Dictionary with String Array as Values Causes Duplicate Keys Error

Question

I'd like to use a Dictionary to store multiple strings for a key.

For instance, the Key will be "The Beatles", and the array of Strings Will contain all "Records" I own. The next key will be "Rolling Stones" and this will hold all the records I own for this band, etc.

As I go through a list of records I own, from a text file, I'd like to update the Dictionary, and add more Records to the Array.

Is this possible? I keep getting "An item with the same key has already been added" error messages.

 Dim Records_I_Own As New Dictionary(Of String, String())

 For each Line in Lines_I_Read

     ....
     ....
     Records_I_Own.Add(Band, Record)
     ....
     ....

 Loop

The error means that the Band variable has the same value two or more times. You should make sure it isn't — Visual Vincent
– Visual Vincent, Commented Mar 14, 2016 at 9:43

Matt Wilko · Accepted Answer · 2016-03-15 08:21:46Z

2

You add a Band and record every time you read a line. When you get to a band that you have already added you will get this error because the whole point of a dictionary is that there are not duplicate keys.

What you need to do is add the record to the string array of the band.

You need to check to see if you already have the Band (key)

So you need to do something like this:

 For each Line in Lines_I_Read

     ....
     ....
     If Records_I_Own.ContainsKey(Band) Then
         'We already have this artist so add a new record to the value collection 
         'get the array
         Dim records as String() = Records_I_Own(Band)
         'extend the array
         ReDim Preserve records(records.Count)
         'add the new record on the end
         records(records.Count - 1) = record
         'copy the array back to the dicationary value
         Records_I_Own(Band) = records
     Else     
         'Add it as a new key and value
         Records_I_Own.Add(Band, Record)
     Endif

     ....
     ....

 Loop

Note that it would be easier to do this if you used a List(of String) instead of a String Array because you can just call.Add instead of having to extend the array

edited Mar 15, 2016 at 8:21

answered Mar 14, 2016 at 10:30

Matt Wilko

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2 Comments

Martin Verjans Over a year ago

Records_I_Own.ContainsKey(Band) does not return a String()... It does return a boolean indicating if the key already exists in the Dictionary...

Matt Wilko Over a year ago

@Martin - well spotted - copy an paste typo. Now fixed

Martin Verjans · Accepted Answer · 2016-03-14 09:43:38Z

2

Use a

Dictionary(of String, List(of String))

instead of an array. So you can add

...
if Not Records_I_Own.containsKey(Band) then
  Records_I_own.Add(Band, New List(of String))
End If
Records_I_Own(Band).Add(Record)
....

answered Mar 14, 2016 at 9:43

Martin Verjans

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