Using IEnumerable(Of String) to read from different kinds of data sources - vb.net

Question

I am using the below variable to store a list of user ID strings. I then use the list to search for each user using an LDAP query.
Dim userIds As IEnumerable(Of String) =
{"testid1", "testid2", "testid3", "testid4", "testid5"}
That works, but the ID's are hard-coded. How do I make it read the ID's from a ListBox control instead? Would it be something like:
Dim userIds As IEnumerable(Of String) = ListBox1???
I would like to use the ListBox because I will plan to load the ListBox with a bunch of ID's from a text file.
Better yet, is it possible to use a TextBox? If it was a TextBox, I could just copy and paste the ID's that I need to query.

Answer 1

The contents of a ListBox control can be accessed using the ListBox.Items property. It returns a ListBox.ObjectCollection object, which implements IList, ICollection, and IEnumerable.
This is assuming you've added the contents programmatically, rather than binding to a DataSource. If you bound to a DataSource, as LarsTech suggests, you should use ListBox.DataSource.
If you wanted to use a TextBox control, you'd have to manually delimit each ID somehow. You could do this by putting only one ID per line, and then use the Split method to get each ID:
Dim ids as String() = myTextBox.Text.Split(new String() { Environment.NewLine }, StringSplitOptions.RemoveEmptyEntries);

Answer 2

I was going to write this as a comment, but it got a bit long and started involving code examples so I figured it would be better to post it as an answer, even though there is already an accepted answer which is very good.
Since I'm the one who originally gave you the code that used IEnumerable I feel like I should explain why I used it... IEnumerable is the lowest level interface that is implemented by all lists, collections, dictionaries, arrays, etc. Basically, anything that stores multiple data which can be looped through by a For Each loop, implements the IEnumerable interface. In fact, essentially the only thing that the IEnumerable interface supports is the ability to enumerate through its contents with a For Each loop. IEnumerable is just an interface, it's not a concrete object type. Therefore, when you create an IEnumerable variable, that means that variable can be used to reference (i.e. point to) any object that implements that interface (i.e. any object that can be enumerated with a For Each loop.
Therefore, in the following line, it's not creating an IEnumerable type of object. Or at least not in the concrete type sense. It's creating a specific type of object which happens to implement the IEnumerable interface and then sets the ids variable to point to it.
Dim userIds As IEnumerable(Of String) = {"1", "2", "3"}
The phrase {"1", "2", "3"} is actually a literal expression to represent an array of strings. In other words, that literal expression is the equivalent of doing the following:
Dim stringArray(2) As String
stringArray(0) = "1"
stringArray(1) = "2"
stringArray(2) = "3"
So, since the object containing the list of ID's is actually an array of strings, it could have been done like this:
Dim userIds() As String = {"1", "2", "3"}
However, since I wanted the code to work, regardless of the data source, I used the more general IEnumerable interface. Since the only thing that I actually required of the data was that it could be enumerated with a For Each loop, I didn't want to limit the flexibility of the code by requiring the input list of ID's to be of some higher-level specific object type. I didn't really care that the ID's were specifically an array, or a list, or a dictionary, or a collection. As long as it was something that I could loop through, that's all I cared about. By doing so, that made the code more flexible so that you could set the variable to any enumerable data source, such as the Items property of the ListBox. For instance, all of the following would have worked, without changing the rest of the code:
Dim userIds As IEnumerable(Of String) = {"1", "2", "3"}
Or
Dim userIdsArray() As String = {"1", "2", "3"}
Dim userIds As IEnumerable(Of String) = userIdsArray
Or
Dim userIdsArray(2) As String
userIdsArray(0) = "1"
userIdsArray(1) = "2"
userIdsArray(2) = "3"
Dim userIds As IEnumerable(Of String) = userIdsArray
Or
Dim userIds As IEnumerable(Of String) = ListBox1.Items.OfType(Of String)()
Or
Dim userIds As IEnumerable(Of String) = File.ReadAllLines("IDs.txt")
Or
Dim userIds As IEnumerable(Of String) = TextBox1.Text.Split({Environment.NewLine}, StringSplitOptions.RemoveEmptyEntries)
Etc.
Since all of those above data sources implement the IEnumerable interface, the same userIds variable can be used to reference all of them.