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Accessing a Dictionary Using Multiple Keys

Dictionaries and some other collection objects allow you to store an item along with a key value. This allows you to retrieve the item later, either by position or by key value. This code stores a value under the key "PHVIS," and finishes by setting the variable Result to "Peter Vogel":

Dim Items As New Dictionary(Of String,  String)
Items.Add("PHVIS", "Peter  Vogel")
Dim Result As String
Result = Items("PHVIS")  

But what if you want to store something by two values -- storing sales amounts by CustomerName and SaleDate, for instance? You could just concatenate two values together to create a single key, but a better solution would be to define a class to hold the two values to use as the key. Unfortunately, using a class won't work. When testing for equality, these collections will use an object's hashcode, returned from the class' GetHashCode method, to determine if two objects are identical. Two different objects, even if they're from the same class and have identical values in their properties, are considered to be different under this test.

But if you use a Structure instead,, everything works. Structures are scalar types rather than reference types like objects, so their values are compared to determine equality. A structure to hold two values might look like this:

Structure TwoValue
  Public CustomerId As String
  Public SaleDate As DateTime
End Structure

To use this structure, you might store a customer's sales under the customer's identifier, and the date that the sale was made:

Dim Items As New Dictionary(Of TwoValue, Integer)
Dim Key As TwoValue
Key.CustomerId = "A123"
Key.SaleDate =  Date.Parse("11/01/2010")
Items.Add(Key, 132)

Retrieving the value just consists of assembling a matching key and passing the key to the Dictionary. This example will retrieve the value and puts it in the variable Result: 

Dim Result As Integer
Dim Key2 As TwoValue
Key2.CustomerId = "A123"
Key2.SaleDate =  Date.Parse("11/01/2010")

Result = items(Key2)

Posted by Peter Vogel on 04/04/2012 at 1:16 PM


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