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massive string array (40000) elements in listbox

I know this is a stupid question, I need to let you guys know that I am fully aware that it is useless in 99% of situations to make a listbox with this many elements in c#:
That being said I need this to be done...is there any way to populate a listbox with 40000 elements without it completely destroying performance/freezing up, thanks!
note: I have tried it, this is per the exact requirements of a professor...when adding 40000 elements through a DataSource and DataBind the application freezes up

You tell me.
for(i=0;i<40000;i++)
{
listBox1.Items.Add("click me");
}

Even if is possible (I never tried it), the usability for this form will be 0.
In that cases a more usable implementation is via lookup text-boxes and lists, where the user can enter a text to search record that matches this text and displays them in a any kind of list.

It is of course possible to do it, but not very practicable.
When using a desktop technology like WinForms or WPF, for a large number of items like this you are better off using something like an auto complete textbox, and have it set to filter/search after the user has typed two or three characters. In this case you can also use a control that offers scrolling virtualisation - this means that there is only a limited number of UI elements created in the scrolling portion of the dropdown, and those elements get reused when a scroll occurs. If you don't use virtualisation then a new element gets created for every list item that gets scrolled in to view. (Note that Silverlight controls have this functionality - just in case it's an option).
For ASP.NET though I would suggest that you do not want to do anything that would cause a large transfer of data (large items, or small items but lots of them) as it won't be performant. Instead you should look to do what Google does - retrieve search results in a paged fashion.

User sortable list control

I need to give to a windows form user (C# 4.0 application) a way to act on a control showing a list containing several items. What is needed is to give to the user a way to select an item and then to move it upside or downside because changing the order. A possible solution is in the following image that shows a possible implementation using a couple of buttons (labeled + and -) to change a sort key value for each element then giving the reorder responsibility to an override of the alphabetical sort provided by a ListBox control. The change must persists until form disposal
I am wondering if there is a better or simpler way to get the same result; maybe there is some control featuring capabilities that I am unaware of. Thanks

There's no builtin control for this. The solution you suggest can easily be implemented using the Remove and Insert methods for the ListBox.Items collection.
Another solution could be to allow the user to drag & drop the items. Look at CodeCaster's link in the comments to your question.

Search Contract ListView always selecting first item in list

My Search Contract populates a SearchContractResultsPages resultsList with objects, however when I try to select an item (which opens other page and passes the selection as a navigation parameter), the first item is always selected and passed as the parameter. I simply have no idea what to do to fix this, or what code needs changing, the resultsListView.SelectedItem is always the item opened, no matter what I actually select. The selection logic is carried out on the DoubleTapped event.
I don't know what code to post, so if anyone has any ideas, I will happily post I relevant parts you need to see.

Bit of a cop-out before I answer, but it depends...
If you are navigating to another page on SelectedItemChanged, the selected item should probably already be the correct one and you can use that as your navigation parameter. If this isn't working we will need to see more (come) code.
If you have an ItemClick event handler on the list view, you should use e.ClickedItem to get the correct reference back. Again, if this is what you are doing and it isn't working, we will need to see code.
Ah, I see you are switching selections on Double tapping the item. This could be the cause of your problem. Double-clicking to select is not one of the usual ways to select an item in a list. You should probably stick with the "small swipe" to select, as virtually all other apps in the ecosystem do. Not only will this let you verify what selection you have made before doing anything with it, but it will not confuse users since they're already used to the paradigm.

Empty expandable treenode in C#

I want to create an expandable empty treenode in C#, i.e. a treenode which is empty and has the [+] sign beside it. The reason is because initially it is empty, but once a node is clicked, I want to populate it with many child nodes.
The only problem I am facing is that empty treenodes aren't expandable, so I don't know what to do. Is there a way to solve this problem, or are there any workarounds?

You have to redraw the tree itself, or create an empty node and simply remove it when the parent node is expanded.
Personally, I'd go for option b). I've done this before, a while ago and thanks to the events raised by the TreeView it pretty easy to accomplish.
You can give the empty node a value like 'Loading...' so it gives some feedback to the user as well. :)

Add a dummy child node, and remove it when you expand.

Have a look at Josh Smiths excellent tutorial on treeviews. It allows lazy loading of child tree nodes by having a dummy node that is removed upon expansion.

Slow treeview in C#

I have a legacy application that is written in C# and it displays a very complex treeview with 10 to 20 thousand elements.
In the past I encountered a similar problem (but in C++) that i solved with the OWNERDATA capability offered by the Win32 API.
Is there a similar mechanism in C#?
EDIT: The plan is to optimize the creation time as well as browsing time. The method available through Win32 API is excellent in both of these cases as it reduce initialization time to nothing and the number of requests for elements are limited to only the ones visible at any one time.
Joshl: We are actually doing exactly what you suggest already, but we still need more efficiency.

One technique for improving performance is to load TreeNodes as the user expands the treeview. Normally a user will not require 20,000 nodes to be open on their screen at once. Only load the level that the user needs to see, along with whatever child information you need to properly display affordances to the user (expand icon if children exist, counts, icons, etc). As the user expands nodes, load children just in time.
Helpful hint from Keith: With the winforms TreeView you need to have at least one child node or it won't show the expand [+], but then you handle the TreeNodeExpanded event to remove that dummy node and populate the children.

In our main WinForm app, we have a treeview loaded all in one shot:
BeginUpdate()
Load 20.000 nodes
EndUpdate()
and so far the performance is still nice. It is actually one of the few components we are not replacing with third party ones.
The TreeView performance, in my experience, gets slow when you load nodes (in one shot, or on demand) without calling Begin/EndUpdate(), especially if your nodes are sorted, but if you call Begin/EndUpdate() correctly, you shouldn't really get performance issues related to the component itself.

I don't believe the .NET TreeView supports what you want, although this type of model is supported by .NET's DataGridView (see DataGridView's VirtualMode property). The TreeView will let you draw your own nodes but it won't let you populate them from some virtual store.
If possible, you might want to consider the use of a DataGridView for your application. If not, managing the nodes manually (like joshl mentions above) might work if you can get around some issues with refreshing the screen properly when nodes are expanded. Outside of that, you might want to check out some of the third party vendors, like this one (Divelements SandGrid), that might (emphasis on might) support your desired mode of operation.
NOTE: The SandGrid is not supported by Divelements as of the end of July 2013.

NOTE: This answer is invalidated by an edit by the questioner saying he already does this kind of thing, but I decided to still post it for future reference by others searching on this topic
When I've done similar things in the past, I've tended to opt for the naive lazy-loading style.
Use the TreeNode.Tag property to hold a reference that you can use to look-up the children
Use the TreeView.BeforeExpand event to populate the child nodes
Optionally use the TreeView.AfterCollapse event to remove them.
To get the [+]/[-] boxes to appear, the best way I've found is to create a singleton dummy TreeNode that gets added as a child to all unpopulated Nodes, and you check for its existence before populating with BeforeExpand.

There is one way to make the TreeView perform much better and that is to create all sub-nodes and hook them together and then add the nodes to the TreeView. If it's the graphical performance we are talking about.
TreeView tree = new TreeView();
TreeNode root = new TreeNode("Root");
PopulateRootNode(root); // Get all your data
tree.Nodes.Add(root);
Otherwise, load them node by node using OnTreeNodeExpanded.

For large data in Windows C# programming, whether it be in WPF or WinForms, I have traditionally added nodes dynamically. I load the initial tree root + children + grandchildren deep. When any node is expanded, I load the tree nodes that would represent the grandchildren of the expanding node, if any.
This pattern also works well with data retrieval. If you are truly loading data from a source of thousands or millions of records you probably don't want to load those all up front. No user wants to wait for that to be loaded, and there is not reason to load data that may never be viewed.
I have typically loaded the grandchildren or the great-grandchildren node data as needed on a background thread, then marshal those data back to the UI thread and create and add the nodes. This leaves the UI responsive. You can visually decorate tree nodes to indicate they are still loading for the case where a user gets ahead of your IO to the data store.

This works for me (CSharp):
Visible = false;
...
Visible = true;
In my case(2000 nodes), it takes only 1~2 seconds to load the tree, which is much more quicker than any other ways.
It might work well in C++.