Convert Dictionary, List

In C# a Dictionary can be converted into a List. It is converted to a List of pairs—this requires the ToList extension method.

The ToList method is part of the System.Linq extensions to the C# language. It can be used for many requirements, and helps with conversions.

To start, we include the System.Collections.Generic and System.Linq namespaces. We create a Dictionary. Next we call ToList() on that Dictionary, yielding a List of KeyValuePair instances.

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        Dictionary<string, int> dictionary = new Dictionary<string, int>();
        dictionary["cat"] = 1;
        dictionary["dog"] = 4;
        dictionary["mouse"] = 2;
        dictionary["rabbit"] = -1;

        // Call ToList.
        List<KeyValuePair<string, int>> list = dictionary.ToList();

        // Loop over list.
        foreach (KeyValuePair<string, int> pair in list)
        {
            Console.WriteLine(pair.Key);
            Console.WriteLine("   {0}", pair.Value);
        }
    }
}
cat
   1
dog
   4
mouse
   2
rabbit
   -1

Example 2

Here we populate a Dictionary with the values in a List. Look closely how you can use ContainsKey in the loop in which you add the elements to the Dictionary.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Part 1: create new List.
        List<string> list = new List<string>();
        list.Add("Olympics");
        list.Add("Nascar");
        list.Add("Super Bowl");
        list.Add("Wimbledon");

        // Part 2: put List values into Dictionary.
        var exampleDictionary = new Dictionary<string, int>();
        foreach (string value in list)
        {
            if (!exampleDictionary.ContainsKey(value))
            {
                exampleDictionary.Add(value, 1);
            }
        }

        // Part 3: display Dictionary.
        foreach (var pair in exampleDictionary)
        {
            Console.WriteLine(pair);
        }
    }
}
[Olympics, 1]
[Nascar, 1]
[Super Bowl, 1]
[Wimbledon, 1]

Keys, Values

Often we may need to obtain a List of the keys in a Dictionary. We can also get a List of the values. We must access the Keys, and Values, properties.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Part 1: example Dictionary.
        Dictionary<string, int> birdDictionary = new Dictionary<string, int>();
        birdDictionary.Add("parakeet", 3);
        birdDictionary.Add("parrot", 5);
        birdDictionary.Add("finch", 7);
        birdDictionary.Add("lovebird", 9);

        // Part 2: get List of keys.
        List<string> keyList = new List<string>(birdDictionary.Keys);
        // Display them.
        foreach (var value in keyList)
        {
            Console.WriteLine(value);
        }

        // Part 3: get List of values.
        List<int> valueList = new List<int>(birdDictionary.Values);
        // Display them.
        foreach (var value in valueList)
        {
            Console.WriteLine(value);
        }
    }
}
parakeet
parrot
finch
lovebird

3
5
7
9

Notes, ToList

The ToList extension method acts upon an IEnumerable generic collection. The Dictionary implements the IEnumerable interface with type parameter KeyValuePair.

Notes, collections

There may be other methods in your program that require a List not a Dictionary. And the List has some superior performance characteristics.

We converted a Dictionary into a List of KeyValuePair struct values. We explored the interaction between ToList() and the Dictionary type's implementation of IEnumerable.