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 is fastest and if you only iterate over ___.Values, it is also faster

Answer 2

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 3

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 4

Simplest form to iterate a dictionary:

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

Answer 5

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

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

Answer 6

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);
}