CODEWITHSUNDEEP
c#
Farid
Farid

Reputation: 9

Object Creation: How to preserve control in place of keyword 'new'

I want to add value in a Hashtable which is used by two functions when I use 'new' keyword all the values of Hashtable become zero or the table become empty. I m using the following code:

Hashtable ht; 

public void AddToList(string ItemNo)
{
    ht = Hashtable();
    ht.Add(ItemNo, ItemNo);
    // Below lines are only for testing
    IDictionaryEnumerator en = ht.GetEnumerator();
    while (en.MoveNext())
    {
        string str = en.Value.ToString();
        MessageBox.Show(str);
    }
}

And for removing the data I use:

public void RemoveFromList(string ItemNo)
{
    Hashtable ht = new Hashtable();
    ht.Remove(ItemNo);
}

I suppose I have to use the keyword 'preserve' or something like this instead of new keyword.

Upvotes: 0

Views: 173

Answers (4)

abatishchev
abatishchev

Reputation: 100308

class FooBar
{
    private Hashtable ht = null; 

    public void AddToList(string ItemNo)
    {
        if (ht == null)
            ht = new Hashtable();

        ht.Add(ItemNo, ItemNo);    
    }

    public void RemoveFromList(string ItemNo)
    {
        if (ht == null)
            ht = new Hashtable();

        ht.Remove(ItemNo);
    }
}

or (most preferred):

class FooBar
{
    private Hashtable ht = new Hashtable();

    public void AddToList(string ItemNo)
    {
        ht.Add(ItemNo, ItemNo);    
    }

    public void RemoveFromList(string ItemNo)
    {
        ht.Remove(ItemNo);
    }
}

Upvotes: 0

Dan
Dan

Reputation: 7744

Yes, "new" will create a new object - in this case an empty Hashtable. So the solution is to only use "new" once so that you only ever create one object.

With the code you have there, the simplest way would be to change it to:

Hashtable ht = new Hashtable(); 

public void AddTOList(string ItemNo)
{
    ht.Add(ItemNo, ItemNo);
}

public void RemoveFromList(string ItemNo)
{
    ht.Remove (ItemNo);
}

However, I have to ask why you want to use a Hashtable here at all. If the actual code you are writing isn't more complicated then a generic list would probably be a better fit, or an ArrayList if you can't use a new enough version of .Net.

Upvotes: 1

Henk Holterman
Henk Holterman

Reputation: 273494

You should not create a new HashTable inside the Add method.

Replace with something like:

Hashtable ht = new HashTable();  // create 1x
public void AddTOList(string ItemNo)
{
    ht.Add(ItemNo, ItemNo);
}

The code you posted would be valid when using for instance a DataContext, because that's just a portal to a backingstore. Your Hashtable is the backingstore, so don't create a new (empty) one each time.

Upvotes: 2

Matt Greer
Matt Greer

Reputation: 62057

Can you show the entire class?

In your RemoveFromList, you are creating a new local HashTable, instead of using the same one in AddToList. If you already created a HashTable and assigned it to ht, and if ht is a member variable of the class, then you don't need to create it every time.

If I have assumed your code correctly (please post the entire class), then just remove this line from RemoveFromList:

HashTable ht = new HashTable();

Upvotes: 1

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