A HashTabe items are key-value pair, so to add the item you should have a unique key and a value. Before adding a pair to hashtable just check if key already exist and if exist do not insert it otherwise it will through a exception.
Suppose you already have a hashtable
Hashtable EmpHashTable = new EmpHashTable();
EmpHashTable.Add("Key1", "Omer");
EmpHashTable.Add("Key2", "Berke");
EmpHashTable.Add("Key3", "Cebi");
Now in a button click, we will check if that key is already there or not, if it is there then we will increment the value of the key and insert it
protected void btnAdd_Click(object sender, EventArgs e)
{
if (txtValue.Text.Trim() != "")
{
string strKey = "";
if (txtKey.Text.Trim() == "")
strKey = "Key" + (EmpHashTable.Keys.Count + 1);
if (!EmpHashTable.ContainsKey(strKey))
{
EmpHashTable.Add(strKey, txtValue.Text.Trim());
ListBoxCloths.DataSource = EmpHashTable;
ListBoxCloths.DataBind();
lblMessage.Text = "Hashtable Item Added";
}
else
{
lblMessage.Text = "Key Already Exist, Enter Another Key";
}
}
else
{
lblMessage.Text = "Please Enter an Item to Add";
}
}