AhmadiRamin

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If you have a .Net Core/Standard solution or you don't want to use PnP Authentication Manager, this is the best approach to get SharePoint App Only Permission.

1- Create an App Registration in Azure
2- Navigate to http://<SharePointWebSite>/_layouts/15/AppInv.aspx and follow the steps from the link below to give your application access:
  https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/add-in-permissions-in-sharepoint
3- Add this class to your solution and install Microsoft.SharePointOnline.CSOM NuGet Package.
4- Update Client Id, Client Secret and Tenant Id in this class
5- Get the SharePoint context and do what your application needs to do: