ConfigurationManager not found
I had to Install NuGet package System.Configuration.ConfigurationManager
in my Visual Studio 2019 C# project.
E.g.,
Install-Package System.Configuration.ConfigurationManager -Version 4.7.0
I had this problem myself and actually included the reference to System.Configuration, and it STILL wouldn't show up.
I had to go to the Build menu, select Clean, and then rebuild.
Posting because in six months when I see this again I'll probably forget what I did...
Make sure you have a reference to the assembly System.Configuration.dll
Project -> Add References -> .NET -> select System.Configuration