
ArrayList list2 = (ArrayList)bf.Deserialize(fsin);
fsin.Close();
foreach (object o in list2)
Console.WriteLine(o.GetType());
Currently i have an application that reads and writes several properties from one or two basic classes to a .txt file using the Binary Serializer.
I've opened up the .txt file in NotePad and as it's formatted for the application it's not very readable to the human eye, not for me anyway =D
I've heard of using XML but pretty much most of my searches seem to overcomplicate things.
The kind of data im trying to save is simply a collection of "Person.cs" classes,nothing more than a name and address, all private strings but with properties and marked as Serializable.
What would be the best way to actually save my data in a way that can be easily read by a person? It would also make it easier to make small changes to the application's data directly in the file instead of having to load it, change it and save it.
Edit:
I have added the current way i am saving and loading my data, my _userCollection is as it suggests and the nUser/nMember are an integer.
#region I/O Operations
public bool SaveData()
{
try
{
//Open the stream using the Data.txt file
using (Stream stream = File.Open("Data.txt", FileMode.Create))
{
//Create a new formatter
BinaryFormatter bin = new BinaryFormatter();
//Copy data in collection to the file specified earlier
bin.Serialize(stream, _userCollection);
bin.Serialize(stream, nMember);
bin.Serialize(stream, nUser);
//Close stream to release any resources used
stream.Close();
}
return true;
}
catch (IOException ex)
{
throw new ArgumentException(ex.ToString());
}
}
public bool LoadData()
{
//Check if file exsists, otherwise skip
if (File.Exists("Data.txt"))
{
try
{
using (Stream stream = File.Open("Data.txt", FileMode.Open))
{
BinaryFormatter bin = new BinaryFormatter();
//Copy data back into collection fields
_userCollection = (List<User>)bin.Deserialize(stream);
nMember = (int)bin.Deserialize(stream);
nUser = (int)bin.Deserialize(stream);
stream.Close();
//Sort data to ensure it is ordered correctly after being loaded
_userCollection.Sort();
return true;
}
}
catch (IOException ex)
{
throw new ArgumentException(ex.ToString());
}
}
else
{
//Console.WriteLine present for testing purposes
Console.WriteLine("\nLoad failed, Data.txt not found");
return false;
}
}
Replace your BinaryFormatter with XMLSerializer and run the same exact code.
The only change you need to make is the BinaryFormatter takes an empty constructor, while for the XMLSerializer you need to declare the type in the constructor:
XmlSerializer serializer = new XmlSerializer(typeof(Person));
Using XmlSerializer is not really complicated. Have a look at this MSDN page for an example: http://msdn.microsoft.com/en-us/library/system.xml.serialization.xmlserializer.aspx
You could implement your own PersonsWriter, that takes a StreamWriter as constructor argument and has a Write method that takes an IList<Person> as input to parse out a nice text representation.
For example:
public class PersonsWriter : IDisposable
{
private StreamWriter _wr;
public PersonsWriter(IList<Person> persons, StreamWriter writer)
{
this._wr = writer;
}
public void Write(IList<Persons> people) {
foreach(Person dude in people)
{
_wr.Write(#"{0} {1}\n{2}\n{3} {4}\n\n",
dude.FirstName,
dude.LastName,
dude.StreetAddress,
dude.ZipCode,
dude.City);
}
}
public void Dispose()
{
_wr.Flush();
_wr.Dispose();
}
}
YAML is another option for human readable markup that is also easy to parse. there are libraries available for c# as well as almost all other popular languages. Here's a sample of what yaml looks like:
invoice: 34843
date : 2001-01-23
bill-to: &id001
given : Chris
family : Dumars
address:
lines: |
458 Walkman Dr.
Suite #292
city : Royal Oak
state : MI
postal : 48046
Frankly, as a human, I don't find XML to be all that readable. In fact, it's not really designed to be read by humans.
If you want a human readable format, then you have to build it.
Say, you have a Person class that has a First Name, a last Name and a SSN as properties. Create your file, and have it write out 3 lines, with a description of the field in the first fifty (random number from my head) and then with character 51 have the value start being written.
This will produce a file that looks like:
First Name-------Stephen
Last Name -------Wrighton
SSN -------------XXX-XX-XXXX
Then, reading it back in, your program would know where the data begins on each line, and what each line is for (the program would know that Line 3 is the SSN value).
But remember, to truly gain human readability, you sacrifice data portability.
Try the DataContractSerializer
It serializes objects to XML and is very easy to use
Write a CSV reader writer if you want a good compromise between human and machine readable in a Windows environment
Loads into Excel too.
There's a discussion about it here:
http://knab.ws/blog/index.php?/archives/3-CSV-file-parser-and-writer-in-C-Part-1.html
EDIT
That is a C# article... it just confusingly has "C" in the URL.
I really think you should go with XML (look into DataContractSerializer). Its not that complicated. You could probably even just replace BinarySerializer with XMLSerializer and go.
If you still don't want to do that, though, you can write a delimited text file. Then you'll have to write your own reader method (although, it could almost just use the split method).
//Inside the Person class:
public override string ToString()
{
List<String> propValues = new List<String>();
// Get the type.
Type t = this.GetType();
// Cycle through the properties.
foreach (PropertyInfo p in t.GetProperties())
{
propValues.add("{0}:={1}", p.Name, p.GetValue(o, null));
}
return String.Join(",". propValues.ToArray())
}
using (System.IO.TextWriter tw = new System.IO.StreamWriter("output.txt"))
{
tw.WriteLine(person.ToString());
}