Place the file you want to share across projects in a Solution Folder. Then, in each project, choose to Add Existing Item ..
In the Add Existing Item dialog, choose the common file. Instead of clicking the Add button, click the drop-down arrow on the right side of the Add button and choose Add As Link from the drop-down menu.
If this is for a Settings file, you need to set the Custom Tool property for the file:
If your file is Settings.settings
you can open your project properties and go to the Settings tab and Visual Studio will set this Custom Tool property for you automatically.
Once the Custom Tool is set, you will see a Designer.cs file created underneath your settings file. This Designer.cs file contains generated code to make your settings accessible programatically.
As you can see, the Designer.cs file is also a linked file, and it's physical location will be the same as your settings file.
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