(转载)C# Dictionary

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Fast lookups are critical.The Dictionary type provides fast lookups with keys to get values.With it we use keys and values of any type, including ints and strings.Dictionary requires a special syntax form.

IntString

Note:Dictionary is used when we have many different elements.We specify its key type and its value type.It provides good performance.

Add

To get started, we add four keys with values to a Dictionary instance.Afterwards, we look inside the Dictionary using Visual Studio's debugger.You will see that the Dictionary is composed of separate keys and values.

Here:We see the Dictionary code.The example program has no output.It does nothing useful.

Program that uses Dictionary Add method: C#using System;using System.Collections.Generic;class Program{    static void Main()    {Dictionary<string, int> dictionary =    new Dictionary<string, int>();dictionary.Add("cat", 2);dictionary.Add("dog", 1);dictionary.Add("llama", 0);dictionary.Add("iguana", -1);    }}

Next,we see what the above code looks like in memory.The Dictionary instance is represented by a collection of key and value pairs.It is fun (and perhaps helpful) to open and close the data structure elements.

Get values

Next, you can check to see if a given string is present in a Dictionary with string keys.We look at more types of Dictionaries further on, but here is the ContainsKey method.It returns true if the key was found.

ContainsKey

Tip:There is a more efficient method called TryGetValue.You should definitely use it when possible.

And:As its name implies, it tests for the key and then returns the value if it finds the key.

TryGetValue

Program that uses ContainsKey: C#using System;using System.Collections.Generic;class Program{    static void Main()    {Dictionary<string, int> dictionary = new Dictionary<string, int>();dictionary.Add("apple", 1);dictionary.Add("windows", 5);// See whether Dictionary contains this string.if (dictionary.ContainsKey("apple")){    int value = dictionary["apple"];    Console.WriteLine(value);}// See whether Dictionary contains this string.if (!dictionary.ContainsKey("acorn")){    Console.WriteLine(false);}    }}Output1False

Also,we sometimes encounter a KeyNotFoundException.This happens when we access a key that does not exist.With Dictionary we must test keys for existence first,with ContainsKey or TryGetValue.

KeyNotFoundException

KeyValuePair

When Dictionary, or any object that implements IDictionary,is used in a foreach-loop, it returns an enumeration.In the case of Dictionary, this enumeration is in the form of KeyValuePair values.

KeyValuePair

Loop

Here we use foreach syntax and KeyValuePair generics in the foreach loop.With collections like Dictionary, we must always know the value types.With each KeyValuePair, there is a Key member and Value member.

Foreach

Program that uses foreach on Dictionary: C#using System;using System.Collections.Generic;class Program{    static void Main()    {// Example Dictionary againDictionary<string, int> d = new Dictionary<string, int>(){    {"cat", 2},    {"dog", 1},    {"llama", 0},    {"iguana", -1}};// Loop over pairs with foreachforeach (KeyValuePair<string, int> pair in d){    Console.WriteLine("{0}, {1}",pair.Key,pair.Value);}// Use var keyword to enumerate dictionaryforeach (var pair in d){    Console.WriteLine("{0}, {1}",pair.Key,pair.Value);}    }}Outputcat, 2dog, 1llama, 0iguana, -1cat, 2dog, 1llama, 0iguana, -1

The code createsa Dictionary with string keys and int values.The Dictionary stores animal counts.The program has a ShowDictionaryPair method.This method shows the foreach-loop and the KeyValuePair declaration.

Tip:In the foreach-loop,each KeyValuePair has two members, a string Key and an int Value.

The final loopin the code shows how to make the syntax for looping simplerby using the var keyword.This is not desirable in some projects.But for KeyValuePair, it reduces source code size.It makes code easier to read.

VarVar Dictionary

Keys

Here we use the Keys property.We then look through each key and lookup the values.This method is slower but has the same results.Using the Keys collection and putting it in an array or List is sometimes effective.

Tip:The Keys property returns a collection of type KeyCollection,not an actual List.We can convert it into a List.

Program that gets Keys: C#using System;using System.Collections.Generic;class Program{    static void Main()    {Dictionary<string, int> d = new Dictionary<string, int>(){    {"cat", 2},    {"dog", 1},    {"llama", 0},    {"iguana", -1}};// Store keys in a ListList<string> list = new List<string>(d.Keys);// Loop through listforeach (string k in list){    Console.WriteLine("{0}, {1}",k,d[k]);}    }}Outputcat, 2dog, 1llama, 0iguana, -1

Benchmark

Using foreach on KeyValuePairs is several times faster than using Keys.KeyValuePair allows us to look through each pair.This avoids lookups.And it avoids changes to the garbage-collected heap.

Note:These figures are not perfect.I made a small change to the Keys version,so these figures are only general.

Benchmark for KeyValuePair foreach-loopKeyValuePair: 125 msNote:         This loops through the pairs in the Dictionary.Keys loop:    437 msNote:         This gets the Keys, then loops through them.      It does another lookup for the value.

Sort

If you need to sort the values in a Dictionary, you may be perplexed at firstand wonder how to order the keys.I have an article about how to do this.It is not optimal but it works.

Info:A dictionary cannot be sorted.But we can extract its keys and values and then sort those.

Sort Dictionary

Types

Dictionary is a generic class.This means it requires you to specify a type for it to use.So, you can use an int key, just as easily as a string key.In this program, we see an example of a Dictionary with int keys.

Tip:For advanced developers,you can use the GetHashCode method and override it to create Dictionaries or hashes with the class.

Program that uses int keys: C#using System;using System.Collections.Generic;class Program{    static void Main()    {// Use a dictionary with an int key.Dictionary<int, string> dict = new Dictionary<int, string>();dict.Add(100, "Bill");dict.Add(200, "Steve");// You can look up the int in the dictionary.if (dict.ContainsKey(200)){    Console.WriteLine(true);}    }}OutputTrue

LINQ

Extension methods can be used with Dictionary.We use the ToDictionary method.This is an extension method on IEnumerable.It places keys and values into a new Dictionary.The program uses lambda expressions.

Here:The example uses ToDictionary, from System.Linq, on the string array.It creates a lookup table for the strings.

ToDictionary

Note:This step has an initial cost.The program will require more time to start up.But it optimizes later performance.

Program that uses LINQ: C#using System;using System.Collections.Generic;using System.Linq;class Program{    static void Main()    {string[] arr = new string[]{    "One",    "Two"};var dict = arr.ToDictionary(item => item, item => true);foreach (var pair in dict){    Console.WriteLine("{0}, {1}",pair.Key,pair.Value);}    }}OutputOne, TrueTwo, True

ContainsValue

Dictionary has a method called ContainsValue.This method does not enjoy the constant-time lookup speed of ContainsKey.It instead searches the entire collection.It is linear in complexity.

This examplewill loop through all elements in the Dictionaryuntil it finds a match, or there are no more elements to check.MSDN states that "this method is an O(N) operation, where N is Count."

ContainsValue

Program that uses ContainsValue: C#using System;using System.Collections.Generic;class Program{    static void Main()    {Dictionary<string, int> d = new Dictionary<string, int>();d.Add("cat", 1);d.Add("dog", 2);if (d.ContainsValue(1)){    Console.WriteLine(true); // true}    }}OutputTrue

Indexer

Instead of calling Add on a Dictionary to add a new value,you can use the indexer with the "[" and "]" brackets.This syntax can also be used to get the value at the key.There are some advantages to this.

Caution:If you try to get a value at a key that doesn't exist, an exception is thrown.

Note:With the indexer, an exception is not thrown when you assign to a key that already has a value.But with Add, an exception is thrown.

Program that uses Dictionary indexer: C#using System;using System.Collections.Generic;class Program{    static void Main()    {Dictionary<int, int> dictionary = new Dictionary<int, int>();// You can assign with the indexer.dictionary[1] = 2;dictionary[2] = 1;dictionary[1] = 3; // Reassign.// You can read with the indexer.// ... If you read an element that doesn't exist, you get an exception.Console.WriteLine(dictionary[1]);Console.WriteLine(dictionary[2]);    }}Output31

Clear

You can erase all pairs by using the Clear method.Or you can assign the variable to null.The difference between Clear and null is not important for memory usage.In either case, the entries are garbage-collected.

Internally:We find that Clear calls Array.Clear, which is not managed code.Dictionary is implemented with arrays.

Clear

Count

The Count method is an effective way to compute the total number of keys in the instance.This is much simpler than accessing the Keys property,or looping over the Dictionary to count it.

Count

Remove

We can eliminate an entry, not just by setting its value to null or string.Empty,but by also removing the key itself.We use the Remove method.No remnants of the key-value pair are kept.

Note:Running the code in Visual Studio, no exceptions are thrown.When you remove a key that doesn't exist, nothing happens.

However:Remove throws System.ArgumentNullException with a null parameter.You cannot remove null.

Program that uses Remove: C#using System;using System.Collections.Generic;class Program{    static void Main()    {Dictionary<string, int> d = new Dictionary<string, int>();d.Add("cat", 1);d.Add("dog", 2);d.Remove("cat"); // Removes catd.Remove("nothing"); // Doesn't remove anything    }}Result    The key "cat" is removed.

Copy

The Dictionary class has a constructor that copies allvalues and keys into a new Dictionary instance.You can write the logic yourself.But using this constructor improves code reuse and simplicity.

Note:This site has more information on the Dictionary copy constructor.We demonstrate how it writes into separate memory.

Copy Dictionary

Return

It is also possible to use a Dictionary asan argument to methods or as a return value from methods or properties.The Dictionary type is defined as a class.It is always passed as a reference type.

And:This means only 32-64 bits will be copied on the method invocation.The same principles apply when with return values.

Return

List vs. Dictionary

I suggest you almost always use Dictionary for lookups.If you use List and you need to look up a key, your program may freeze if you haveexcess elements.With Dictionary your program recovers from pathological, edge cases.

List

On the other hand,it is faster to loop through all the elements in a List than in a Dictionary.If looping through elements is the most common operation, a List is superior.Usually looping speed is not critical.

Dictionary vs. List Loops

Also:You will find other collections, such as SortedDictionary, in the .NET Frameworkavailable for you to use.

But:I have found it is hard to get equivalent performance as you can with Dictionary.

SortedDictionary

Composite keys

You can sometimes use multiple variables in a key by creating a special functionthat transforms those variables into a string, serializing them.So, you could use the string "1,2" to mean the ints 1 and 2.

Tip:This is similar to how composite names in programming languagesuse a period: "Type.Member".

Field

Sometimes it is useful to have a Dictionary at the class level,not in a method or constructor.And if you have a static class then you should initialize your Dictionaryat the class level.

Also:Avoid the static constructor—static constructors have performancepenalties, which I have measured.

Program that uses Dictionary with class: C#using System;using System.Collections.Generic;class Program{    static void Main()    {Example e = new Example();Console.WriteLine(e.GetValue());    }}class Example{    Dictionary<int, int> _d = new Dictionary<int, int>()    {{1, 1},{2, 3},{3, 5},{6, 10}    };    public int GetValue()    {return _d[2]; // Example only    }}Output3

GetEnumerator

It is possible to loop over a Dictionary without a foreach loop.We use the GetEnumerator method and then a while-loop on the MoveNext method.This improves performance slightly, but adds complexity.

GetEnumerator

IEqualityComparer

The Dictionary type uses an IEqualityComparer to determine the hashcode for its keys.You can implement this interface with a class.This can improve performance in some programs, but often has limited benefits.

IEqualityComparer Dictionary

Performance

We revisit here Dictionary performance.The Dictionary is well-optimized by the .NET Framework developers.But there are still some techniques that influence performance.Knowing how Dictionary works is one key to improvement.

1. Change add order.The order we add keys to a Dictionary influences how fast those keysare looked up in the future.

Change Add Order

2. Increase capacity.We can increase the capacity to about four times what it needs to be to accommodateall elements.

Increase Capacity

3. Use smaller collections.If you have two different sets of data, using two different Dictionariesmay help.

Use Smaller Dictionary

4. Shorten lookup keys.For a Dictionary with string keys, short lookup keyscan improve performance.

Shorten Key Length

5. Use faster comparer.Also for Dictionaries with string keys, you can use a faster StringComparer

Use StringComparer

6. Test keys for existence.It is best to use ContainsKey to see if a key exists before changing it.

Test Before Adding

7. Measure memory usage.When optimizing a program, you are possibly wasting your time unlessyou measure your progress.

Measure Memory UsageBenchmark

Comparisons

These articles set Dictionaryversus other collections such as an array and a List.In some program contexts, arrays and Lists can be used instead of a Dictionary.Some aspects of performance are more important than others.

Array vs. DictionaryList vs. Dictionary

Caution:Some of the optimization tips do not apply to most programs.And some tips may cause minimal benefits or even performance loss.

Also:We see examples of the classic tradeoff in computerscience: faster lookups but with less memory efficiency.

Misc.

Not all requirements you may have with your collections are satisfied by the built-in methods.You will require some custom methods.These methods provide a sample of the custom routines you can construct.

Dictionary Binary FileDictionary Equals MethodCombine Dictionary Keys

Also:These examples show Dictionary instances used in different contexts.They may be less useful.

Case-Insensitive DictionaryStatic DictionaryStopword Dictionary

Summary

We explored the Dictionary type.We further used KeyValuePair to look through all pairs in the Dictionary.And we tested the ContainsKey and TryGetValue methods, which check key existence.

Thus:With its performance advantages,Dictionary fascinates, but is also profoundly useful.