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    <p>Okay, I've worked out <em>some</em> of what's going on.</p> <p>It's got relatively little to do with extension methods per se. It's more about how VB handles <code>ByRef</code> in general, and some odd behaviour of <code>TreeView.Nodes</code> by the looks of it.</p> <p>In particular, you'll get the <em>exact</em> same behaviour if you change this:</p> <pre><code>treeTest.Nodes(0).Test() </code></pre> <p>to:</p> <pre><code>ExtModule.Test(treeTest.Nodes(0)) </code></pre> <p>... even if you remove the <code>ExtensionAttribute</code>.</p> <p>Here's some C# code which demonstrates the same effect, without using <code>ref</code> parameters or extension methods at all:</p> <pre><code>using System.Drawing; using System.Windows.Forms; class Test { static void Main() { TreeView tree = new TreeView { Nodes = { "a" } }; Form form = new Form { Controls = { tree } }; form.Load += delegate { TreeNode node = tree.Nodes[0]; tree.Nodes[0] = node; }; Application.Run(form); } } </code></pre> <p>The important lines are these ones:</p> <pre><code>TreeNode node = tree.Nodes[0]; tree.Nodes[0] = node; </code></pre> <p>When your empty extension method has a <code>ByRef</code> parameter, your code is equivalent to the above C# code - because VB fakes "real" <code>ByRef</code> behaviour by using a temporary variable and then assigning back to the original property.</p> <p>When your empty extension method has a <code>ByVal</code> parameter, your code is just equivalent to:</p> <pre><code>TreeNode node = tree.Nodes[0]; // Do nothing </code></pre> <p>... and that <em>doesn't</em> create a second node.</p>
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    1. COIsn't it a bug in VB? I mean self-assignment should not cause such issues normally.
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    2. CO@Neolisk: Well, this is how VB handles `ByRef` parameters. I don't like it personally, but it's the designed behaviour. It looks more like a bug in `TreeNodeCollection` to me, given the C# code.
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    3. COYou are probably right, btw, here is a TL;DR version of your post: `treeTest.Nodes(0) = treeTest.Nodes(0)` creates another node, which it should not. Hence all the problems. Thanks for digging it up. +1
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