Urgent help required--C# equivalent to Java HashMap

Question

Java HashMap equivalent in C# is HashTable.
But in java the HashMap can have a null key ie key is a null reference and there can be duplicate keys ie basically keys with the same name.
I want a class that provides the above feature ie
1)stores in key value pairs
2) key can be null
3)Keys with the same name can be added.

or if there is another util class that provides the above mentioned features.

Please help me with this.

Thanks a lot in advance

Answer 1

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

Answer 2

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

Answer 3

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);

Answer 4

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

Answer 5

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.

Answer 6

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.)

Answer 7

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 :

---

public class NullableDictionnary : Dictionary<string, string>{
    string null_value;

    public StringDictionary this[string key]{
        get{
            if ( key == null )
                {return null_value;}
            return base[key];
        }
        set{
            if ( key == null )
                {null_value = value;}
            else
                {base[key] = value;}
        }
    }
}

---

Hope this helps someone in the future.

Answer 8

DonkeyMaster,

This did help me and I posted your answer to another forum I found seeking a similar resolve:

 

http://stackoverflow.com/questions/1273139/c-java-hashmap-equivalent/4855802#4855802

 

Thx