public static class TestEntityChange
{
    //实体备份方法，在实体为修改前进行备份
    public static void EntityCopy<T>(this T t, Guid relid)
    {
        var test = new TestPropertyChangeLog();
        test.Relid = relid;
        test.Data = JsonConvert.SerializeObject(t);
        test.Create();
    }
    //将Json格式的数据转化成相应的实体
    public static T GetEntityCopy<T>(Guid relid)
    {
        var entity = TestPropertyChangeLog.GetAll().Find(x => x.Relid == relid);
        T a = (T)JsonConvert.DeserializeObject(entity.Data, typeof(T));
        return a;
    }
}

TestA a = TestEntityChange.GetEntityCopy<TestA>(new Guid(EntityId));