Difference between Dictionary and Hashtable

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Why Dictionary is preferred over hashtable in C#?

What is the difference between Dictionary and Hashtable. How to decide which one to use?

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Simply, Dictionary<TKey,TValue> is a generic type, allowing:

static typing (and compile-time verification)

use without boxing

If you are .NET 2.0 or above, you should prefer Dictionary<TKey,TValue> (and the other generic collections)

A subtle but important difference is that Hashtable supports multiple reader threads with a single writer thread, while Dictionary offers no thread safety. If you need thread safety with a generic dictionary, you must implement your own synchronization or (in .NET 4.0) use ConcurrentDictionary<TKey, TValue> .

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Lets give an example that would explain the difference between hashtable and dictionary.

Here is a method that implements hashtable

public void MethodHashTable()
{
    Hashtable objHashTable = new Hashtable();
    objHashTable.Add(1, 100);    // int
    objHashTable.Add(2.99, 200); // float
    objHashTable.Add('A', 300);  // char
    objHashTable.Add("4", 400);  // string

    lblDisplay1.Text = objHashTable[1].ToString();
    lblDisplay2.Text = objHashTable[2.99].ToString();
    lblDisplay3.Text = objHashTable['A'].ToString();
    lblDisplay4.Text = objHashTable["4"].ToString();


    // ----------- Not Possible for HashTable ----------
    //foreach (KeyValuePair<string, int> pair in objHashTable)
    //{
    //    lblDisplay.Text = pair.Value + " " + lblDisplay.Text;
    //}
}

The following is for dictionary

  public void MethodDictionary()
  {
    Dictionary<string, int> dictionary = new Dictionary<string, int>();
    dictionary.Add("cat", 2);
    dictionary.Add("dog", 1);
    dictionary.Add("llama", 0);
    dictionary.Add("iguana", -1);

    //dictionary.Add(1, -2); // Compilation Error

    foreach (KeyValuePair<string, int> pair in dictionary)
    {
        lblDisplay.Text = pair.Value + " " + lblDisplay.Text;
    }
  }

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There is one more important difference between a HashTable and Dictionary. If you use indexers to get a value out of a HashTable, the HashTable will successfully return null for a non-existent item, whereas the Dictionary will throw an error if you try accessing a item using a indexer which does not exist in the Dictionary

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