Dictionary C# with Example

Dictionary is a map. For a given key there can be one value in the dictionary. 
using System.Collections.Generic; 
var people = new Dictionary 
{ 
{ "John", 30 }, {"Mary", 35}, {"Jack", 40} 
}; 
// Reading data 
Console.WriteLine(people["John"]); // 30 
Console.WriteLine(people["George"]); // throws KeyNotFoundException 
int age; 
if (people.TryGetValue("Mary", out age)) 
{ 
Console.WriteLine(age); // 35 
} 
// Adding and changing data 
people["John"] = 40; //  Overwriting  values  this way  is ok 
people.Add("John", 40); //  Throws  ArgumentException  since  "John"  already  exists 
// Iterating through contents 
foreach(KeyValuePair person in people) 
{ 
Console.WriteLine("Name={0}, Age={1}", person.Key, person.Value); 
} 
foreach(string name in people.Keys) 
{ 
Console.WriteLine("Name={0}", name); 
} 
foreach(int age in people.Values) 
{ 
Console.WriteLine("Age={0}", age); 
} 
 

Duplicate key when using collection initialization 
var people = new Dictionary 
{ 
{ "John", 30 }, {"Mary", 35}, {"Jack", 40}, {"Jack", 40} 
}; // throws ArgumentException since "Jack" already exists