People often tell me not to use ArrayList for making my arrays in VB.NET.
I would like to hear opinions about that, why shouldn't I? What is the best method for creating and manipulating array contents, dimensions etc?
Thanks.
Use generic lists instead. ArrayList is not typed, meaning that you can have a list with strings, numbers, +++. Rather you should use a generic list like this:
Dim list1 As New List(Of String) ' This beeing a list of string
The lists-class also allows you to expand the list on the fly, however, it also enforces typing which helps write cleaner code (you don't have to typecast) and code that is less prone to bugs.
ArrayList is gennerally speaking just a List(Of Object).
ArrayLists are not type checked so you will need to do a lot of boxing/unboxing. Use a .net collection instead that support generics like List.
Because List does not have to unbox your objects it boasts a surprisingly better performance than Arraylist.
ArrayLists are less performant and memory-extensive:
Dim list1 As New ArrayList
For i As Integer = 1 To 100000000
list1.Add(i)
Next
' --> OutOfMemoryException after 13.163 seconds, having added 67.108.864 items
Dim list2 As New List(Of Integer)
For i As Integer = 1 To 100000000
list2.Add(i)
Next
' --> finished after 1.778 seconds, having added all values
Because its not strongly typed. Use a List(Of T) which T is your type.
Related
I've had to update a vb.net project from .NetFramework 4 to .NetFramework 4.7.2. In the process the following code is now throwing an error
Dim actuatorModelsArr = DirectCast(retNumberList, Array) Dim
Dim actuatorModels = actuatorModelsArr.Cast(Of ACTUATORMODELS)().ToList()
The error is System.ArrayTypeMismatchException: Source array type cannot be assigned to destination array type.
retNumberList is an Integer array
ACTUATORMODELS is an Enum
in the .netFramework 4 version actuatorModels is a list of the Enum
{System.Collections.Generic.List`1[FisherIECLib.ACTUATORMODELS]}
The list is used later in the module via linq to grab one of the Enums as a return value.
Is there a way around this or a way to create a list of the Enum?
Thanks in advance,
Hank
I think it's interesting that it stopped working. You can replace the code with a Select and cast to make it work
Dim retNumberList = {1, 2, 3, 4}
' either of these will produce a List of your Enum
Dim a = retNumberList.Select(Function(i) DirectCast(i, ACTUATORMODELS)).ToList()
Dim b = retNumberList.Select(Function(i) CType(i, ACTUATORMODELS)).ToList()
As for the original not working:
Dim c = retNumberList.Cast(Of ACTUATORMODELS)().ToList()
The literature suggests this is the equivalent of a (type)obj c# style cast, but both versions of vb.net cast work in the select. I am not sure why.
djv's answer helps fix your problem. Hopefully this answer will explain what's going wrong. If you look at the reference source for Cast(Of T) on an untyped IEnumerable, you'll find that the first thing that happens is the C# equivalent of this:
Dim asTyped = TryCast(source, IEnumerable(Of TResult))
If asTyped IsNot Nothing Then Return asTyped
Surprisingly, this cast will work for Integer() to IEnumerable(Of ACTUATORMODELS). This sets the issue in motion, because when it comes to making a List(Of T) out of the resulting sequence, it turns out that Integer() and ACTUATORMODELS() are actually not interchangeable, but Cast has already treated the sequence as though they are.
Based on some testing, this issue seems to arise out of the interaction of this corner case with a corner case in how the List(Of T) range constructor works and the more general corner case of Integer() vs TEnum().
In the general case of iterating over Integer() as if it were IEnumerable(Of TEnum), it works. You can make a For Each loop over the sequence, the enumeration variable will have TEnum as the type, and you'll see the values as if they were TEnum.
The problem comes in the range constructor for List(Of T), and an optimization there for a source that implements ICollection(Of T). In that case, the ctor will try to use ICollection(Of T).CopyTo to copy the items into the list's internal storage. This is where the error ultimately occurs, because (going back to the implementation of Cast(Of T)) the source is still the Integer() array, and Array.Copy (via Array.CopyTo) is not OK with trying to do that with a destination of TEnum().
I feel like this is a bug somewhere, though I'm not sure if it's in Cast(Of T), the List(Of T) range ctor, or in the array copy handling. I'm also not sure it's something that will get fixed, since it's a corner case that hits what look like significant optimizations and would require a very specialized check to catch.
I am using myArrayList.Contains(myString) and myArrayList.IndexOf(myString) to check if arrayList contains provided string and get its index respectively.
But, How could I check if contains a substring?
Dim myArrayList as New ArrayList()
myArrayList.add("sub1;sub2")
myArrayList.add("sub3;sub4")
so, something like, myArrayList.Contains("sub3") should return True
Well you could use the ArrayList to search for substrings with
Dim result = myArrayList.ToArray().Any(Function(x) x.ToString().Contains("sub3"))
Of course the advice to use a strongly typed List(Of String) is absolutely correct.
As far as your question goes, without discussing why do you need ArrayList, because array list is there only for backwards compatibility - to select indexes of items that contain specific string, the best performance you will get here
Dim indexes As New List(Of Integer)(100)
For i As Integer = 0 to myArrayList.Count - 1
If DirectCast(myArrayList(i), String).Contains("sub3") Then
indexes.Add(i)
End If
Next
Again, this is if you need to get your indexes. In your case, ArrayList.Contains - you testing whole object [string in your case]. While you need to get the string and test it's part using String.Contains
If you want to test in non case-sensitive manner, you can use String.IndexOf
I have declared an ArrayList like this
Dim List1 As ArrayList = New ArrayList
Adding a ListItem to it
Dim Item As String = ""
List1.Add(New ListItem(Item))
Is there is any limit how many characters the ListItem can contain?
"Is there is any limit how many characters Item can contains."
It's limited by the limit of the length of a String and your memory.
The theoretical limit may be 2,147,483,647, but the practical limit is
nowhere near that. Since no single object in a .Net program may be
over 2GB and the string type uses unicode (2 bytes for each
character), the best you could do is 1,073,741,823, but you're not
likely to ever be able to allocate that on a 32-bit machine.
https://stackoverflow.com/a/140749/284240
Apart from that, always use a strongly typed List(Of ListItem) instead of an ArrayList.
Dim List1 = New List(Of ListItem)
List1.Add(New ListItem("Foo1"))
c# When should I use List and when should I use arraylist?
I am very new to VB.NET, and I come from an Objective-C background.
In Objective-C, I could create an NSMutableDictionary which could hold any types of values.
Here with VB.NET, I managed to make a Dictionary, but when I initialized an instance of it, I am asked for a specific value type. How can I enable any value types within one single dictionary?
You can write a dictionary to hold objects like this:
Dim myDictionary As New Dictionary(Of String, Object)
And then to supply it:
myDictionary.Add("keyA", New Button)
myDictionary.Add("keyB", "Test")
This is probably not considered best programming practice. It's better to take advantage of generics and have a dictionary hold a common class or interface of items.
Having multiple dictionaries for each class or interface of items would be more efficient and easier to maintain.
You can use the Object type to hold any value. Example:
Dim myDict as New Dictionary(Of String, Object)()
Can I have a List containing one string and two numbers? Or I can only have one type of element?
If that's the kind of functionality you want, then I would look at the non-generic System.Collections.ArrayList class.
Update
For those of you who aren't going to read the huge comment chain...it looks like Adam Robinson is on to something using List<object> over ArrayList. Both will work but on large collections it seems like List<object> is measurably faster than ArrayList.
You can. A list of Objects can do that. But, you lose type safety with that and also design time intelliSense.
What do you want to do? You could also use a class with 3 members.
No, containers like List(Of T) store exactly one type T of elements. You can, though, make this one type consist of one string and two numbers.
Structure Foo
Public Desc As String
Public x As Integer, y As Integer
End Structure
Dim List = New List(Of Foo)
Yes, you can.
dim myVehicles as new list(of object)
dim myCar as new car
dim myBike as new bike
dim mySecondCar as new car
myVehicles.add(myCar)
myVehicles.add(myBike)
myVehicles.add(mySecondCar)