The ToolStripComboBox doesn't have a SelectedValue changed - it only has the SelectedIndexChanged event.
ToolStripComboBox Events (System.Windows.Forms)[
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Even if it did, the first click would raise it in the same way that it raises an index change.
To avoid the "first call" if it's a problem, add a bool to your container class and check it in the event handler then reset it.