If you need a working way to use development settings like .net core does with appsettings.json and appsettings.Development.json, you can follow this steps.
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Install nuget package MSBuild.Microsoft.VisualStudio.Web.targets dependency on your executable project to transform config files.
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Edit your .csproj file on executable project.
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Add your App.Debug.config as below (in case of being a web application, Web.config & Web.Debug.config). This adds a nice tree list on VS of these two files: