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Sunday, June 3, 2012
What is the best way to iterate over a Dictionary in C#?
I've seen a few different ways to iterate over a Dictionary in C#. Is there a standard way?
Depends on whether you're after the keys or the values...
From the MSDN Dictionary<(Of <(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); }
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(); }
foreach(KeyValuePair<String,String> entry in MyDic)
ReplyDelete{
// do something with entry.Value or entry.Key
}
Depends on whether you're after the keys or the values...
ReplyDeleteFrom the MSDN Dictionary<(Of <(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);
}
If you are trying to use a generic Dictionary in C# like you would use an associative array in another language:
ReplyDeleteforeach(KeyValuePair<string, string> 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 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 let us use the following:
ReplyDeletefor (int index = 0; index < dictionary.Count; index++) {
var item = dictionary.ElementAt(index);
var itemKey = item.Key;
var itemValue = item.Value;
}
I would say foreach is the standard way, though it obviously depends on what you're looking for
ReplyDeleteforeach(var value in my_dictionary) {
...
}
Is that what you're looking for?
There are plenty of options. My personal favorite is by KeyValuePair
ReplyDeleteDictionary<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
You suggested below to iterate
ReplyDeleteDictionary<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.
I found this method in the documentation for the DictionaryBase class on MSDN:
ReplyDeleteforeach (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.
Use the built in support for the iterator pattern, the foreach key word.
ReplyDeleteIf using a non generic Dictionary, simply use the KeyValuePair type for the different items:
foreach(KeyValuePair item in myDictionary)
{
DoStuffWith(item);
}
The generic version is almost identical, apart from defining the types in the KeyValuePair to be the same as the dictionary:
foreach(KeyValuePair<Tkey, Tvalue> item in myDictionary)
{
DoStuffWith(item);
}
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.
ReplyDeletepublic new IEnumerator<T> GetEnumerator()
{
return this.Values.GetEnumerator();
}