I have a list contains duplicate items.
List<string> filterList = new List<string>()
{
"postpone", "access", "success", "postpone", "success"
};
I get the output which is postpone, access, success by using
List<string> filter = filterList.Distinct().ToList();
string a = string.Join(",", filter.Select(a => a).ToArray());
Console.WriteLine(a);
I had saw other example, they can use groupby to get the latest element since they have other item like ID etc. Now I only have the string, how can I get the latest item in the list which is access, postpone, success? Any suggestion?
One way to do this would be use the Index of the item in original collection along with GroupBy. For example,
var lastDistinct = filterList.Select((x,index)=> new {Value=x,Index=index})
.GroupBy(x=>x.Value)
.Select(x=> x.Last())
.OrderBy(x=>x.Index)
.Select(x=>x.Value);
var result = string.Join(",",lastDistinct);
Output
access,postpone,success
An OrderedDictionary does this. All you have to do is add your items to it with a logic of "if it's in the dictionary, remove it. add it". OrderedDictionary preserves the order of adding so by removing an earlier added one and re-adding it it jumps to the end of the dictionary
var d = new OrderedDictionary();
filterList.ForEach(x => { if(d.Contains(x)) d.Remove(x); d[x] = null; });
Your d.Keys is now a list of strings
access
postpone
success
OrderedDictionary is in the Collections.Specialized namespace
If you wanted the keys as a CSV, you can use Cast to turn them from object to string
var s = string.Join(",", d.Keys.Cast<string>());
Your input list is only of type string, so using groupBy doesn't really add anything. If you consider your code, your first line gives you the distinct list, you only lose the distinct items because you did a string.join on line 2. All you need to do is add a line before you join:
List<string> filter = filterList.Distinct().ToList();
string last = filter.LastOrDefault();
string a = string.Join(",", filter.Select(a => a).ToArray());
Console.WriteLine(a);
I suppose you could make your code more terse because you need neither .Select(a => a) nor .ToArray() in your call to string.Join.
GroupBy would be used if you had a list of class/struct/record/tuple items, where you might want to group by a specific key (or keys) rather than using Distinct() on the whole thing. GroupBy is very useful and you should learn that, and also the ToDictionary and ToLookup LINQ helper functionality.
So why shouldn't you return the first occurrence of "postpone"? Because later in the sequence you see the same word "postpone" again. Why would you return the first occurrence of "access"? Because later in the sequence you don't see this word anymore.
So: return a word if the rest of the sequence does not have this word.
This would be easy in LINQ, with recursion, but it is not very efficient: for every word you would have to check the rest of the sequence to see if the word is in the rest.
It would be way more efficient to remember the highest index on which you found a word.
As an extension method. If you are not familiar with extension methods, see extension methods demystified.
private static IEnumerable<T> FindLastOccurences<T>(this IEnumerable<T> source)
{
return FindLastOccurrences<T>(source, null);
}
private static IEnumerable<T> FindLastOccurences<T>(this IEnumerable<T> source,
IEqualityComparer<T> comparer)
{
// TODO: check source not null
if (comparer == null) comparer = EqualityComparer<T>.Default;
Dictionary<T, int> dictionary = new Dictionary<T, int>(comparer);
int index = 0;
foreach (T item in source)
{
// did we already see this T? = is this in the dictionary
if (dictionary.TryGetValue(item, out int highestIndex))
{
// we already saw it at index highestIndex.
dictionary[item] = index;
}
else
{
// it is not in the dictionary, we never saw this item.
dictionary.Add(item, index);
}
++index;
}
// return the keys after sorting by value (which contains the highest index)
return dictionay.OrderBy(keyValuePair => keyValuePair.Value)
.Select(keyValuePair => keyValuePair.Key);
}
So for every item in the source sequence, we check if it is in the dictionary. If not, we add the item as key to the dictionary. The value is the index.
If it is already in the dictionary, then the value was the highest index of where we found this item before. Apparently the current index is higher, so we replace the value in the dictionary.
Finally we order the key value pairs in the dictionary by ascending value, and return only the keys.
Currently coding in C#, I wonder if there is a way to factor the code as presented below
Entity1 = GetByName("EntityName1");
Entity2 = GetByName("EntityName2");
Entity3 = GetByName("EntityName3");
The idea would be to get a single call in the code, factoring the code by placing the entities and the strings in a container and iterating on this container to get a single "GetByName()" line. Is there a way to do this?
You can use LINQ:
var names=new[]{"EntityName1","EntityName2","EntityName3",.....};
var entities=names.Select(name=>GetByName(name)).ToArray();
Without ToArray, Select will return an IEnumerable that will be reevalueated each time you enumerate it - that is, GetByName will be called each time you enumerate the enumerable.
ToArray() or ToList will create an array (or list) you can use multiple times.
You can also call ToDictionary if you want to be able to access the entities by name:
var entities=names.ToDictionary(name=>name,name=>GetByName(name));
All this assumes that the entities don't already exist or that GetByName has to do some significant work to retrieve them. If the entities exist you can simply put them in a Dictionary<String,Entity>. If the entities have a Name property you can use ToDictionary to create the dictionary in one statement:
var entities=new []{entity1,entity2,entity3,...};
var entitiesDict=entities.ToDictionary(entity=>entity.Name,entity=>entity);
Do you mean something like the below (where entities is the collection of Entity1, Entity1 & Entity3):
var results = entities.Select(e => GetByName(e.Name));
It depends on what you're looking for. If you need to set the variables in a single line, that won't work. You could play with reflection if you're dealing with fields or properties, but honestly that seems messier than what you've got already.
If the data-structure doesn't matter, and you just need the data and are able to play with it as you see so fit, I'd probably enumerate it into a dictionary. Of course, that's pretty tightly coupled to what you've got now, which looks like it's a fake implementation anyway.
If you want to do that, it's pretty straight-forward. It's your choice how you create the IEnumerable<string> that's represented below as entityNames. You could use an array initializer as I do, you could use a List<string> that you build over time, you could even yield return it in its own method.
var entityNames = new[] { "EntityName1", "EntityName2", "EntityName3" };
var dict = entityNames.ToDictionary(c => c, c => GetByName(c));
Then it's just a matter of checking those.
var entity1 = dict["EntityName1"];
Or enumerating over the dictionary.
foreach(var kvp in dict)
{
Console.WriteLine("{0} - {1}", kvp.Key, kvp.Value);
}
But realistically, it's hard to know whether that's preferable to what you've already got.
Ok, here is an idea.
You can declare this function.
IReadOnlyDictionary<string, T> InstantiateByName<T>(
Func<string, T> instantiator
params string[] names)
{
return names.Distinct().ToDictionary(
name => name,
name => instantiator(name))
}
which you could call like this,
var entities = InstantiateByName(
GetByName,
"EntityName1",
"EntityName2",
"EntityName3");
To push the over-engineering to the next level,
you can install the Immutable Collections package,
PM> Install-Package Microsoft.Bcl.Immutable
and modify the function slightly,
using Microsoft.Immutable.Collections;
IReadOnlyDictionary<string, T> InstantiateByName<T>(
Func<string, T> instantiator
params string[] names,
IEqualityComparer<string> keyComparer = null,
IEqualityComparer<T> valueComparer = null)
{
if (keyComparer == null)
{
keyComparer = EqualityComparer<string>.Default;
}
if (valueComparer == null)
{
valueComparer = EqualityComparer<T>.Default;
}
return names.ToImmutableDictionary(
name => name,
name => instantiator(name),
keyComparer,
valueComparer);
}
The function would be used in the exactly the same way. However, the caller is responsible for passing unique keys to the function but, an alternative equality comparer can be passed.
I have a hash table that contains values of a^j. j is the key and a^j is the value.
I am now calculating another value a^m. I basically want to see if a^m is in the hash table.
I used the ContainsValue fn. to find the value. How would i go about finding out the key of the value?
Here is a little snippet of where i want to implement the search for the value.
Dictionary<BigInteger, BigInteger> b = new Dictionary<BigInteger, BigInteger>();
***add a bunch of BigIntegers into b***
for(int j=0; j < n; j++)
{
z = q* BigInteger.ModPow(temp,j,mod);
***I want to implement to search for z in b here****
}
Does this change anything? the fact that i am searching while inside a for loop?
The fastest way is probably to iterate through the hashtable's DictionaryEntry items to find the value, which in turn gives you the key. I don't see how else to do it.
Firstly, you should absolutely be using Dictionary<TKey, TValue> instead of Hashtable - if you're using BigInteger from .NET 4, there's no reason not to use generic collections everywhere you can. Chances are for the most part you'd see no difference in how it's used - just create it with:
Dictionary<BigInteger, BigInteger> map =
new Dictionary<BigInteger, BigInteger>();
to start with. One thing to watch out for is that the indexer will throw an exception if the key isn't present in the map - use TryGetValue to fetch the value if it exists and a bool to say whether or not it did exist.
As for finding the key by value - there's no way to do that efficiently from a Dictionary. You can search all the entries, which is most easily done with LINQ:
var key = map.Where(pair => pair.Value == value)
.Select(pair => pair.Key)
.First();
but that will iterate over the whole dictionary until it finds a match, so it's an O(n) operation.
If you want to do this efficiently, you should keep two dictionaries - one from a to a^j and one from a^j to a. When you add an entry, add it both ways round. Somewhere on Stack Overflow I've got some sample code of a class which does this for you, but I doubt I'd be able to find it easily. EDIT: There's one which copes with multiple mappings here; the "single mapping" version is in the answer beneath that one.
Anyway, once you've got two dictionaries, one in each direction, it's easy - obviously you'd just lookup a^m as a key in the second dictionary to find the original value which created it.
Note that you'll need to consider whether it's possible for two original keys to end up with the same value - at that point you obviously wouldn't be able to have both mappings in one reverse dictionary (unless it was a Dictionary<BigInteger, List<BigInteger>> or something similar).
Edit: Changed to use Dictionary<TKey, TValue>
Dictionary<TKey, TValue> is an IEnumerable<KeyValuePair<TKey, TValue>>. If you do a foreach over it directly, you can get both the key and value for each entry.
class SomeType
{
public int SomeData = 5;
public override string ToString()
{
return SomeData.ToString();
}
}
// ...
var blah = new Dictionary<string, SomeType>();
blah.Add("test", new SomeType() { SomeData = 6 });
foreach (KeyValuePair<string, SomeType> item in blah)
{
if(e.Value.SomeData == 6)
{
Console.WriteLine("Key: {0}, Value: {1}", item.Key, item.Value);
}
}
If you have a newer version of the .Net framework, you could use Linq to find your matches, and place them in their own collection. Here's a code sample showing a little bit of Linq syntax:
using System;
using System.Collections;
using System.Linq;
class SomeType
{
public int SomeData = 5;
public override string ToString()
{
return SomeData.ToString();
}
}
class Program
{
static void Main(string[] args)
{
var blah = new Dictionary<string, SomeType>();
blah.Add("test", new SomeType() { SomeData = 6 });
// Build an enumeration of just matches:
var entriesThatMatchValue = blah
.Where(e => e.Value.SomeData == 6);
foreach (KeyValuePair<string, SomeType> item in entriesThatMatchValue)
{
Console.WriteLine("Key: {0}, Value: {1}", item.Key, item.Value);
}
// or: ...
// Build a sub-enumeration of just keys from matches:
var keysThatMatchValue = entriesThatMatchValue.Select(e => e.Key);
// Build a list of keys from matches in-line, using method chaining:
List<string> matchingKeys = blah
.Where(e => e.Value.SomeData == 6)
.Select(e => e.Key)
.ToList();
}
}
private object GetKeyByValue(object searchValue)
{
foreach (DictionaryEntry entry in myHashTable)
{
if (entry.Value.Equals(searchValue))
{
return entry.Key;
}
}
return null;
}
I've seen a few different ways to iterate over a dictionary in C#. Is there a standard way?
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.
As already pointed out on this answer, KeyValuePair<TKey, TValue> implements a Deconstruct method starting on .NET Core 2.0, .NET Standard 2.1 and .NET Framework 5.0 (preview).
With this, it's possible to iterate through a dictionary in a KeyValuePair agnostic way:
var dictionary = new Dictionary<int, string>();
// ...
foreach (var (key, value) in dictionary)
{
// ...
}
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
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;
}
}
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}");
}
foreach is fastest and if you only iterate over ___.Values, it is also faster
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
Simplest form to iterate a dictionary:
foreach(var item in myDictionary)
{
Console.WriteLine(item.Key);
Console.WriteLine(item.Value);
}
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.
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
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););
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];
}
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
}
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 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
}
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));
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);
}
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
}