Use of HashTable in C #

I. Brief introduction to the hash table (Hashtable)
In. in the. NET Framework, Hashtable is System. A container provided by the Collections namespace is used to process and present key-value pairs similar to key/value. The key is usually used for quick search, and the key is case sensitive; value is used to store the value corresponding to the key. In Hashtable, key/value pairs are of the object type, so Hashtable can support any type of key/value pairs.
Ii. Simple operations on Hash Tables
Add a key/value pair in the hash table: HashtableObject. Add (key, value );
Remove a key/value pair in the hash table: HashtableObject. Remove (key );
Remove all elements from the hash table: HashtableObject. Clear ();
Determine whether the hash table Contains the specified key: HashtableObject. Contains (key );
The following Windows program will include all the above operations:
 
[Csharp]
Private void button#click (object sender, System. EventArgs e)
{
Hashtable ht = new Hashtable (); // create a Hashtable instance
 
Ht. Add ("E", "e"); // Add key/value pairs
Ht. Add ("A", "");
Ht. Add ("C", "c ");
Ht. Add ("B", "B ");
 
MessageBox. Show ("the value of key A is:" + (string) ht ["A"]);
MessageBox. show ("whether the key E:" + ht. contains ("E "). toString (); // determines whether the hash table contains a specific key. The return value is true or false.

Ht. Remove ("C"); // Remove a key/value pair
MessageBox. Show ("C key removed:" + ht ["C"]);
 
Ht. Clear (); // remove all elements
MessageBox. Show ("all elements have been removed" + ht ["A"]); // No output will be provided here
}

 
3. traverse the hash table
DictionaryEntry Object is required to traverse a hash table. The Code is as follows:
[Csharp]
Foreach (DictionaryEntry de in ht) // ht is a Hashtable instance.
{
MessageBox. Show (de. Key. ToString (); // de. Key corresponds to key/value pairs
MessageBox. Show (de. Value. ToString (); // de. Key corresponds to the key/value Key-value Pair Value
}

 
4. Sorting hash tables
The definition of sorting hash tables here is to re-arrange the keys in key/value pairs according to certain rules, but in fact this definition cannot be implemented, because we cannot re-arrange keys directly in Hashtable, if Hashtable needs to provide output of certain rules, we can adopt a work und:
[Csharp]
ArrayList akeys = new ArrayList (ht. Keys); // do not forget to import System. Collections
Akeys. Sort (); // Sort by letter
Foreach (string skey in akeys)
{
MessageBox. Show (skey + ":");
MessageBox. Show (ht [skey]. ToString (); // output after sorting
}

 
[Csharp]
Public void InitData ()
{
String SQL = @ "select * from orders ";
 
SqlConnection con = new SqlConnection ("server =.; uid = sa; pwd = sa; database = NorthWind ");
SqlDataAdapter sda = new SqlDataAdapter (SQL, con );
DataTable dt = new DataTable ();
Sda. Fill (dt );
 
Hashtable htt = new Hashtable (5 );
 
Foreach (DataRow dr in dt. Rows)
{
Htt. Add (dr ["orderID"], dr ["mermerid"]);
 
}
ArrayList al = new ArrayList (htt. Keys );
Al. Sort ();
Foreach (DictionaryEntry de in htt)
{
If (de. Value. ToString = "ALFKI ")
{
Response. Write (de. Key + "Next .");
}
}
GridView1.DataSource = dt;
GridView1.DataBind ();
}