class Sample
{
  void SomeMethodCore<T>(List<T> list)
  {
    // 这个地方你喜欢作什么就作什么，并且知道T的具体类型
    foreach(T item in list)
    {
       Console.WriteLine(item);
    }
  }

  // 使用时，调用这个方法
  void SomeMethod(object itShouldBeAList)
  {
    if (itShouldBeAList == null)
      throw new ArgumentNullException("itShouldBeAList");
    if (!itShouldBeAList.GetType().IsGenericType)
      throw new ArgumentException("itShouldBeAList is not a List<T>");
    if (itShouldBeAList.GetType().GetGenericTypeDefinition() != typeof(List<>))
      throw new ArgumentException("itShouldBeAList is not a List<T>");
    typeof(Sample).GetMethod("SomeMethodCore", BindingFlags.Instance | BindingFlags.NonPublic).MakeGenericMethod(itShouldBeAList.GetType().GetGenericArguments()).Invoke(this, new object[] { itShouldBeAList });
  }

  // 一个模拟的调用
  void Caller()
  {
    object list = new List<int>{1,2,3};
    SomeMethod(list);
  }
}