Dictionary

The C# Dictionary is a collection that we can use to map keys to values. Each key must have the same type (like string), and all values must have a type as well.

For remembering things, the Dictionary is both fast and effective. We specify the key and value types. The Add method inserts into the Dictionary.

First example

Here we add 4 keys (each with an int value) to 2 separate Dictionary instances. Every Dictionary has pairs of keys and values.

using System;
using System.Collections.Generic;

// Version 1: create a Dictionary, then add 4 pairs to it.
var dictionary = new Dictionary<string, int>();
dictionary.Add("cat", 2);
dictionary.Add("dog", 1);
dictionary.Add("llama", 0);
dictionary.Add("iguana", -1);
// The dictionary has 4 pairs.
Console.WriteLine("DICTIONARY 1: " + dictionary.Count);

// Version 2: create a Dictionary with an initializer.
var dictionary2 = new Dictionary<string, int>()
{
    {"cat", 2},
    {"dog", 1},
    {"llama", 0},
    {"iguana", -1}
};
// This dictionary has 4 pairs too.
Console.WriteLine("DICTIONARY 2: " + dictionary2.Count);
DICTIONARY 1: 4
DICTIONARY 2: 4

TryGetValue

This is often the most efficient lookup method. In my experience, most of the time we want to use TryGetValue—so learn how to use it right away when learning Dictionary.

using System;
using System.Collections.Generic;

var values = new Dictionary<string, string>();
values.Add("A", "uppercase letter A");
values.Add("c", "lowercase letter C");

// Part 1: local variable result.
string result;
if (values.TryGetValue("c", out result))
{
    Console.WriteLine(result);
}

// Part 2: use inline "out string."
if (values.TryGetValue("c", out string description))
{
    Console.WriteLine(description);
}
lowercase letter C
lowercase letter C

Foreach

Let us loop over the data in a Dictionary. With each KeyValuePair, there are Key and Value properties—this gives us all data stored.

using System;
using System.Collections.Generic;

// Part 1: initialize data.
Dictionary<string, int> data = new Dictionary<string, int>()
{
    {"cat", 2},
    {"dog", 1},
    {"llama", 0},
    {"iguana", -1}
};

// Part 2: loop over pairs with foreach.
foreach (KeyValuePair<string, int> pair in data)
{
    Console.WriteLine("FOREACH KEYVALUEPAIR: {0}, {1}", pair.Key, pair.Value);
}

// Part 3: use var keyword to enumerate Dictionary.
foreach (var pair in data)
{
    Console.WriteLine("FOREACH VAR: {0}, {1}", pair.Key, pair.Value);
}
FOREACH KEYVALUEPAIR: cat, 2
FOREACH KEYVALUEPAIR: dog, 1
FOREACH KEYVALUEPAIR: llama, 0
FOREACH KEYVALUEPAIR: iguana, -1
FOREACH VAR: cat, 2
FOREACH VAR: dog, 1
FOREACH VAR: llama, 0
FOREACH VAR: iguana, -1

Keys

The Keys property returns a collection of type KeyCollection, not an actual List. We can convert it into a List. This property is helpful in many programs.

using System;
using System.Collections.Generic;

var data = new Dictionary<string, int>()
{
    {"cat", 2},
    {"dog", 1},
    {"llama", 0},
    {"iguana", -1}
};
// Store keys in a List.
var list = new List<string>(data.Keys);
// Loop through the List.
foreach (string key in list)
{
    Console.WriteLine("KEY FROM LIST: " + key);
}
KEY FROM LIST: cat
KEY FROM LIST: dog
KEY FROM LIST: llama
KEY FROM LIST: iguana

Count and Remove

Count returns the number of keys in a dictionary. And Remove eliminates a key (and its value) from the dictionary—if the key is found.

using System;
using System.Collections.Generic;

var lunch = new Dictionary<string, int>() { {"carrot", 1}, {"pear", 4}, {"apple", 6}, {"kiwi", 3} };
Console.WriteLine("COUNT: " + lunch.Count);

// Remove pear.
lunch.Remove("pear");
Console.WriteLine("COUNT: " + lunch.Count);
COUNT: 4
COUNT: 3

GetValueOrDefault

This method is available on Dictionary in .NET 5. It safely (with no possible exceptions) gets a value from the Dictionary, or the default value for the value's type.

using System;
using System.Collections.Generic;

var cats = new Dictionary<string, int>() { { "mittens", 5 } };
// Has value 5.
Console.WriteLine(cats.GetValueOrDefault("mittens"));
// Not found.
Console.WriteLine(cats.GetValueOrDefault("?"));
5
0

TryAdd

With this method (part of .NET 5) we add an entry only if the key is not found. It returns a bool that tells us whether the key was added.

using System;
using System.Collections.Generic;

var items = new Dictionary<string, int>();
// Part 1: add the string with value 1.
bool result = items.TryAdd("test", 1);
Console.WriteLine("Added: " + result);

// Part 2: the value already exists, so we cannot add it again.
bool result2 = items.TryAdd("test", 2);
Console.WriteLine("Added: " + result2);

// Part 3: the value is still 1.
Console.WriteLine(items.GetValueOrDefault("test"));
Added: True
Added: False
1

ContainsKey

This commonly-called method sees if a given string is present in a Dictionary. We use string keys here—we look at more types of Dictionaries further on.

using System;
using System.Collections.Generic;

var dictionary = new Dictionary<string, int>();
dictionary.Add("apple", 1);
dictionary.Add("windows", 5);

// See whether Dictionary contains this string.
if (dictionary.ContainsKey("apple"))
{
    int value = dictionary["apple"];
    Console.WriteLine(value);
}

// See whether it contains this string.
if (!dictionary.ContainsKey("acorn"))
{
    Console.WriteLine(false);
}
1
False

Int keys

Dictionary is a generic class. To use it, we must specify a type. This is a good feature—it means we can use an int key just as easily as a string key.

using System;
using System.Collections.Generic;

// Use int key type.
var data = new Dictionary<int, string>();
data.Add(100, "color");
data.Add(200, "fabric");

// Look up the int.
if (data.ContainsKey(200))
{
    Console.WriteLine("CONTAINS KEY 200");
}
CONTAINS KEY 200

LINQ

Extension methods can be used with Dictionary. We use the ToDictionary method—this is an extension method on IEnumerable. It places keys and values into a new Dictionary.

using System;
using System.Linq;

string[] values = new string[]
{
    "One",
    "Two"
};
// Create Dictionary with ToDictionary.
// ... Specify a key creation method, and a value creation method.
var result = values.ToDictionary(item => item, item => true);
foreach (var pair in result)
{
    Console.WriteLine("RESULT PAIR: {0}, {1}", pair.Key, pair.Value);
}
RESULT PAIR: One, True
RESULT PAIR: Two, True

ContainsValue

This method tries to find a value in the Dictionary (not a key). It searches the entire collection, as no hashing is available on values.

using System;
using System.Collections.Generic;

var data = new Dictionary<string, int>();
data.Add("cat", 1);
data.Add("dog", 2);
// Use ContainsValue to see if the value is present with any key.
if (data.ContainsValue(1))
{
    Console.WriteLine("VALUE 1 IS PRESENT");
}
VALUE 1 IS PRESENT

Indexer

We do not need to use Add to insert into a Dictionary. We can instead use the indexer with square brackets. This syntax also gets the value at the key.

using System;
using System.Collections.Generic;

var dictionary = new Dictionary<int, int>();

// We can assign with the indexer.
dictionary[1] = 2;
dictionary[2] = 1;
dictionary[1] = 3; // Reassign.

// Read with the indexer.
// ... An exception occurs if no element exists.
Console.WriteLine(dictionary[1]);
Console.WriteLine(dictionary[2]);
3
1

Copy

Dictionary provides a constructor that copies all values and keys into a new Dictionary instance. This constructor improves code reuse. It makes copying simpler.

using System;
using System.Collections.Generic;

var test = new Dictionary<int, int>();
test[20] = 30;

// Copy the dictionary, and add another key, so we now have 2 keys.
var copy = new Dictionary<int, int>(test);
copy[30] = 40;

Console.WriteLine("TEST COUNT: {0}", test.Count);
Console.WriteLine("COPY COUNT: {0}", copy.Count);
TEST COUNT: 1
COPY COUNT: 2

Clear

We can erase all pairs with the Clear method. Or we can assign the Dictionary variable to null. This causes little difference in memory usage—the entries are garbage-collected.

using System;
using System.Collections.Generic;

// Create a 1-key dictionary.
var d = new Dictionary<string, bool>();
d.Add("dog", true);
Console.WriteLine(d.Count);

// Clear the dictionary.
d.Clear();
Console.WriteLine(d.Count);
1
0

Dictionary field

Sometimes it is useful to have a Dictionary at the class level, as a field. And if we have a static class, we can initialize the Dictionary at the class level.

using System;
using System.Collections.Generic;

class Example
{
    Dictionary<int, int> _lookup = new Dictionary<int, int>()
    {
        {1, 1},
        {2, 3},
        {3, 5},
        {6, 10}
    };
    public int GetValue()
    {
        return _lookup[2]; // Example only.
    }
}

class Program
{
    static void Main()
    {
        // Create Example object instance.
        var example = new Example();
        // Look up a value from the Dictionary field.
        Console.WriteLine("RESULT: " + example.GetValue());
    }
}
RESULT: 3

KeyNotFoundException

This error happens when we access a nonexistent key. With Dictionary we should test keys for existence first, with ContainsKey or TryGetValue.

using System.Collections.Generic;

class Program
{
    static void Main()
    {
        var colors = new Dictionary<string, int>() { { "blue", 10 } };
        // Use ContainsKey or TryGetValue instead.
        int result = colors["fuchsia"];
    }
}
Unhandled Exception: System.Collections.Generic.KeyNotFoundException:
The given key was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at Program.Main() in ...

Benchmark, StringComparer

When you do a lookup on a Dictionary of string keys, the string must be hashed. If we pass StringComparer.Ordinal to our Dictionary, we have a faster algorithm.

using System;
using System.Collections.Generic;
using System.Diagnostics;

const int _max = 10000000;
var data1 = new Dictionary<string, bool>();
data1["Las Vegas"] = true;
var data2 = new Dictionary<string, bool>(StringComparer.Ordinal);
data2["Las Vegas"] = true;

// Version 1: do lookups on Dictionary with default string comparisons.
Stopwatch s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    if (!data1.ContainsKey("Las Vegas"))
    {
        return;
    }
}
s1.Stop();
// Version 2: do lookups in Dictionary that has StringComparer.Ordinal.
Stopwatch s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    if (!data2.ContainsKey("Las Vegas"))
    {
        return;
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
16.40 ns    ContainsKey, default comparer
15.17 ns    ContainsKey, StringComparer.Ordinal

Benchmark, Keys

Here we compare loops. A foreach-loop on KeyValuePairs is faster than the looping over Keys and accessing values in the loop body.

using System;
using System.Collections.Generic;
using System.Diagnostics;

const int _max = 1000000;
var test = new Dictionary<string, int>();
test["bird"] = 10;
test["frog"] = 20;
test["cat"] = 60;
int sum = 0;

// Version 1: use foreach loop directly on Dictionary.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    foreach (var pair in test)
    {
        sum += pair.Value;
    }
}
s1.Stop();

// Version 2: use foreach loop on Keys, then access values.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    foreach (var key in test.Keys)
    {
        sum += test[key];
    }
}
s2.Stop();
Console.WriteLine(s1.Elapsed.TotalMilliseconds);
Console.WriteLine(s2.Elapsed.TotalMilliseconds);
22.5618 ms    Dictionary foreach
72.231  ms    Keys foreach

Summary

The C# Dictionary, like all hash tables, is fascinating. It is fast—and useful in many places. Once we get a handle on its syntax, it is easy to use.