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?



Source: Tips4all

10 comments:

  1. foreach(KeyValuePair<String,String> entry in MyDic)
    {
    // do something with entry.Value or entry.Key
    }

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  2. 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);
    }

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

    foreach(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)

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  4. In some cases you may need a counter that may be provided by for-loop implementation. For that LINQ let us use the following:

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

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

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


    Is that what you're looking for?

    ReplyDelete
  6. 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

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  7. 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.

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  8. 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.

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  9. Use the built in support for the iterator pattern, the foreach key word.

    If 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);
    }

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  10. 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();
    }

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