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Sample Image - Phonebook.jpg

Introduction

This article introduces and describes the use of hash table and serialization in .NET using C#. The sample application used here is a Phone Book application. Phone Book application is a console application that allows a user to add, view, list, and delete entries containing names and phone numbers.

A hash table is a collection of key-value pairs. In .NET, the class that implements a hash table is the Hashtable class. Elements can be added to the hash table by calling the Add method passing in the key-value pairs that you want to add. The objects used as keys must implement Object.Equals and Object.GetHashCode methods.

private Hashtable table = new Hashtable();
		
public void AddEntry(BookEntry entry)
{
   table.Add( entry.GetPerson(), entry );
}

Once the hash table is populated, you can search and retrieve elements in it by calling the indexer for the Hashtable class.

public BookEntry GetEntry(Person key)
{
   return (BookEntry) table[key];
}

Entries can be removed from the hash table by calling the Remove method which takes a key identifying the element you want to remove.

public void DeleteEntry(Person key)
{
   table.Remove( key );
}

The populated hash table can be saved to a file by using serialization. Serialization is the process of converting an object into a linear sequence of bytes for either storage or transmission to another location. This task can be performed using BinaryFormater class which serializes the hash table object to the file stream.

public void Save()
{
   Stream s = File.Open("Phone.bin", FileMode.Create, FileAccess.ReadWrite);
   BinaryFormatter b = new BinaryFormatter();
   b.Serialize(s, table);
   s.Close();      
}

The hash table object can be recovered back from the file by using Deserializemethod as shown below.

    s = File.Open("Phone.bin", FileMode.Open, FileAccess.Read);
    BinaryFormatter b = new BinaryFormatter();
    table = (Hashtable) b.Deserialize(s);

I hope you enjoy this brief introduction on hash table and serialization in .NET.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

About the Author

Liong Ng

Occupation:	Web Developer
Location:	United States

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Date

has anyone found a Deserialization error and fixed it?

desnu

14:44 9 Jul '04

PhoneBook example is great.
I copied the class handling code into my own C#.NET project, copied the classes, but when I run it from my own project I get the following:
An unhandled exception of type 'System.InvalidOperationException' occurred in system.xml.dll
Additional information: There is an error in XML document (2, 2).
Unhandled Exception: System.InvalidOperationException: There is an error in XML document (2, 2). --->

System.InvalidOperationException: was not expected.
at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReader1.Read5_ArrayOfCCommand()
--- End of inner exception stack trace ---
at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle)
at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader)

at the line in the source:
Contact[] contacts = (Contact[]) x.Deserialize( reader );

Anyone else get this too? and if so, can you please tell me how to fix this?

mucho appreciated

desnu

1.50/5 (3 votes)

Re: has anyone found a Deserialization error and fixed it?

hack.root

16:04 5 Dec '05

Check [Serializable] attribute in all serialized members of Contact inner classes and public parameterless constuctors, most common bug is that one of the classes can't be deserialized because of protected or none existent parameterless constructor, Regards, Artur Mustafin. mailto: hack.root@mail.ru
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Removing within a ForEach not possible

aberglas

9:10 6 Jun '03

How do you make the following code work correctly? foreach (elem in myhastable) if (elem.Value % 2 = 1) myhashtable.Remove(elem.Key); (You cannot Remove within a foreach.) (When writing articles do not just repeat the purile documentation MS produces. Think about the hard questions.)
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Re: Removing within a ForEach not possible leppie 10:00 6 Jun '03

aberglas wrote:
How do you make the following code work correctly?

You dont!

aberglas wrote:
Think about the hard questions

Yes, try do that next time. The article is a beginners article, nothing wrong with that. Your question on the other hand is totally off topic. If you did infact read thru MSDN, you would have noticed that a collection is NOT modifiable via its Enumarator (which a foreach is infact).

But here follows an answer anyways:

Best will be to make a copy of the key values then foreach thru that and remove the entries like that.

leppie::AllocCPArticle(Generic DFA State Machine for .NET);

Re: Removing within a ForEach not possible

hack.root

19:48 5 Dec '05

It is not a problem for good programmer.

The main idea is not to REALLY try to remove from collection, but force behave it as if it didn't never had that elements. Delete elements only at Compact is als a good idea.

The code here:

    public struct HashtableKeyPair<T, U>
    {
      T key;
      U val;
 
      public HashtableKeyPair(T key, U val)
      {
        this.key = key;
        this.val = val;
      }
 
      public T Key { get { return key; } }
      public U Value { get { return val; } }
    }
 
    public class Hashtable<T, U>: System.Collections.Hashtable
    {
      public Hashtable() : base() { }
 
      public Hashtable(IEnumerable<HashtableKeyPair<T, U>?> keyPairs)
      {
        foreach (HashtableKeyPair<T, U>? keyPair in keyPairs)
          if (keyPair.HasValue)
            this[keyPair.Value.Key] = keyPair.Value.Value;
      }
 
      public IEnumerable<HashtableKeyPair<T, U>> KeyPairs
      {
        get
        {
          foreach (HashtableKeyPair<T, U> keyPair in base.Values)
              yield return keyPair;
        }
      }
 
      public U this[T key]
      {
        get { return ((HashtableKeyPair<T, U>base[key]).Value; }
        set
        {
          base[key] = new HashtableKeyPair<T, U>(key, value);
        }
      }
 
      public HashtableKeyPair<T, U>? this[T key, Predicate<HashtableKeyPair<T, U>?> match]
      {
        get
        {
          if (base[key] != null)
          {
            HashtableKeyPair<T, U> obj = (HashtableKeyPair<T, U>base[key];
            if (match(obj))
              return obj;
          }
          return null;
        }
      }
 
      public Hashtable<T, U> GetMatchedHashtable(Predicate<HashtableKeyPair<T, U>?> match)
      {
        return new Hashtable<T, U>(Match(match));
      }
 
      public IEnumerable<HashtableKeyPair<T, U>?> Match(Predicate<HashtableKeyPair<T, U>?> match)
      {
        foreach (T key in Keys)
          if (this[key, match].HasValue)
            yield return this[key, match].Value;
      }
 
      public void Remove(T key)
      {
        base[key] = null;
      }
 
      private Predicate<HashtableKeyPair<T, U>?> hasValues = delegate(HashtableKeyPair<T, U>? obj) { return obj.HasValue; };
 
      public void Compact()
      {      
        Hashtable<T, U> hashtable = new Hashtable<T, U>(Match(hasValues));
 
        base.Clear();
 
        foreach (HashtableKeyPair<T, U> keyPair in hashtable.KeyPairs)
          this[keyPair.Key] = keyPair.Value;
      }
    }

Usage:

      Hashtable<int, int> myhashtable = new Hashtable<int, int>();
 
      myhashtable[0] = 1;
      myhashtable[1] = 4;
      myhashtable[2] = 9;
      myhashtable[3] = 16;
 
      Predicate<HashtableKeyPair<int, int>?> even = delegate(HashtableKeyPair<int, int>? obj) { return obj.HasValue && (obj.Value.Value % 2 == 1) == true; };
      foreach (HashtableKeyPair<int, int> keyPair in myhashtable.GetMatchedHashtable(even).KeyPairs)
      {
        myhashtable.Remove(keyPair.Key);
      }
 
      myhashtable.Compact();

"^" What does this mean ?

Cypher

1:11 21 Feb '03

In your code, you override the GetHashCode() for Person. but what does this sign "^" mean ? public override int GetHashCode() { return firstName.GetHashCode() ^ lastName.GetHashCode(); } \|\|\|Cypher\|\|\|
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Re: "^" What does this mean ?

Liong

11:45 23 Feb '03

The sign "^" means XOR or exclusive OR.
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While your at it, explain the HCP

Soliant

16:03 27 Nov '02

Maybe explain how overriding the HCP and Equals methods of Hashtable can enhance performance. Good overview though. Soliant \| email "The 'B' in Visual Basic means Beginner" - R. Bischoff
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