<p>I have a legacy application that is written in C# and it displays a very complex treeview with 10 to 20 thousand elements.</p>

<p>In the past I encountered a similar problem (but in C++) that i solved with the OWNERDATA capability offered by the Win32 API.</p>

<p>Is there a similar mechanism in C#?</p>

<p>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.</p>

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.