the name configurationmanager does not exist in the current context

We were developing a console application to connect to our SharePoint online sites. Here we have save the credentials inside App.Config file and we were retrieving like below:

  private static string GetSPOAccountName()
        {
            try
            {
                return ConfigurationManager.AppSettings["SPOAccount"];
            }
            catch
            {
                throw;
            }
        }

But it shows the error like below:

The name 'ConfigurationManager' does not exists in the current context.

Solution:
To resolve this issue we need to add System.Configuration dll. If you have .Net framework installed, then we can add it from local drive from the .NET tab.

But here I have added from NuGet package. Right click on the References folder -> Manage NuGet Packages... like below:

Then in the NuGet screen, search for System.Configuration in the Browse tab and then from there add select System.Configuration.Abstractions and click on Install like below:

Then the error will not come. Hope this will be helpful.