GetHashCode method - Hashtable and Dictionary Collection Types - C#

Hi,

Can you please let me know about:

GetHashCode method in Hashtable and Dictionary Collection Types - C#

Please explain to me

1) what it is ?
2) Why we need it ?
3) How to use it ?

Thanks
milani_lucieAsked:
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BuggyCoderConnect With a Mentor Commented:
public class Customer
{
private long CustomerId;
private string CustomerName;

public Customer(long id, string Name)
{
CustomerId=id;
CustomerName=Name;
}

public override int GetHashCode()
{
return CustomerId.GetHashCode();
}
}

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0
 
BuggyCoderCommented:
in short every key value collection internally operates on an array to achieve o(1) time complexity. Each key you pass in is converted to an array index using technique called hashing. The value is then stored at this index. next time when you retrieve it again by passing the key, the key again get hashed to return the index and the value is retreived from that index.

Now Hashtables and Dictionaries can even treat instances of your types(objects) as keys. Now since hashing works on an assumption that item to be hashed has to be integer so we need a way for our objects to return an integer when they are to be used as keys. When you override gethashcode, you work an algorithm to provide key(int) for your object and so it get hashed and can be used as key.

Here are more details:-
http://msdn.microsoft.com/en-us/library/ms379571%28v=vs.80%29.aspx#datastructures20_2_topic5
http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx
0
 
milani_lucieAuthor Commented:
Can you please provide me the code: How to implement it ?

Thanks
0
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milani_lucieAuthor Commented:
Any help with sample code - Thanks
0
 
milani_lucieAuthor Commented:
How to declare the Dictionary Key of type Object (Customer) ? Can you please provide some sample code how to declare it ?

Thanks
0
 
milani_lucieAuthor Commented:
Dictionary<Customer, string> dictionary = new Dictionary<Customer, string>();

Am i correct ?

Thanks
0
 
milani_lucieAuthor Commented:
Complete Code:

public class Customer
{
      private long CustomerId;
      private string CustomerName;

      public Customer(long id, string Name)
      {
            CustomerId=id;
            CustomerName=Name;
      }

      public override int GetHashCode()
      {
            return CustomerId.GetHashCode();
      }
}

Dictionary<Customer, string> dict = new Dictionary<Customer, string>();

dict.Add(cust1, "Hello World");
dict.Add(cust2, "Bye World");

Am i correct ?

Thanks
0
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