Hashtable error ..

Question

hi....
Hashtable hashtable;
object obj2;
(hashtable = this.invoicetable).set_Item(obj2 = num3, hashtable.get_Item(obj2) + this.txtinvoice.Text.Substring(m, 1));

I am working with Hashtable but it does not allows set_item() and get_item() methods in 2.0 framework....

error:
System.Collections.Hashtable.this[object].set': cannot explicitly call operator or accessor..
also same error for get method....

any body help me...