Urgent help required--C# equivalent to Java HashMap

Urgent help required--C# equivalent to Java HashMap

• Timpie

  I needed a Dictionary which accepted a "null" key, but there seems to be no native one, so I have written my own. It's very simple, actually. I inherited from Dictionary, added a private field to hold the value for the "null" key, then overwritten the indexer. It goes like this :
  

• SKCM

  There's no .net class that provides this functionality. Hower, you can write your own "HashMap" class with the logic you need, by implementing ICollection interface.
  
  http://msdn.microsoft.com/library/default.asp url=/library/en-us/cpref/html/frlrfsystemcollectionsicollectionclasstopic.asp
  
  

• ChadLee

  Java's HashMap doesn't actually allow multiple keys - it just allows you to replace the value for a particular key, which is the same for Hashtable in .NET, if you use the indexer rather than the Add method.
  
  As for supporting a null key - you could implement your own IDictionary which allows this, using a Hashtable for everything other than the null key.
  
  Jon
  
  
  
  

• Dhanasu

  D'oh! I read the OP's request for "multiple keys" to mean "multiple values per key". My brain couldn't conceve the idea of having anything OTHER than multiple keys in a collection, so I misread the intent.
  
  Anyway, unfortunately there isn't a plain Dictionary<> class in PowerCollections, presumably because there's already one in the standard libraries. Alas, the standard one doesn't allow null keys (as we already discussed), so we can't use that...
  
  Using a MultiDictionary<> to store single key/value pairs seems a little overkill, but perhaps it'll work.
  
  
  

• SUPAXE

  What you want is the Wintellect Power Collections, a set of collection classes developed by Peter Gould (who helped to design the C#/.Net libraries). This is an officially sanctioned library.
  
  Get it at http://www.wintellect.com/powercollections
  
  I can't speak too highly of that library!
  
  For what you want, you'd use a "MultiDictionary". The code goes like this:
  
  

  Wintellect.PowerCollections.MultiDictionary<string, string> test =     new Wintellect.PowerCollections.MultiDictionary<string, string>(true); test.Add(null, "Null Test1"); test.Add(null, "Null Test2"); test.Add("Key", "Key Test1"); test.Add("Key", "Key Test2"); ICollection<string> null_key_values = test[null]; foreach (string s in null_key_values)     MessageBox.Show(s, Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Information); ICollection<string> key_values = test["Key"]; foreach (string s in key_values)     MessageBox.Show(s, Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Information);

  
  
  

• Kent Chenery

  
  Data
  
  MySchool        StudentID     Key              Value
  West College      10              Name          Ricky
  West College      10              Surmane      Martin
  West College      10              Adress         DownStreet
  West College      10              Zip              555   
  West College      20              Name          Sharone
  West College      20              Surmane      Stone
  West College      20              Adress         5th Ave.
  West College      20              Zip              444   
  East  College      30              Name          Marry
  East College       30              Surmane      Christmas
  East College       30              Adress         UpStreet
  East College       30              Zip              111   
  
  _____________________________
  
  HashTable Structure
  
  myHashTable [ MySchool, HashTable2]
  myHasTable2 [ StudentID,HashTable3]
  myHashTable3 [Key,Value] 
  ___________________________
  
  CODE
  

  IDictionaryEnumerator listEnumerator = myHastable.GetEnumerator();

  while (listEnumerator.MoveNext())

  {

  Response.Write(listEnumerator.Key.ToString());

  Response.Write("--->");

  Response.Write(listEnumerator.Value.ToString());

  Response.Write("<br>");

  }
  
  ___________________________________________
  OutPut
  West College   ---> System.Collections.Hashtable 
  East  College   --->  System.Collections.Hashtable
  ____________________________________________
  
  Question / BrainStorming
  
  How  inner hashtable datas can be retrieved into a GridView
  
  ( That is the difference Java HashMap vs to C# HashTables.)
  

  
  

• SHELMAN

  I think we need to check with the OP where he actually wants multi-valued collections or not - Java's HashMap is not multivalued - but its put() method doesn't complain if the key is already present (which Hashtable's Add method does). In this case, just using the indexer instead is the right way to go.
  
  However, it looks like the Power Collections support null keys, so it solves that problem neatly.
  
  Jon
  
  
  
  
• Urgent help required--C# equivalent to Java HashMap