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

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)

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;
}

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);
}

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)!

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}");
}

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?

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

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

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.

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

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 key, out TVal value)
{
   key = pair.Key;
   value = 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}");
}

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 the 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;
    }
}

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

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.

If you want to use a for loop, you can do as below:

var keyList=new List<string>(dictionary.Keys);
for (int i = 0; i < keyList.Count; i++)
{
    var key= keyList[i];
    var value = dictionary[key];
}

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();
}

The best answer is of course: Think, if you could use a more appropriate data structure than a dictionary if you plan to iterate over it- as Vikas Gupta mentioned already in the (beginning of the) discussion under the question. But that discussion as this whole thread still lacks surprisingly good alternatives. One is:

SortedList<string, string> x = new SortedList<string, string>();

x.Add("key1", "value1");
x.Add("key2", "value2");
x["key3"] = "value3";
foreach( KeyValuePair<string, string> kvPair in x )
            Console.WriteLine($"{kvPair.Key}, {kvPair.Value}");

Why it could be argued a code smell of iterating over a dictionary (e.g. by foreach(KeyValuePair<,>) ?

A basic principle of Clean Coding: "Express intent!" Robert C. Martin writes in "Clean Code": "Choosing names that reveal intent". Obviously naming alone is too weak. "Express (reveal) intent with every coding decision" expresses it better.

A related principle is "Principle of least surprise" (=Principle of Least Astonishment).

Why this is related to iterating over a dictionary? Choosing a dictionary expresses the intent of choosing a data structure which was made for primarily finding data by key. Nowadays there are so much alternatives in .NET, if you want to iterate through key/value pairs that you could choose something else.

Moreover: If you iterate over something, you have to reveal something about how the items are (to be) ordered and expected to be ordered! Although the known implementations of Dictionary sort the key collection in the order of the items added- AFAIK, Dictionary has no assured specification about ordering (has it?).

But what are the alternatives?

TLDR:
SortedList: If your collection is not getting too large, a simple solution would be to use SortedList<,> which gives you also full indexing of key/value pairs.

Microsoft has a long article about mentioning and explaining fitting collections:
Keyed collection

To mention the most important: KeyedCollection<,> and SortedDictionary<,> . SortedDictionary<,> is a bit faster than SortedList for only inserting if it gets large, but lacks indexing and is needed only if O(log n) for inserting is preferenced over other operations. If you really need O(1) for inserting and accept slower iterating in exchange, you have to stay with simple Dictionary<,>. Obviously there is no data structure which is the fastest for every possible operation..

Additionally there is ImmutableSortedDictionary<,>.

And if one data structure is not exactly what you need, then derivate from Dictionary<,> or even from the new ConcurrentDictionary<,> and add explicit iteration/sorting functions!

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

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

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 } );

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
}