xamarinmac 
namespace System.Configuration.Internal
{
    public class DelegatingConfigHost : IInternalConfigHost
    {
        public virtual void WriteCompleted(string streamName, bool success, object writeContext);
    }
}
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Reference this API
.NET
5.06.07.08.09.010.0
.NET Core
2.02.12.23.03.1
.NET Framework
2.03.03.54.04.54.5.14.5.24.64.6.14.6.24.74.7.14.7.24.84.8.1
.NET Standard
2.02.1
Information specific to xamarinmac
Assemblies

System.Configuration.ConfigurationManager , Version=9.0.0.0, PublicKeyToken=cc7b13ffcd2ddd51

System.Configuration.ConfigurationManager , Version=10.0.0.0, PublicKeyToken=cc7b13ffcd2ddd51

Referencing

Your project needs a package reference to System.Configuration.ConfigurationManager.

Package System.Configuration.ConfigurationManager (9.0.3) netstandard2.0
Platform Restrictions

This API is supported on any platform except for these:

  • Browser
  • Built-in API
  • Package-provided API