{"id":641,"date":"2022-05-07T11:15:54","date_gmt":"2022-05-07T04:15:54","guid":{"rendered":"https:\/\/csharptutorial.net\/?page_id=641"},"modified":"2023-07-08T20:57:14","modified_gmt":"2023-07-08T13:57:14","slug":"csharp-dictionary","status":"publish","type":"page","link":"https:\/\/www.csharptutorial.net\/csharp-collections\/csharp-dictionary\/","title":{"rendered":"C# Dictionary<TKey, TValue>"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to use the C# Dictionary<Tkey, TValue><\/code> class to manage dictionaries effectively.<\/p>\n\n\n\n

Introduction to the C# Dictionary<TKey,TValue><\/h2>\n\n\n\n

The Dictionary<TKey,TValue><\/code> class represents a collection of keys and values. It belongs to the System.Collections.Generic<\/code> namespace. The TKey<\/code> and TValue<\/code> are the types of keys and values in the dictionary.<\/p>\n\n\n\n

Every key in a dictionary must be unique. Also, a key cannot be null<\/code> if its type is a reference type. If a key is an object, it must not change in any way that affects its uniqueness (or hash value).<\/p>\n\n\n\n

Because C# implements Dictionary<TKey, TValue)<\/code> as a hashtable, retrieving a value by key is very fast.<\/p>\n\n\n\n

Create a dictionary<\/h2>\n\n\n\n

The following creates a dictionary in which both keys and values are strings:<\/p>\n\n\n

Dictionary<string<\/span>,string<\/span>> states = new<\/span> Dictionary<string<\/span>,string<\/span>>();<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Alternatively, you can use the var<\/code> keyword:<\/p>\n\n\n
var<\/span> states = new<\/span> Dictionary<string<\/span>, string<\/span>>();<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Populating dictionary<\/h2>\n\n\n\n
To add the specified key and value to a dictionary, you use the Add()<\/code> method. For example, the following adds three key\/value pairs to the states<\/code> dictionary:<\/p>\n\n\n
var<\/span> states = new<\/span> Dictionary<string<\/span>,string<\/span>>();\n\nstates.Add(\"CA\"<\/span>, \"California\"<\/span>);\nstates.Add(\"TX\"<\/span>, \"Texas\"<\/span>);\nstates.Add(\"NY\"<\/span>, \"New York\"<\/span>);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the keys are the state abbreviation and the values are the state names.<\/p>\n\n\n\n
Also, you can use a collection initializer to add key\/value pairs to a dictionary when you create it. For example:<\/p>\n\n\n
var<\/span> states = new<\/span> Dictionary<string<\/span>, string<\/span>>()\n{\n    {\"CA\"<\/span>,\"California\"<\/span> },\n    {\"TX\"<\/span>,\"Texas\"<\/span> },\n    {\"NY\"<\/span>, \"New York\"<\/span> }\n};<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this syntax, you place a key\/value pair inside curly braces and wrap all elements in outer curly braces.<\/p>\n\n\n\n
Retrieving an element from a dictionary<\/h2>\n\n\n\n
To access an element, you use a square bracket and the key of the element like this:<\/p>\n\n\n
dict<\/span>[key]<\/span><\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
For example, the following shows how to access the elements with keys “CA” and “NY”:<\/p>\n\n\n
var<\/span> states = new<\/span> Dictionary<string<\/span>, string<\/span>>()\n{\n    {\"CA\"<\/span>,\"California\"<\/span> },\n    {\"TX\"<\/span>,\"Texas\"<\/span> },\n    {\"NY\"<\/span>, \"New York\"<\/span> }\n};\n\nConsole.WriteLine(states[\"CA\"<\/span>]);\nConsole.WriteLine(states[\"NY\"<\/span>]);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
California\nNew York<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
If you attempt to use a key that doesn’t exist, you’ll get a KeyNotFound<\/code> exception:<\/p>\n\n\n
var<\/span> states = new<\/span> Dictionary<string<\/span>, string<\/span>>()\n{\n    {\"CA\"<\/span>,\"California\"<\/span> },\n    {\"TX\"<\/span>,\"Texas\"<\/span> },\n    {\"NY\"<\/span>, \"New York\"<\/span> }\n};\n\nConsole.WriteLine(states[\"AZ\"<\/span>]); \/\/ Exception<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Fortunately, you can check if a key exists before accessing the element by using the ContainKeys()<\/code> method like this:<\/p>\n\n\n
var<\/span> states = new<\/span> Dictionary<string<\/span>, string<\/span>>()\n{\n    {\"CA\"<\/span>,\"California\"<\/span> },\n    {\"TX\"<\/span>,\"Texas\"<\/span> },\n    {\"NY\"<\/span>, \"New York\"<\/span> }\n};\n\nif<\/span> (states.ContainsKey(\"AZ\"<\/span>))\n{\n    Console.WriteLine(states[\"AZ\"<\/span>]);\n}\nelse<\/span>\n{\n    Console.WriteLine(\"No state with the abbreviation AZ exists\"<\/span>);\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
No state with the abbreviation AZ exists<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Also, you can use the TryGetValue()<\/code> method if you’re unsure whether the key exists:<\/p>\n\n\n
var<\/span> states = new<\/span> Dictionary<string<\/span>, string<\/span>>()\n{\n    {\"CA\"<\/span>,\"California\"<\/span> },\n    {\"TX\"<\/span>,\"Texas\"<\/span> },\n    {\"NY\"<\/span>, \"New York\"<\/span> }\n};\n\nstring<\/span> state;\n\nif<\/span>(states.TryGetValue(\"CA\"<\/span>, out<\/span> state))\n{\n    Console.WriteLine(state);\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
California<\/code><\/span><\/pre>\n\n\n
Iterating through a dictionary<\/h2>\n\n\n\n
To iterate all elements of a dictionary, you use the foreach<\/a><\/code> statement. The following example uses the foreach<\/code> statement to iterate over all elements in the states<\/code> dictionary:<\/p>\n\n\n
var<\/span> states = new<\/span> Dictionary<string<\/span>, string<\/span>>()\n{\n    {\"CA\"<\/span>,\"California\"<\/span> },\n    {\"TX\"<\/span>,\"Texas\"<\/span> },\n    {\"NY\"<\/span>, \"New York\"<\/span> }\n};\n\nforeach<\/span> (var<\/span> state in<\/span> states)\n{\n    Console.WriteLine($\"{state.Key}<\/span>: {state.Value}<\/span>\"<\/span>);\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In each iteration, the foreach<\/code> statement assigns the current dictionary’s element to the state variable. <\/p>\n\n\n\n
The type of each element is the struct KeyValuePair<TKey,TValue><\/code> which contains two properties: Key<\/code> and Value<\/code>.<\/p>\n\n\n\n
To access all the keys of a dictionary, you use the Keys<\/code> property. For example:<\/p>\n\n\n
var<\/span> states = new<\/span> Dictionary<string<\/span>, string<\/span>>()\n{\n    {\"CA\"<\/span>,\"California\"<\/span> },\n    {\"TX\"<\/span>,\"Texas\"<\/span> },\n    {\"NY\"<\/span>, \"New York\"<\/span> }\n};\n\nforeach<\/span> (var<\/span> stateAbbr in<\/span> states.Keys)\n{\n    Console.WriteLine(stateAbbr);\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
CA\nTX\nNY<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
To access only the values of a dictionary, you use the Values<\/code> property. For example:<\/p>\n\n\n
var<\/span> states = new<\/span> Dictionary<string, string>()\n{\n    {\"CA\"<\/span>,\"California\"<\/span> },\n    {\"TX\"<\/span>,\"Texas\"<\/span> },\n    {\"NY\"<\/span>, \"New York\"<\/span> }\n};\n\nforeach<\/span> (var<\/span> stateName in states.Values)\n{\n    Console.WriteLine(stateName);\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
California\nTexas\nNew<\/span> York<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Using a dictionary to simplify the if…else statement<\/h2>\n\n\n\n
The following GetDay()<\/code> method returns the day name based on a day number from 1 to 7 using an if...else<\/a><\/code> statement:<\/p>\n\n\n
string<\/span> GetDay<\/span>(int<\/span> day<\/span>)<\/span>\n{\n    if<\/span> (day == 1<\/span>) \n    {\n        return<\/span> \"Sunday\"<\/span>; \n    }\n    else<\/span> if<\/span> (day == 2<\/span>) \n    { \n        return<\/span> \"Monday\"<\/span>; \n    }\n    else<\/span> if<\/span> (day == 3<\/span>) \n    { \n        return<\/span> \"Tuesday\"<\/span>; \n    }\n    else<\/span> if<\/span> (day == 4<\/span>) \n    {\n        return<\/span> \"Wednesday\"<\/span>; \n    }\n    else<\/span> if<\/span> (day == 5<\/span>) \n    { \n        return<\/span> \"Thursday\"<\/span>; \n    }\n    else<\/span> if<\/span> (day == 6<\/span>) \n    {\n        return<\/span> \"Friday\"<\/span>; \n    }\n    else<\/span> if<\/span> (day == 7<\/span>) \n    {\n        return<\/span> \"Saturday\"<\/span>; \n    }\n    else<\/span>\n    { \n        return<\/span> \"Invalid\"<\/span>; \n    }\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
To simplify the code, you can use a dictionary for looking up the day name based on a day number like this:<\/p>\n\n\n
string<\/span> GetDay<\/span>(int<\/span> day<\/span>)<\/span>\n{\n    var<\/span> days = new<\/span> Dictionary<int<\/span>, string<\/span>>()\n    {\n        {1<\/span>, \"Sunday\"<\/span> },\n        {2<\/span>, \"Monday\"<\/span> },\n        {3<\/span>, \"Tuesday\"<\/span> },\n        {4<\/span>, \"Wednesday\"<\/span> },\n        {5<\/span>, \"Thursday\"<\/span> },\n        {6<\/span>, \"Friday\"<\/span> },\n        {7<\/span>, \"Saturday\"<\/span> },\n    };\n\n    if<\/span> (days.ContainsKey(day))\n    {\n        return<\/span> days[day];\n    }\n    return<\/span> \"Invalid\"<\/span>;\n\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary<\/h2>\n\n\n\n
    \n
  • Use the Dictionary<TKey,TValue><\/code> to manage dictionaries effectively.<\/li>\n\n\n\n
  • Use the ContainsKey<\/code> or TryGetValue<\/code> method if you’re unsure whether the key exists or not.<\/li>\n<\/ul>\n
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