a.cs

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

internal static class Program
{
    private static void Main()
    {
        var asm = Assembly.Load("ClassLibrary1");
        var type = asm.GetType("Class");
        var obj = Activator.CreateInstance(type);
        var method = type.GetMethod("Func", BindingFlags.NonPublic | BindingFlags.Instance);
        method.Invoke(obj, null);
    }
}

/*
using System;

class Class
{
    void Func()
    {
        Console.WriteLine("Test");
    }
}
*/