Differences between hashtable and dictionary Indexer

Hashtable indexer Definition:

         // Summary:          // Gets or sets the value associated with the specified key.          //          // Parameters:          // Key:          // The Key whose value to get or set.          //          // Returns:          // The value associated with the specified key. If the specified key is not          // Found, attempting to get it returns NULL, and attempting to set it creates          // A new element using the specified key.          //          // Exceptions:         // System. argumentnullexception:          // Key is null.          //          // System. notsupportedexception:          // The property is set and the system. Collections. hashtable is read-only.-or-          // The property is set, key does not exist in the collection, and the system. Collections. hashtable          // Has a fixed size.          Public   Virtual   Object   This [ Object Key] {Get; set ;}

It is explicitly written in returns that if the specified key cannot be found, the return value is null. So you can use the followingCodeFragment is desirable:

 hashtable HS =  New  hashtable ();  // Add some elements   If  (HS [ "key" ] =  null ) { // The specified key does not found in hashtable }

Dictionary definition:

         // Summary:          // Gets or sets the value associated with the specified key.          //          // Parameters:          // Key:         // The Key of the value to get or set.          //          // Returns:          // The value associated with the specified key. If the specified key is not          // Found, a get operation throws a system. Collections. Generic. keynotfoundexception,          // And a set operation creates a new element with the specified key.          //          // Exceptions:          // System. argumentnullexception:          // Key is null.          //          // System. Collections. Generic. keynotfoundexception:          // The property is retrieved and key does not exist in the collection.         Public Tvalue This [Tkey key] {Get; set ;}

Note: When the specified key does not exist, system. Collections. Generic. keynotfoundexception is thrown and a new element is automatically created with the current key.

Therefore, when using a dictionary, you cannot use hashtable to determine whether a key exists. Instead, you should use the following code snippet:

 dictionary  string ,  string  dict =  New  dictionary  string ,  string  ();  // Add some elements   If  (! Dict. containskey ( "key"  )) { // The specified key does not found in dictionary }