using System;
using System.Linq;
using System.Reflection;

public class Test
{
	public static void Main()
	{
		var derived = new Derived();
		var baseType = derived.GetType().BaseType;
		
		var classCField = baseType.GetField("_classC", BindingFlags.NonPublic | BindingFlags.Instance);
				
		Console.WriteLine(classCField.GetValue((Base)derived));
	}
}

public class ClassC
{
}

public abstract class Base//<A, B> : IBase<A, B> where A : class where B : class, new()
{
	private readonly ClassC _classC = new ClassC();
}

public class Derived : Base//<ClassC, ClassD>
{
}