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Summary: An example of how the System.Collections.Generic namespace can allow us to create a Property which can be added to, referred to by index and also iterated.
I recently came across a situation where I wanted to add a list of objects to a class and then refer to them by index. The System.Collections.Generic namespace came to the rescue, specifically via it's List Generic class.To demonstrate the situation, imagine we have a box which can contain multiple files. Firstly, I'd need to create a box class (perhaps with an ID so that we can reference it later) and then I'd need a Files property to which I could add multiple files. I'll also have to create a File class which I'll add an ID to as well.To represent this in code, we would create a File class with one property for the ID e.g. Public Class File Sub New(ByVal ID As Integer) _FileID = ID End Sub Public Property FileID() As Integer Get Return _FileID End Get Set(ByVal value As Integer) _FileID = value End Set End Property Private _FileID As Integer End Class Then, we need to look at how we create our Box class and this is where the Generics come in. Unlike our File class above, we need to be able to create a list of files rather than just one variable such as the ID. To do this, we can use the List class and the IList interface. When we create our Files property we need to tell it that it is actually a list and also what the list contains. In this case it will contain a list of our Files class e.g. Imports System.Collections.Generic Public Class Box Sub New(ByVal ID As Integer) _BoxID = ID _Files = New List(Of File) End Sub Public Property BoxID() As Integer Get Return _BoxID End Get Set(ByVal value As Integer) _BoxID = value End Set End Property Public Property Files() As IList(Of File) Get Return _Files End Get Set(ByVal value As IList(Of File)) _Files = value End Set End Property Private _BoxID As Integer Private _Files As IList(Of File) End Class Now, if I want to create a new box and add some files to it, I can do so very easily like this: ' Create a box Dim b As New Box(1) ' Add a few files b.Files.Add(New File(1)) b.Files.Add(New File(2)) b.Files.Add(New File(3)) Also, if I need to go back and reference a file by it's index I can do so like this: ' Get a file by index Dim myFile As File myFile = b.Files(2) And, if needed, I can loop through the entire list of files: ' Loop through the collection of files For Each f As File In b.Files Response.Write(f.FileID) Next
Posted on 16/05/2007 02:14:00
1. Chris Davis 18/05/2007 02:44:04
instead of:For Each f As File In b.Files Response.Write(f.FileID) Next why not use (c#): - i think the syntax is correct?b.Files.foreach(delegate(File f) {Response.Write(f.FileID);})
2. Will Asrari 13/06/2007 19:58:38
Cosign what Chris Davis wrote. With that 1 line of code you can dump the contents of an entire collection.
3. Jeff 13/07/2007 13:48:23
Okay so you're saying that if a List<T> variable has ten items or ten thousand items I can dump the whole collection using that one line of code? That's pretty smooth. But there's a term in there I'm not familiar with, delegate.In this case though he's only going after the file Id, what if your list had multiple items, like FileName, and LastUpdatedDate. Can you still use this to dump the collection? And if so would you then restructure the code thusly:b.Files.foreach(delegate(File f) { Response.Write(f); })or how would you do it so it dumped the entire collection of items no matter how many elements the the collection in question (in this case File f) had in it?
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