添加参数时未将对象引用设置为对象的实例

我想在单击save按钮时更新一些信息。

我收到一个错误

打开：命令。参数.Add（"@doctorID"，SqlDbType.Int）.Value=居民博士.DoctorID；说明：对象引用未设置为对象的实例。

我想我需要创建某种对象？

按钮代码：

  private void btnSave_Click(object sender, RoutedEventArgs e)
    {
        Resident hello = new Resident();
        hello.Doctor = new Doctor();
        Resident residentID;
        txtAdditionalInformation.Text = hello.addtionalInformation;
        txtForename.Text = hello.FirstName;
        txtSurname.Text = hello.Surname;
        txtTitle.Text = hello.Title;

        ResidentData.Update(hello);
    }

更新代码（ResidentData类）：

   public static void Update(Resident resident, SqlConnection connection,                  SqlTransaction transaction)
    {
        StringBuilder sqlString = new StringBuilder();
        SqlCommand command;
        sqlString.Append("UPDATE [Resident] SET ");
        sqlString.Append("title = @title, ");
        sqlString.Append("firstName = @firstName, ");
        sqlString.Append("surname = @surname, ");
        sqlString.Append("dateOfBirth = @dateOfBirth, ");
        sqlString.Append("photo = @photo, ");
        sqlString.Append("doctorID = @doctorID, ");
        sqlString.Append("roomID = @roomID, ");
        sqlString.Append("allergies = @allergies, ");
        sqlString.Append("additionalInformation = @additionalInformation ");
        sqlString.Append("WHERE residentID = @residentID ");
        command = new SqlCommand(sqlString.ToString(), connection);
        if ((transaction != null)) command.Transaction = transaction;
        command.Parameters.Add("@residentID", SqlDbType.Int).Value = resident.ResidentID;
        command.Parameters.Add("@title", SqlDbType.VarChar, 50).Value = Helper.GetValue(resident.Title);
        command.Parameters.Add("@firstName", SqlDbType.VarChar, 100).Value = Helper.GetValue(resident.FirstName);
        command.Parameters.Add("@surname", SqlDbType.VarChar, 100).Value = Helper.GetValue(resident.Surname);
        command.Parameters.Add("@dateOfBirth", SqlDbType.DateTime).Value = Helper.GetValue(resident.DateOfBirth);
        command.Parameters.Add("@photo", SqlDbType.Image, 2147483647).Value = Helper.GetValue(resident.Photo);
        command.Parameters.Add("@doctorID", SqlDbType.Int).Value = resident.Doctor.DoctorID;
        command.Parameters.Add("@roomID", SqlDbType.Int).Value = resident.Room.RoomID;
        command.Parameters.Add("@allergies", SqlDbType.NText).Value = resident.Allergies;
        command.Parameters.Add("@additionalInformation", SqlDbType.NText).Value = resident.addtionalInformation;
        int rowsAffected = command.ExecuteNonQuery();
        if (!(rowsAffected == 1))
        {
            throw new Exception("An error has occurred while updating Resident details.");
        }
    }

*住宅类别：

 {
    public class Resident
    {
    private int residentID;
    private string title;
    private string firstName;
    private string surname;
    private string searchText;
    private System.DateTime dateOfBirth;
    private byte[] photo;
    private Room room;
    private Doctor doctor;
    private string nhs;
    private string residentBarcode;
    private string allergies;
    private string additionalInformation;
    public Resident()
        : base()
    {
    }
    public Resident(int newResidentID, string newTitle, string newFirstName, string newSurname, string newSearchText, System.DateTime newDateOfBirth, byte[] newPhoto, Room newRoom, Doctor newDoctor, string newNhs, string newResidentBarcode, string newAllergies, string newAdditionalInformation)
        : base()
    {
        residentID = newResidentID;
        title = newTitle;
        firstName = newFirstName;
        surname = newSurname;
        searchText = newSearchText;
        dateOfBirth = newDateOfBirth;
        photo = newPhoto;
        room= newRoom;
        doctor = newDoctor;
        nhs = newNhs;
        residentBarcode = newResidentBarcode;
        allergies = newAllergies;
        additionalInformation = newAdditionalInformation;
    }
    public int ResidentID
    {
        get { return residentID; }
        set { residentID = value; }
    }
    public string Title
    {
        get { return title; }
        set { title = value; }
    }
    public string FirstName
    {
        get { return firstName; }
        set { firstName = value; }
    }
    public string Surname
    {
        get { return surname; }
        set { surname = value; }
    }
    public string SearchText
    {
        get { return searchText; }
        set { searchText = value; }
    }
    public System.DateTime DateOfBirth
    {
        get { return dateOfBirth; }
        set { dateOfBirth = value; }
    }
    public byte[] Photo
    {
        get { return photo; }
        set { photo = value; }
    }

    public Room Room
    {
        get { return room; }
        set { room = value; }
    }
    public Doctor Doctor
    {
        get { return doctor; }
        set { doctor = value; }
    }
    public string NHS
    {
        get { return nhs; }
        set { nhs = value; }
    }
    public string ResidentBarcode
    {
        get { return residentBarcode; }
        set { residentBarcode = value; }
    }
    public string Allergies
    {
        get { return allergies; }
        set { allergies = value; }
    }
    public string addtionalInformation{
        get { return additionalInformation; }
        set { additionalInformation = value; }
    } 

}

}

Resident hello = new Resident();
ucMedicationCheckIn.SaveCheckedInMedication();
ResidentData.Update(hello);
ucMedicationCheckIn.HideDetails();

command.Parameters.Add("@doctorID", SqlDbType.Int).Value = resident.Doctor.DoctorID;

public Resident()
{
    this.Doctor = new Doctor();
    this.Room = new Room();
    //etc...   
}

Resident hello = new Resident();
hello.Doctor = new Doctor();
hello.Room = new Room();