Class accessor level - how to make this possible?

CSharp · Console

/Program.cs

using System;

public class Program
{
	// Properties you want others to have access too
	public interface ICredentialsValidator
	{
		bool IsTest(string userName);
	}

	public static void Main()
	{
		var b = new PublicClass().GetCredentialsValidator();

		Console.WriteLine(b.IsTest("test"));
		Console.WriteLine(b.IsTest("blah"));
	}

	public class PublicClass
	{
		public ICredentialsValidator GetCredentialsValidator()
		{
			return new PrivateClass();
		}

		private class PrivateClass : ICredentialsValidator
		{
			public bool IsTest (string userName)
			{
				return userName == "test";
			}
		}
	}
}