The Class Diagram item is missing in Visual Studio.
Note: I ran into problem starting in VS2019.
For some reason this is not installed by default in Visual Studio, so we simply need to install it.
1. In Visual Studio click Tools > Get Tools and Features…
2. Close Visual Studio.
3. In Visual Studio Installer:
- Click Individual components
- Search for Class Designer
- Select Class Designer
- Click Modify (bottom-right button)
4. Wait for it to finish installing, and then launch Visual Studio.
Now you’ll be able to add Class Diagram items again.