创建Person对象的实例时，您缺少一个ID字段。由于这个实体框架无法找到现有的人。

You are missing an Id field when creating an instance of Person object. Because of this Entity Framework is not able to find an existing Person.

您的代码应如下所示：

public static void Update(int id, string name, string family)
{
    var _person = new Person() { Id = id , FirstName = name, LastName = family };

    using (var newContext = new MyDbContext())
    {
        newContext.Persons.Attach(_person);
        newContext.Entry(_person).Property(X => X.LastName).IsModified = true;
        newContext.SaveChanges();
    }