DictionaryIn VB.NET the Dictionary allows fast key lookups. A generic type, it can use any types for its keys and values. Its syntax is at first confusing.
Compared to alternatives, a Dictionary is easy to use and effective. It has many functions (like ContainsKey and TryGetValue) that do lookups.
Most Dictionary collections we use will be populated with the Add() Subroutine. Usually we create an empty Dictionary and then Add() keys and values.
Dictionary with String keys, and Integer values. The Dictionary is empty here.Dictionary. For the arguments, we pass the key we want to add, and the value.Count of this dictionary, after four Add() calls have run, is 4—a key and value are counted together as one entry.Module Module1
Sub Main()
' Step 1: create a Dictionary.
Dim dictionary As New Dictionary(Of String, Integer)
' Step 2: add 4 entries.
dictionary.Add("bird", 20)
dictionary.Add("frog", 1)
dictionary.Add("snake", 10)
dictionary.Add("fish", 2)
' Step 3: display count.
Console.WriteLine("DICTIONARY COUNT: {0}", dictionary.Count)
End Sub
End ModuleDICTIONARY COUNT: 4TryGetValueOften we test a value through a key in a Dictionary collection. Then we act upon that value. The TryGetValue function combines the two steps—this enables optimizations.
Dictionary with string keys and values, and then add two pairs to it. We then invoke TryGetValue and print the result.GetValueOrDefault for a more convenient way to access values in a VB.NET dictionary.Module Module1
Sub Main()
Dim values As Dictionary(Of String, String) = New Dictionary(Of String, String)
values.Add("A", "uppercase letter A")
values.Add("c", "lowercase letter C")
' Get value with TryGetValue.
Dim result As String = Nothing
If values.TryGetValue("c", result) Then
Console.WriteLine("RESULT: {0}", result)
End If
End Sub
End ModuleRESULT: lowercase letter CContainsKeyThis function returns a Boolean value, which means you can use it in an If conditional statement. One common use of ContainsKey is to prevent exceptions before calling Add.
Module Module1
Sub Main()
' Declare new Dictionary with String keys.
Dim dictionary As New Dictionary(Of String, Integer)
' Add two keys.
dictionary.Add("carrot", 7)
dictionary.Add("perl", 15)
' See if this key exists.
If dictionary.ContainsKey("carrot") Then
' Write value of the key.
Dim num As Integer = dictionary.Item("carrot")
Console.WriteLine(num)
End If
' See if this key also exists (it doesn't).
If dictionary.ContainsKey("python") Then
Console.WriteLine(False)
End If
End Sub
End Module7If you add keys to the Dictionary and one is already present, you will get an exception. We often must check with ContainsKey that the key is not present.
Module Module1
Sub Main()
Dim lookup As Dictionary(Of String, Integer) =
New Dictionary(Of String, Integer)
lookup.Add("cat", 10)
' This causes an error.
lookup.Add("cat", 100)
End Sub
End ModuleUnhandled Exception: System.ArgumentException:
An item with the same key has already been added.
at System.ThrowHelper.ThrowArgumentException...KeyNotFoundExceptionTo see if a key exists in a Dictionary, we should use ContainsKey or TryGetValue. If we just access the key directly, we might get a KeyNotFoundException.
KeyNotFoundException, but this will be slower than not causing an exception.Module Module1
Sub Main()
Dim dict = New Dictionary(Of String, String)()
' We must use ContainsKey or TryGetValue.
If dict("car") = "vehicle" Then
Return
End If
End Sub
End ModuleUnhandled Exception: System.Collections.Generic.KeyNotFoundException:
The given key was not present in the dictionary.
at System.Collections.Generic.Dictionary`2.get_Item(TKey key)...The Item member is a Property accessor. It sets or gets an element in the Dictionary. It is equivalent to Add when you assign it.
Add() may be a better choice than assigning to a key in a Dictionary—it will warn you with an exception if you have a duplicate.Module Module1
Sub Main()
Dim dictionary = New Dictionary(Of Integer, Integer)()
' Add data by assigning to a key.
dictionary(10) = 20
' Look up value.
Console.WriteLine(dictionary(10))
End Sub
End Module20For Each loopWe can loop over the entries in a Dictionary. It is usually easiest to use the For Each loop. We can access each individual KeyValuePair structure in the loop body.
Dictionary and add 4 color names to it—blue, yellow, green and red.Dictionary keys and values—no casts are required.Module Module1
Sub Main()
' Step 1: create Dictionary with 4 keys.
Dim colors As New Dictionary(Of String, Integer)
colors.Add("blue", 32)
colors.Add("yellow", 16)
colors.Add("green", 256)
colors.Add("red", 100)
' Step 2: use For Each loop over pairs.
For Each pair As KeyValuePair(Of String, Integer) In colors
Console.WriteLine("COLOR: {0}, VALUE: {1}", pair.Key, pair.Value)
Next
End Sub
End ModuleCOLOR: blue, VALUE: 32
COLOR: yellow, VALUE: 16
COLOR: green, VALUE: 256
COLOR: red, VALUE: 100KeysWe can get a List of the Dictionary keys. Dictionary has a Keys property, and we can use this with types like the List that act on IEnumerable.
Dictionary and place 4 String keys (which are all animal names) in it, with Integer values.List constructor on the Keys property. The keys have the same type as that from the source Dictionary.Item() we can access the value from the Dictionary based on the String key.Module Module1
Sub Main()
' Step 1: add 4 string keys.
Dim animals As New Dictionary(Of String, Integer)
animals.Add("bird", 12)
animals.Add("frog", 11)
animals.Add("cat", 10)
animals.Add("elephant", -11)
' Step 2: get List of String Keys.
Dim list As New List(Of String)(animals.Keys)
' Step 3: loop over Keys and print the Dictionary values.
For Each value As String In list
Console.WriteLine("ANIMAL: {0}, VALUE: {1}", value, animals.Item(value))
Next
End Sub
End Moduleplease, 12
help, 11
poor, 10
people, -11Dictionary uses typed keys and values. We specify these types when the Dictionary is declared. Here we use a more complex value type—a String array.
Dictionary. It has Integer keys and String array values—so each int can point to an entire array.ContainsKey. When the key type is Integer, we must pass an Integer to ContainsKey.String array value with Item, and then call String.Join to concatenate its values.Module Module1
Sub Main()
' Step 1: use Integer keys, String array values.
Dim dictionary As New Dictionary(Of Integer, String())
dictionary.Add(100, New String() {"cat", "bird"})
dictionary.Add(200, New String() {"dog", "fish"})
' Step 2: see if key exists.
If dictionary.ContainsKey(200) Then
' Step 3: get array value, join elements, and print it.
Dim value() As String = dictionary.Item(200)
Console.WriteLine("RESULT: {0}", String.Join(",", value))
End If
End Sub
End ModuleRESULT: dog,fishContainsValueThis returns a Boolean that tells whether any value in the Dictionary is equal to the argument. It is implemented as a For-loop over the entries in the Dictionary.
ContainsValue has no performance advantage over a List that uses linear searching. Accessing keys in the Dictionary is faster.Module Module1
Sub Main()
' Create new Dictionary with Integer values.
Dim dictionary As New Dictionary(Of String, Integer)
dictionary.Add("pelican", 11)
dictionary.Add("robin", 21)
' See if Dictionary contains the value 21 (it does).
If dictionary.ContainsValue(21) Then
' Prints true.
Console.WriteLine(True)
End If
End Sub
End ModuleTrueRemoveHere we use Remove. You must pass one parameter to this method, indicating which key you want to have removed from the Dictionary instance.
Module Module1
Sub Main()
' Create Dictionary and add two keys.
Dim dictionary As New Dictionary(Of String, Integer)
dictionary.Add("fish", 32)
dictionary.Add("microsoft", 23)
' Remove two keys.
dictionary.Remove("fish") ' Will remove this key.
dictionary.Remove("apple") ' Doesn't change anything.
End Sub
End ModuleIt is possible to copy the entire contents of a Dictionary. You can do this by declaring a new Dictionary reference and using the copy constructor.
Dictionary constructor, pass the Dictionary you want to copy as the parameter.Module Module1
Sub Main()
Dim source = New Dictionary(Of String, Integer)()
source.Add("bird", 20)
' Copy the Dictionary.
Dim copy = New Dictionary(Of String, Integer)(source)
' Write some details.
Console.WriteLine("COPY: {0}, COUNT = {1}", copy("bird"), copy.Count)
End Sub
End ModuleCOPY: 20, COUNT = 1We use a Dictionary in a class. We store it as Private member variable, and then access it through Public methods on the enclosing class.
Main Sub. We create a new instance of the Example class.New() Sub in the Example class. The Dictionary field is created. We add 3 entries to the Dictionary.GetValue() on the Example instance. This returns a value from the Dictionary that was stored with the String key "make."Module Module1
Class Example
Private _dictionary
Public Sub New()
' Part 2: allocate and populate the field Dictionary.
Me._dictionary = New Dictionary(Of String, Integer)
Me._dictionary.Add("make", 55)
Me._dictionary.Add("model", 44)
Me._dictionary.Add("color", 12)
End Sub
Public Function GetValue() As Integer
' Return value from private Dictionary.
Return Me._dictionary.Item("make")
End Function
End Class
Sub Main()
' Part 1: allocate an instance of the class.
Dim example As New Example
' Part 3: write a value from the class.
Console.WriteLine(example.GetValue())
End Sub
End Module55CountYou can count the number of entries with the Count property. Internally, the Count property subtracts 2 integers—it does no looping.
Module Module1
Sub Main()
Dim dictionary As New Dictionary(Of String, Integer)
dictionary.Add("a", 5)
dictionary.Add("b", 8)
dictionary.Add("c", 13)
dictionary.Add("d", 14)
' Get count.
Console.WriteLine(dictionary.Count)
End Sub
End Module4ToDictionaryWe can quickly construct a new Dictionary from a collection (array, List) with the ToDictionary extension method. ToDictionary returns a Dictionary.
ToDictionary has two lambda arguments. They both receive one argument: the String from the source array.String (for the first, key selector function) and an Integer (for the value selector function).Module Module1
Sub Main()
' Create an array of four string literal elements.
Dim array() As String = {"dog", "cat", "rat", "mouse"}
' Use ToDictionary.
' ... Use each string as the key.
' ... Use each string length as the value.
Dim dict As Dictionary(Of String, Integer) =
array.ToDictionary(Function(value As String)
Return value
End Function,
Function(value As String)
Return value.Length
End Function)
' Display dictionary.
For Each pair In dict
Console.WriteLine(pair)
Next
End Sub
End Module[dog, 3]
[cat, 3]
[rat, 3]
[mouse, 5]ContainsKeyDictionary is an optimization for key lookups. It can make a slow program many times faster. Consider this benchmark, which tests Dictionary and List.
ContainsKey on a Dictionary with 1000 String keys.Contains to find a String key in a List that exists. The element is the 900th one in the List.Dictionary is faster. In the List, 900 items are looped over, but with hashing in the Dictionary, fewer items are scanned.Module Module1
Sub Main()
' Create Dictionary and List.
Dim lookup As Dictionary(Of String, Integer) = New Dictionary(Of String, Integer)
For i As Integer = 0 To 1000
lookup.Add(i.ToString(), 1)
Next
Dim list As List(Of String) = New List(Of String)
For i As Integer = 0 To 1000
list.Add(i.ToString())
Next
Dim m As Integer = 1000
' Version 1: search Dictionary.
Dim s1 As Stopwatch = Stopwatch.StartNew
For i As Integer = 0 To m - 1
If Not lookup.ContainsKey("900") Then
Return
End If
Next
s1.Stop()
Dim s2 As Stopwatch = Stopwatch.StartNew
' Version 2: search List.
For i As Integer = 0 To m - 1
If Not list.Contains("900") Then
Return
End If
Next
s2.Stop()
Dim u As Integer = 1000000
Console.WriteLine(((s1.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
Console.WriteLine(((s2.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
End Sub
End Module 35.10 ns Dictionary ContainsKey (1000 keys)
6801.10 ns List Contains (1000 elements)This generic type is powerful. Designed for super-fast lookups, Dictionary often improves the performance of programs. And it can simplify our logic by checking for duplicates.