Since .NET Core 2.0, you can also call the ConfigurationExtensions.Exists extension method to check if a section exists.
var section = this.Configuration.GetSection("testsection");
var sectionExists = section.Exists();
Since GetSection(sectionKey)
never returns null, you can safely call Exists
on its return value.
It is also helpful to read this documentation on Configuration in ASP.NET Core.