Dictionary Dictionary TryGetValue KeyValuePair Select

Map

How can we map keys to values in a C# program? The C# language has no built-in map type. But it offers a powerful Dictionary type, which we use to map things.

With Dictionary, we must specify the key type (like "string") and the value type. With Add we map a key to a value. With TryGetValue we safely check this mapping.

Map example

Here we map strings to other strings. Each string can only have a mapping to one other string (keys must be unique).

Start We use the Add method to map the "cat" and "dog" strings to some words that indicate color.

Next We loop over all keys and values in the map. We access each KeyValuePair in the map in the foreach loop.

Tip TryGetValue is the best method to access a value in the map from a key. It returns false if the key does not exist.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Use Dictionary as a map.
        var map = new Dictionary<string, string>();

        // ... Add some keys and values.
        map.Add("cat", "orange");
        map.Add("dog", "brown");

        // ... Loop over the map.
        foreach (var pair in map)
        {
            string key = pair.Key;
            string value = pair.Value;
            Console.WriteLine(key + "/" + value);
        }

        // ... Get value at a known key.
        string result = map["cat"];
        Console.WriteLine(result);

        // ... Use TryGetValue to safely look up a value in the map.
        string mapValue;
        if (map.TryGetValue("dog", out mapValue))
        {
            Console.WriteLine(mapValue);
        }
    }
}cat/orange
dog/brown
orange
brown

`Select`

For a map function, we want a method that applies another method to each element in a collection. The Select method, part of LINQ, is ideal for mapping elements.

Note In Python, we call map to transform each element of a list—this code uppercases each string in a list.

Note 2 In C# we call Select() from the System.Linq namespace on the array. We print each uppercase string.

pets = ["bird", "fish"]

# Apply upper method to all elements with map.
for result in map(lambda x: x.upper(), pets):
    print(result)BIRD
FISHusing System;
using System.Linq;

class Program
{
    static void Main()
    {
        string[] array = { "bird", "fish" };
        var result = array.Select(x => x.ToUpper());

        // Print result strings.
        foreach (string value in result)
        {
            Console.WriteLine(value);
        }
    }
}BIRD
FISH

Some notes

Dictionary is an important part of C# programming. It is a map collection. Usually the best way to access it is with TryGetValue—this is safe and does not throw exceptions.

A summary

In this language, no map built-in exists. But we can access a Dictionary, or use the Select method from LINQ for all our mapping needs.