Dictionary<t,t> Key-value pairs collection (dictionary)

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Using system;using system.collections.generic;using system.linq;using system.text;namespace Dictionary Dictionary {class program {static void Main (string[] args) {//hashtable key-value pair Collection declaration: Hashtable HT = new HASHTABL  E (); In the process of declaring a//dictionary key value that does not determine the type of the key and value, the collection differs from the Hashtable key-value pair collection, and the type of the Dictionary key and value has been declared to determine the Dictionary<int , string> dic = new Dictionary<int, string> (); Creates a key-value pair for the collection object, with the key type int.            The type of the value is string. Dic.            ADD (1, "Zhang San"); Dic.            ADD (2, "John Doe");            D.add ("Zhang San", 28) error, because Dictionary<int,string> has determined that its key can only be of type int, the value can only be a string type. Dic. ADD (1, "New Zhang San");//error. The key is unique.            A key value of 1 has been added to the previous one. DIC[1] = "New Zhang San"; This will not be an error. In fact, here is a judgment. That is: If the DIC key value pair has a key value of 1, then it will be updated to "new Zhang San". If there are no key-value pairs for key 1. Then add a key value of 1 for the "New Zhang San" value to//dic. Clear ();            Clears all the elements of DIC, the key-value pair collection. The Hashtable key-value pair collection can only be traversed in this way.            That is, you can traverse keys only. foreach (var item in DIC. Keys)//traverse DIC for all key-value pairs {Console.WriteLine (item);            Output Console.WriteLine ("key is: {0}----value is: {1}", item, Dic[item]);//output: Key is: 1----value is: Zhang Sanxi is: 2----value is: John Doe            }//and dictionary key-value pairs in addition to the above way to traverse, you can also use this generally to traverse.            foreach (Keyvaluepair<int, string> item in DIC)//The DIC key pair is traversed in the form of key-value pairs (a pair of pairs). {Console.WriteLine (item);//output: [1, Zhang San] [2, John Doe] Console.WriteLine ("Key is: {0}----value is: {1}", item. Key, item. value);//output: The key is: 1----value is: Zhang Sanxi is: 2----value is: John Doe}//dictionary key-value pair collection instance: each letter of the STR string as key, each letter out The current number of times as a value.            Added to the key-value pair object.            String str = "Welcome to China";            Dictionary<char, int> dic2 = new Dictionary<char, int> ();            int i = 1; foreach (var item in str)//Traverse str This string {if (item = = ')//If the currently traversed character is ' ' Skip the current loop.                That is: Remove the space {continue;//Skip the current loop and proceed to the next round of loops. } if (Dic2. Keys.contains (item))//If DIC2 this key-value pair contains keyItem This key value pair {dic2[item]++; The value of the key value pair will be increased from the original base. That is, +1} else {Dic2. ADD (item, 1); Otherwise, use item as key. 1 as the default value.                Added to Dic2 in this key-value pair. }} foreach (var item in Dic2) {Console.WriteLine ("Letter: {0}----occurrences: {1} times" , item. Key, item.            Value);        } console.readkey (); }    }}

Dictionary<t,t> Key-value pairs collection (dictionary)