Create a list of Unknown type at compile time

dilkonika

Hello ! It's possible to create a list of unknown type at compile time ? Let's suppose the type is in one variable as a string. And I want to create a list of this type , and of course after I want to add items from this type to the list. I have found this code on C# , but it seems not work on vb.net :

// Create a List<> of unknown type at compile time.
Type customList = typeof(List<>).MakeGenericType(tempType);
IList objectList = (IList)Activator.CreateInstance(customList);

// Copy items from a PropertyInfo list to the object just created
object o = objectThatContainsListToCopyFrom;
PropertyInfo p = o.GetType().GetProperty("PropertyName");
IEnumerable copyFrom = p.GetValue(o, null);
foreach(object item in copyFrom) objectList.Add(item); // Will throw exceptions if the types don't match.

// Iterate and access the items of "objectList"
// (objectList declared above as non-generic IEnumerable)
foreach(object item in objectList) { Debug.WriteLine(item.ToString()); }

I just want to create the list , and after to add items like this :

List1.Items.Add(object).

What can I do ? Thank you !

Sascha Lefevre

That looks like it should work in C#. What problem do you have with the VB-translation?

If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson

dilkonika

I don't know c# , so I have used a code translator. this is the code :

Dim customList As Type = GetType(List(Of )).MakeGenericType(tempType)
Dim objectList As IList = CType(Activator.CreateInstance(customList), IList)

' Copy items from a PropertyInfo list to the object just created
Dim o As Object = objectThatContainsListToCopyFrom
Dim p As PropertyInfo = o.[GetType]().GetProperty("PropertyName")
Dim copyFrom As IEnumerable = p.GetValue(o, Nothing)
For Each item As Object In copyFrom
objectList.Add(item)
Next
' Will throw exceptions if the types don't match.
' Iterate and access the items of "objectList"
' (objectList declared above as non-generic IEnumerable)
For Each item As Object In objectList
Debug.WriteLine(item.ToString())
Next

It shows several errors , but the first is on second line :

Error 103 Too few type arguments to 'System.Collections.Generic.IList(Of T)'

Sascha Lefevre

This works for me:

Option Explicit On
Option Strict On

Imports System.Reflection

Module Module1

Public Class ChildClass
End Class

Public Class SomeClass
    Public Property Children As ICollection(Of ChildClass) = New HashSet(Of ChildClass)
End Class

Sub Main()
    Dim tempType = GetType(ChildClass)
    Dim someClass = New SomeClass
    someClass.Children.Add(New ChildClass)

    Dim customList As Type = GetType(List(Of )).MakeGenericType(tempType)
    Dim objectList As IList = CType(Activator.CreateInstance(customList), IList)

    ' Copy items from a PropertyInfo list to the object just created
    Dim o As Object = someClass
    Dim p As PropertyInfo = o.\[GetType\]().GetProperty("Children")
    Dim copyFrom As IEnumerable = CType(p.GetValue(o, Nothing), IEnumerable)
    For Each item As Object In copyFrom
        objectList.Add(item)
    Next
    ' Will throw exceptions if the types don't match.
    ' Iterate and access the items of "objectList"
    ' (objectList declared above as non-generic IEnumerable)
    For Each item As Object In objectList
        Debug.WriteLine(item.ToString())
    Next
End Sub

End Module

If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson

dilkonika

sorry , but the Ilist type is unknown . Everytime I get an error that :

Error 103 Too few type arguments to 'System.Collections.Generic.IList(Of T)'

I don't know why ?

Sascha Lefevre

Do you get this error also when running my code on its own in a new console-project?

If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson

dilkonika

Yes , Ilist and Ienumerable are unknown and produce a similar error message that I have posted before.

Sascha Lefevre

I wouldn't know why it should be neccessary for you, as it works for me without, but try it with these imports:

Imports System.Collections
Imports System.Collections.Generic

If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson

dilkonika

I add :

Imports System.Collections

and now the errors are resolved. But I have some problems in runtime 1. I want first to initialize an empty list. Because if I have just the :

dim itemlist as Ilist

( Before calling these 2 lines

Dim customList As Type = GetType(List(Of )).MakeGenericType(GetType(MyClass))
itemlist = CType(Activator.CreateInstance(customList), IList)

How can I check if the list is empty or ?? something similar. But I want a general checkin mode , because the same expression must be correct even when I clear the list and I want to check if empty ( before initializing for a new type ). ( I try to use

If itemlist.Count > 0 Then

but doesn't work.

Sascha Lefevre

I have a hard time understanding that - can you explain it a bit more detailed?

If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson

dilkonika

ok. I need a way to detect that this list is empty. But the expression that should do this , must include 2 cases : - The list is not yet created . ( So it's just declared as mylist as Ilist). - The list is created ( with your code ) but the items are deleted ( or has no items ). I try with the code : If mylist.Count..... but this doesn't work.

Sascha Lefevre

If mylist IsNot Nothing AndAlso mylist.Count > 0 Then
End If

?

If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson