How to Fix Error CS0246: "Type or namespace not found" in .NET Core

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The type or namespace name 'XX' could not be found (are you missing a using directive or an assembly reference?). The error was caused by the Class Library in.NET Framework 4.7.2 but the application was in the.NET Framework. It was the patch version of the patched version. The class library version is in the application in.NET. The error is: "Type or. namespace not found" in the Application Explorer or Alt+Enter in the Solution Explorer. You can find the version in the Properties screen of the project.