[WickyNilliams]

.NET doesn't work like that. Why would a server have to diffing? It's outputting a string, no concept of the DOM.

Perhaps you're thinking of components (or "controls" as they're known) in WebForms for .NET? They had a component style architecture but everything about them was difficult to understand and hard to use. Compared that to React where components are cheap and easy to build/compose, to such an extent that almost everything even things which don't actually render their own UI) ends up as components in React.