VB.NET - KeyValuePair Examples

KeyValuePair. A KeyValuePair is a Structure. It has 2 fields of specified types. It stores 2 pieces of data together as a single object.

Data types. KeyValuePair defines a Structure of any kind of key, and any kind of value. Often we use Strings and Integers for the types, but any type is possible—reference or value.

First example. Here we create a List that stores KeyValuePairs. The (Of String, Integer) syntax is used to specify that the key is a String, and the value is an Integer.

Note When creating a KeyValuePair, you must set the key and the value in the constructor.

Finally We loop through all the elements of the list with For-Each. We access Key, and Value.

For

Module Module1 Sub Main() ' Create List of key-value pairs. Dim list As List(Of KeyValuePair(Of String, Integer)) = New List(Of KeyValuePair(Of String, Integer)) list.Add(New KeyValuePair(Of String, Integer)("Cat", 1)) list.Add(New KeyValuePair(Of String, Integer)("Dog", 2)) list.Add(New KeyValuePair(Of String, Integer)("Rabbit", 4)) ' Loop over pairs. For Each pair As KeyValuePair(Of String, Integer) In list ' Display key and value. Console.WriteLine(pair) Next End Sub End Module

[Cat, 1] [Dog, 2] [Rabbit, 4]

Return values. Sometimes you may want to return 2 values at once from a function. You could use the ByRef modifier. But the KeyValuePair can also be returned.

Info GetPair() returns a new instance of the KeyValuePair type that has a specific key and value.

Module Module1 Sub Main() Dim pair As KeyValuePair(Of String, String) = GetPair() Console.WriteLine(pair.Key) Console.WriteLine(pair.Value) End Sub Function GetPair() As KeyValuePair(Of String, String) ' Create and return a KeyValuePair instance. Return New KeyValuePair(Of String, String)("image/avif", "avif") End Function End Module

image/avif avif

Dictionary for-each. The easiest way to loop over the keys and values in a Dictionary instance is with the For-Each loop construct. In this code, we use the KeyValuePair in the loop.

Dictionary

Module Program Sub Main() ' Create and add values to the Dictionary. Dim animals As Dictionary(Of String, Integer) = New Dictionary(Of String, Integer)() animals("Cat") = 2 animals("Dog") = 1 ' Loop over dictionary. For Each pair As KeyValuePair(Of String, Integer) In animals Console.WriteLine(pair) Next End Sub End Module

[Cat, 2] [Dog, 1]

Tuple. The KeyValuePair is restricted to a key and a value, but the Tuple type can have more items. If you need 3 fields grouped together, check out the Tuple type.

Note One drawback of the Tuple type is that is must be allocated on the managed heap as an object instance.

However The KeyValuePair is a structure so it can often be allocated in the stack memory.

Tuple

A summary. KeyValuePair stores a key and a value together in a single object in memory. It is a value type. A common usage is in the For-Each loop in a Dictionary.