`SortedList`

Some collections keep their element sorted as you use them. The SortedList type, for example, always sorts its keys. We add keys and values to SortedList.

Unlike a List, the SortedList stores both keys and values. For a collection with just keys (that is more like a List, except no duplicates) consider a SortedSet.

First example

Here we create a SortedList, and then invoke Add(). In Add, the first argument is the key to add. The second argument is the value at that key.

Start We use the For-Each loop over the SortedList. The KeyValuePair is used (with matching types to the SortedList) in the loop.

Tip The SortedList has no ways to reorder its elements. If you want to change the ordering, you must create a new collection.

Module Module1
    Sub Main()
        ' Create SortedList and add Strings to it.
        Dim sorted As SortedList(Of String, String) =
            New SortedList(Of String, String)

        sorted.Add("dog", "red")
        sorted.Add("cat", "black")
        sorted.Add("zebra", "black and white")
        sorted.Add("ant", "black")

        ' Loop over pairs in the collection.
        For Each pair As KeyValuePair(Of String, String) In sorted
            Console.WriteLine(pair.Key + "/" + pair.Value)
        Next
    End Sub
End Moduleant/black
cat/black
dog/red
zebra/black and white

`IndexOfKey`

We can search for keys and values in the SortedList. To find a key, use IndexOfKey. The special value -1 is returned if the key does not exist—no exception is thrown.

And Use IndexOfValue to find a value in the SortedList. The special value -1 is used here too to mean "missing."

Module Module1
    Sub Main()
        Dim items = New SortedList(Of String, Integer)
        items.Add("bird", 20)
        items.Add("fish", 0)

        ' Display all pairs.
        For Each item As KeyValuePair(Of String, Integer) In items
            Console.WriteLine("ITEM: {0}", item)
        Next

        ' Find this key.
        Dim indexFish = items.IndexOfKey("fish")
        Console.WriteLine("INDEX OF FISH: {0}", indexFish)

        ' Find this value.
        Dim index20 = items.IndexOfValue(20)
        Console.WriteLine("INDEX OF 20: {0}", index20)

        ' If a key or value is missing, the value -1 is returned.
        Dim indexMissing = items.IndexOfKey("carrot")
        Console.WriteLine("INDEX OF CARROT: {0}", indexMissing)
    End Sub
End ModuleITEM: [bird, 20]
ITEM: [fish, 0]
INDEX OF FISH: 1
INDEX OF 20: 0
INDEX OF CARROT: -1

Notes, duplicates

If we try to add a duplicate to the SortedList, an exception is thrown. We cannot call Add without first checking for the key in the List (consider IndexOfKey).

The SortedList is a sorted list of pairs. It is not used in the same way as a List, which makes it harder to apply in programs. A SortedSet is often a better choice.