Dim extensions As New Dictionary(Of String, String) _
from { { "txt", "notepad" },
{ "bmp", "paint" },
{ "doc", "winword" } }
This creates a dictionary and immediately fills it with three KeyValuePairs.
You can also add new values later on by using the Add method:
extensions.Add("png", "paint")
Note that the key (the first parameter) needs to be unique in the dictionary, otherwise an Exception will be thrown.