序列化实体框架生成的类

下午好，我正在使用实体框架。我应用了数据库优先方法，EF 从测试、图片、问题、答案数据库中的表创建了实体。

课堂测试：

[XmlRoot(ElementName = "Test")]
public partial class Test
{
    public Test()
    {
        Questions = new HashSet<Question>();
    }

    [XmlIgnoreAttribute]
    public int ID { get; set; }

    public string Name { get; set; }
    public string Theory { get; set; }
    public bool IsShowTheory { get; set; }
    public string URLonTheory { get; set; }
    public System.DateTime Date { get; set; }
    public int CountRandomQuestion { get; set; }
    public int CountCorrectAnswers { get; set; }
    public Nullable<int> PictureForTheory { get; set; }
    public System.Guid GUID { get; set; }

    [XmlIgnoreAttribute]
    public virtual Picture Picture { get; set; }

    [XmlArray("Questions"), XmlArrayItem("Question")]
    public virtual ICollection<Question> Questions { get; set; }
}

班级图片：

[XmlRoot(ElementName = "Picture")]
public partial class Picture
{
    public Picture()
    {
        this.Tests = new HashSet<Test>();
    }

    [XmlIgnore]
    public int Id { get; set; }

    public byte[] Picture1 { get; set; }

    [XmlIgnore]
    public virtual ICollection<Test> Tests { get; set; }
}

课堂问题

[XmlRoot(ElementName = "Question")]
public partial class Question
{
    [XmlIgnore]
    public int ID { get; set; }

    public int Test { get; set; }
    public string Text { get; set; }

    [XmlIgnore]
    public virtual ICollection<Answer> Answers { get; set; }

    [XmlIgnore]
    public virtual Test Test1 { get; set; }

    public Question()
    {
        this.Answers = new HashSet<Answer>();
    }
}

课堂答案

[XmlRoot(ElementName = "Answer")]
public partial class Answer
{
    [XmlIgnoreAttribute]
    public int Id { get; set; }

    public int QuestionID { get; set; }
    public string Text { get; set; }
    public bool IsTrue { get; set; }

    public virtual Question Question { get; set; }

    private Answer()
    {
    }
}

我需要将测试类和所有相关类序列化到一个 xml 文件中。试图这样做：

private static void SerializeTest(Test test, string path)
{
    XmlSerializer xmlFormat = new XmlSerializer(typeof(Test),
        new Type[] {typeof(Picture) , typeof(Question), typeof(Answer)});

    using (Stream fStream = new FileStream(path,
            FileMode.Create, FileAccess.Write, FileShare.None))
    {
        xmlFormat.Serialize(fStream, test);
    }
}

尝试序列化时，抛出异常并显示消息

无法序列化 System.Collections.Generic.ICollection`1[[TestSystem.Question, TestSystem, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]] 类型的成员 TestSystem.Test.Questions，因为它是一个接口。

如何正确序列化 EF 生成的类？提前致谢！