What is the best way to iterate over a dictionary?

Question

I've seen a few different ways to iterate over a dictionary in C#. Is there a standard way?

Answer 1

foreach(KeyValuePair<string, string> entry in myDictionary)
{
    // do something with entry.Value or entry.Key
}

Answer 2

If you are trying to use a generic Dictionary in C# like you would use an associative array in another language:

foreach(var item in myDictionary)
{
  foo(item.Key);
  bar(item.Value);
}

Or, if you only need to iterate over the collection of keys, use

foreach(var item in myDictionary.Keys)
{
  foo(item);
}

And lastly, if you're only interested in the values:

foreach(var item in myDictionary.Values)
{
  foo(item);
}

(Take note that the var keyword is an optional C# 3.0 and above feature, you could also use the exact type of your keys/values here)

Answer 3

In some cases you may need a counter that may be provided by for-loop implementation. For that, LINQ provides ElementAt which enables the following:

for (int index = 0; index < dictionary.Count; index++) {
  var item = dictionary.ElementAt(index);
  var itemKey = item.Key;
  var itemValue = item.Value;
}

Answer 4

Depends on whether you're after the keys or the values...

From the MSDN Dictionary(TKey, TValue) Class description:

// When you use foreach to enumerate dictionary elements,
// the elements are retrieved as KeyValuePair objects.
Console.WriteLine();
foreach( KeyValuePair<string, string> kvp in openWith )
{
    Console.WriteLine("Key = {0}, Value = {1}", 
        kvp.Key, kvp.Value);
}

// To get the values alone, use the Values property.
Dictionary<string, string>.ValueCollection valueColl =
    openWith.Values;

// The elements of the ValueCollection are strongly typed
// with the type that was specified for dictionary values.
Console.WriteLine();
foreach( string s in valueColl )
{
    Console.WriteLine("Value = {0}", s);
}

// To get the keys alone, use the Keys property.
Dictionary<string, string>.KeyCollection keyColl =
    openWith.Keys;

// The elements of the KeyCollection are strongly typed
// with the type that was specified for dictionary keys.
Console.WriteLine();
foreach( string s in keyColl )
{
    Console.WriteLine("Key = {0}", s);
}

Answer 5

Generally, asking for "the best way" without a specific context is like asking what is the best color?

One the one hand, there are many colors and there's no best color. It depends on the need and often on taste, too.

On the other hand, there are many ways to iterate over a Dictionary in C# and there's no best way. It depends on the need and often on taste, too.

Most straightforward way

foreach (var kvp in items)
{
    // key is kvp.Key
    doStuff(kvp.Value)
}

If you need only the value (allows to call it item, more readable than kvp.Value).

foreach (var item in items.Values)
{
    doStuff(item)
}

If you need a specific sort order

Generally, beginners are surprised about order of enumeration of a Dictionary.

LINQ provides a concise syntax that allows to specify order (and many other things), e.g.:

foreach (var kvp in items.OrderBy(kvp => kvp.Key))
{
    // key is kvp.Key
    doStuff(kvp.Value)
}

Again you might only need the value. LINQ also provides a concise solution to:

• iterate directly on the value (allows to call it item, more readable than kvp.Value)
• but sorted by the keys

Here it is:

foreach (var item in items.OrderBy(kvp => kvp.Key).Select(kvp => kvp.Value))
{
    doStuff(item)
}

There are many more real-world use case you can do from these examples. If you don't need a specific order, just stick to the "most straightforward way" (see above)!

Answer 6

I would say foreach is the standard way, though it obviously depends on what you're looking for

foreach(var kvp in my_dictionary) {
  ...
}

Is that what you're looking for?

Answer 7

You can also try this on big dictionaries for multithreaded processing.

dictionary
.AsParallel()
.ForAll(pair => 
{ 
    // Process pair.Key and pair.Value here
});

Answer 8

There are plenty of options. My personal favorite is by KeyValuePair

Dictionary<string, object> myDictionary = new Dictionary<string, object>();
// Populate your dictionary here

foreach (KeyValuePair<string,object> kvp in myDictionary)
{
     // Do some interesting things
}

You can also use the Keys and Values Collections

Answer 9

I appreciate this question has already had a lot of responses but I wanted to throw in a little research.

Iterating over a dictionary can be rather slow when compared with iterating over something like an array. In my tests an iteration over an array took 0.015003 seconds whereas an iteration over a dictionary (with the same number of elements) took 0.0365073 seconds that's 2.4 times as long! Although I have seen much bigger differences. For comparison a List was somewhere in between at 0.00215043 seconds.

However, that is like comparing apples and oranges. My point is that iterating over dictionaries is slow.

Dictionaries are optimised for lookups, so with that in mind I've created two methods. One simply does a foreach, the other iterates the keys then looks up.

public static string Normal(Dictionary<string, string> dictionary)
{
    string value;
    int count = 0;
    foreach (var kvp in dictionary)
    {
        value = kvp.Value;
        count++;
    }

    return "Normal";
}

This one loads the keys and iterates over them instead (I did also try pulling the keys into a string[] but the difference was negligible.

public static string Keys(Dictionary<string, string> dictionary)
{
    string value;
    int count = 0;
    foreach (var key in dictionary.Keys)
    {
        value = dictionary[key];
        count++;
    }

    return "Keys";
}

With this example the normal foreach test took 0.0310062 and the keys version took 0.2205441. Loading all the keys and iterating over all the lookups is clearly a LOT slower!

For a final test I've performed my iteration ten times to see if there are any benefits to using the keys here (by this point I was just curious):

Here's the RunTest method if that helps you visualise what's going on.

private static string RunTest<T>(T dictionary, Func<T, string> function)
{            
    DateTime start = DateTime.Now;
    string name = null;
    for (int i = 0; i < 10; i++)
    {
        name = function(dictionary);
    }
    DateTime end = DateTime.Now;
    var duration = end.Subtract(start);
    return string.Format("{0} took {1} seconds", name, duration.TotalSeconds);
}

Here the normal foreach run took 0.2820564 seconds (around ten times longer than a single iteration took - as you'd expect). The iteration over the keys took 2.2249449 seconds.

Edited To Add: Reading some of the other answers made me question what would happen if I used Dictionary instead of Dictionary. In this example the array took 0.0120024 seconds, the list 0.0185037 seconds and the dictionary 0.0465093 seconds. It's reasonable to expect that the data type makes a difference on how much slower the dictionary is.

What are my Conclusions?

• Avoid iterating over a dictionary if you can, they are substantially slower than iterating over an array with the same data in it.
• If you do choose to iterate over a dictionary don't try to be too clever, although slower you could do a lot worse than using the standard foreach method.

Answer 10

C# 7.0 introduced Deconstructors and if you are using .NET Core 2.0+ Application, the struct KeyValuePair<> already include a Deconstruct() for you. So you can do:

var dic = new Dictionary<int, string>() { { 1, "One" }, { 2, "Two" }, { 3, "Three" } };
foreach (var (key, value) in dic) {
    Console.WriteLine($"Item [{key}] = {value}");
}
//Or
foreach (var (_, value) in dic) {
    Console.WriteLine($"Item [NO_ID] = {value}");
}
//Or
foreach ((int key, string value) in dic) {
    Console.WriteLine($"Item [{key}] = {value}");
}

Answer 11

You suggested below to iterate

Dictionary<string,object> myDictionary = new Dictionary<string,object>();
//Populate your dictionary here

foreach (KeyValuePair<string,object> kvp in myDictionary) {
    //Do some interesting things;
}

FYI, foreach doesn't work if the value are of type object.

Answer 12

With .NET Framework 4.7 one can use decomposition

var fruits = new Dictionary<string, int>();
...
foreach (var (fruit, number) in fruits)
{
    Console.WriteLine(fruit + ": " + number);
}

To make this code work on lower C# versions, add System.ValueTuple NuGet package and write somewhere

public static class MyExtensions
{
    public static void Deconstruct<T1, T2>(this KeyValuePair<T1, T2> tuple,
        out T1 key, out T2 value)
    {
        key = tuple.Key;
        value = tuple.Value;
    }
}

Answer 13

Simplest form to iterate a dictionary:

foreach(var item in myDictionary)
{ 
    Console.WriteLine(item.Key);
    Console.WriteLine(item.Value);
}

Answer 14

Using C# 7, add this extension method to any project of your solution:

public static class IDictionaryExtensions
{
    public static IEnumerable<(TKey, TValue)> Tuples<TKey, TValue>(
        this IDictionary<TKey, TValue> dict)
    {
        foreach (KeyValuePair<TKey, TValue> kvp in dict)
            yield return (kvp.Key, kvp.Value);
    }
}

And use this simple syntax

foreach (var(id, value) in dict.Tuples())
{
    // your code using 'id' and 'value'
}

Or this one, if you prefer

foreach ((string id, object value) in dict.Tuples())
{
    // your code using 'id' and 'value'
}

In place of the traditional

foreach (KeyValuePair<string, object> kvp in dict)
{
    string id = kvp.Key;
    object value = kvp.Value;

    // your code using 'id' and 'value'
}

The extension method transforms the KeyValuePair of your IDictionary<TKey, TValue> into a strongly typed tuple, allowing you to use this new comfortable syntax.

It converts -just- the required dictionary entries to tuples, so it does NOT converts the whole dictionary to tuples, so there are no performance concerns related to that.

There is a only minor cost calling the extension method for creating a tuple in comparison with using the KeyValuePair directly, which should NOT be an issue if you are assigning the KeyValuePair's properties Key and Value to new loop variables anyway.

In practice, this new syntax suits very well for most cases, except for low-level ultra-high performance scenarios, where you still have the option to simply not use it on that specific spot.

Check this out: MSDN Blog - New features in C# 7

Answer 15

Sometimes if you only needs the values to be enumerated, use the dictionary's value collection:

foreach(var value in dictionary.Values)
{
    // do something with entry.Value only
}

Reported by this post which states it is the fastest method: http://alexpinsker.blogspot.hk/2010/02/c-fastest-way-to-iterate-over.html

Answer 16

I found this method in the documentation for the DictionaryBase class on MSDN:

foreach (DictionaryEntry de in myDictionary)
{
     //Do some stuff with de.Value or de.Key
}

This was the only one I was able to get functioning correctly in a class that inherited from the DictionaryBase.

Answer 17

I will take the advantage of .NET 4.0+ and provide an updated answer to the originally accepted one:

foreach(var entry in MyDic)
{
    // do something with entry.Value or entry.Key
}

Answer 18

The standard way to iterate over a Dictionary, according to official documentation on MSDN is:

foreach (DictionaryEntry entry in myDictionary)
{
     //Read entry.Key and entry.Value here
}

Answer 19

As of C# 7, you can deconstruct objects into variables. I believe this to be the best way to iterate over a dictionary.

Example:

Create an extension method on KeyValuePair<TKey, TVal> that deconstructs it:

public static void Deconstruct<TKey, TVal>(this KeyValuePair<TKey, TVal> pair, out TKey, out TVal val)
{
   key = pair.Key;
   val = pair.Value;
}

Iterate over any Dictionary<TKey, TVal> in the following manner

// Dictionary can be of any types, just using 'int' and 'string' as examples.
Dictionary<int, string> dict = new Dictionary<int, string>();

// Deconstructor gets called here.
foreach (var (key, value) in dict)
{
   Console.WriteLine($"{key} : {value}");
}

Answer 20

If say, you want to iterate over the values collection by default, I believe you can implement IEnumerable<>, Where T is the type of the values object in the dictionary, and "this" is a Dictionary.

public new IEnumerator<T> GetEnumerator()
{
   return this.Values.GetEnumerator();
}

Answer 21

var dictionary = new Dictionary<string, int>
{
    { "Key", 12 }
};

var aggregateObjectCollection = dictionary.Select(
    entry => new AggregateObject(entry.Key, entry.Value));

Answer 22

Just wanted to add my 2 cent, as the most answers relate to foreach-loop. Please, take a look at the following code:

Dictionary<String, Double> myProductPrices = new Dictionary<String, Double>();

//Add some entries to the dictionary

myProductPrices.ToList().ForEach(kvP => 
{
    kvP.Value *= 1.15;
    Console.Writeline(String.Format("Product '{0}' has a new price: {1} $", kvp.Key, kvP.Value));
});

Altought this adds a additional call of '.ToList()', there might be a slight performance-improvement (as pointed out here foreach vs someList.Foreach(){}), espacially when working with large Dictionaries and running in parallel is no option / won't have an effect at all.

Also, please note that you wont be able to assign values to the 'Value' property inside a foreach-loop. On the other hand, you will be able to manipulate the 'Key' as well, possibly getting you into trouble at runtime.

When you just want to "read" Keys and Values, you might also use IEnumerable.Select().

var newProductPrices = myProductPrices.Select(kvp => new { Name = kvp.Key, Price = kvp.Value * 1.15 } );

Answer 23

I wrote an extension to loop over a dictionary.

public static class DictionaryExtension
{
    public static void ForEach<T1, T2>(this Dictionary<T1, T2> dictionary, Action<T1, T2> action) {
        foreach(KeyValuePair<T1, T2> keyValue in dictionary) {
            action(keyValue.Key, keyValue.Value);
        }
    }
}

Then you can call

myDictionary.ForEach((x,y) => Console.WriteLine(x + " - " + y));

Answer 24

I know this is a very old question, but I created some extension methods that might be useful:

    public static void ForEach<T, U>(this Dictionary<T, U> d, Action<KeyValuePair<T, U>> a)
    {
        foreach (KeyValuePair<T, U> p in d) { a(p); }
    }

    public static void ForEach<T, U>(this Dictionary<T, U>.KeyCollection k, Action<T> a)
    {
        foreach (T t in k) { a(t); }
    }

    public static void ForEach<T, U>(this Dictionary<T, U>.ValueCollection v, Action<U> a)
    {
        foreach (U u in v) { a(u); }
    }

This way I can write code like this:

myDictionary.ForEach(pair => Console.Write($"key: {pair.Key}, value: {pair.Value}"));
myDictionary.Keys.ForEach(key => Console.Write(key););
myDictionary.Values.ForEach(value => Console.Write(value););

Answer 25

Dictionary< TKey, TValue > It is a generic collection class in c# and it stores the data in the key value format.Key must be unique and it can not be null whereas value can be duplicate and null.As each item in the dictionary is treated as KeyValuePair< TKey, TValue > structure representing a key and its value. and hence we should take the element type KeyValuePair< TKey, TValue> during the iteration of element.Below is the example.

Dictionary<int, string> dict = new Dictionary<int, string>();
dict.Add(1,"One");
dict.Add(2,"Two");
dict.Add(3,"Three");

foreach (KeyValuePair<int, string> item in dict)
{
    Console.WriteLine("Key: {0}, Value: {1}", item.Key, item.Value);
}

Answer 26

in addition to the highest ranking posts where there is a discussion between using

foreach(KeyValuePair<string, string> entry in myDictionary)
{
    // do something with entry.Value or entry.Key
}

or

foreach(var entry in myDictionary)
{
    // do something with entry.Value or entry.Key
}

most complete is the following because you can see the dictionary type from the initialization, kvp is KeyValuePair

var myDictionary = new Dictionary<string, string>(x);//fill dictionary with x

foreach(var kvp in myDictionary)//iterate over dictionary
{
    // do something with kvp.Value or kvp.Key
}

Answer 27

Dictionaries are special lists, whereas every value in the list has a key which is also a variable. A good example of a dictionary is a phone book.

   Dictionary<string, long> phonebook = new Dictionary<string, long>();
    phonebook.Add("Alex", 4154346543);
    phonebook["Jessica"] = 4159484588;

Notice that when defining a dictionary, we need to provide a generic definition with two types - the type of the key and the type of the value. In this case, the key is a string whereas the value is an integer.

There are also two ways of adding a single value to the dictionary, either using the brackets operator or using the Add method.

To check whether a dictionary has a certain key in it, we can use the ContainsKey method:

Dictionary<string, long> phonebook = new Dictionary<string, long>();
phonebook.Add("Alex", 415434543);
phonebook["Jessica"] = 415984588;

if (phonebook.ContainsKey("Alex"))
{
    Console.WriteLine("Alex's number is " + phonebook["Alex"]);
}

To remove an item from a dictionary, we can use the Remove method. Removing an item from a dictionary by its key is fast and very efficient. When removing an item from a List using its value, the process is slow and inefficient, unlike the dictionary Remove function.

Dictionary<string, long> phonebook = new Dictionary<string, long>();
phonebook.Add("Alex", 415434543);
phonebook["Jessica"] = 415984588;

phonebook.Remove("Jessica");
Console.WriteLine(phonebook.Count);