using System.Collections;
using System.Collections.Generic;

//add one key-value：
HashtableObject.Add(key, value);
//remove one key-value：
HashtableObject.Remove(key);
//remove all：           
HashtableObject.Clear();
//judge：
HashtableObject.Contains(key);

private void moverepeat(DataTable tb)
{
    String[] val = new String[];

    //delete repeat key reactant
    Hashtable ht = new Hashtable();
    int kk = 0;//as the index
    foreach (DataRow dr in tb_KeyReactant.Rows) {
        string s = dr[1].ToString();
        if (!ht.ContainsKey(dr[1].ToString())) {
            ht.Add(dr[1].ToString(), kk.ToString());
            kk++;
        }
    }

    //sort by alpha
    ArrayList akeys = new ArrayList(ht.Keys);
    akeys.Sort();

    ICollection keys = ht.Keys;
    //get keys
    int k = 0
    foreach (string key in keys) {
        val[k] = key;
        k++;
    }

    //search keys
    String nameOfTar = "example";
    int indexOfTar = ht[nameOfTar];
}