Nunit System.NullReferenceException：Object Reference

Question

我正在尝试更多地使用Nunit，而且我是一个完整的菜鸟。我写了一些测试方法，似乎认为有空引用。

测试类

using LibraryBL.Loan;
using LibraryBLMock.Mock;
using LibraryBLModel.Model;
using NUnit.Framework;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LibraryBLTests
{
    [TestFixture]
    public class LoanManagementTests
    {
        [Test]
        public void CheckReservedFalse() // Passes if reserved = false
        {
            LoanManagement LoanManagement = new LoanManagement();
            MockPublications MockPublications = new LibraryBLMock.Mock.MockPublications();
            List<Publication> l = new List<Publication>();
            l = MockPublications.MockListPublications();
        }

        [Test]
        public void InitCheckOutTest() // Passes if reserved = false
        {
            LoanManagement LoanManagement = new LoanManagement();
            LoanManagement.InitCheckOut("0-330-28498-4");
        }

    }
}

目前我没有使用资产方法测试任何值，但对于CheckReservedFalse，我想测试我的方法返回false。

我想测试的类方法

public class LoanManagement
    {
        #region ReservePublication
        public void InitCheckOut(string ISBN)
        {
            if (!CheckReserved(ISBN))
            {

            }
        }

        public bool CheckReserved(string ISBN)
        {
            List<Publication> p = GetPublicationsByISBN("0-330-28498-4");

            if (p.Count == 0)
            {
                return false;
            }
            else
            {
                return true;
            }
        }

        public List<Publication> GetPublicationsByISBN(string ISBN)
        {
             LibraryBLMock.Mock.MockPublications m = new LibraryBLMock.Mock.MockPublications(); // Use MOCK instead of data layer
             return m.MockListPublications()
                .Where(i => i.ISBN == ISBN && i.reserved == false).ToList();

        }

        public void SetPublication(string Status)
        {
            switch(Status)
            {
                case "CheckOut":
                    // DB method to set book as reserved
                    break;
            }
        }

        #endregion

        public void CheckOutPublication()
        {
            string s = "";
            if(CheckReserved("0-330-28498-4"))
            {
                s = "This publication is not available at this current time";
            }
            else
            {
                //s = "There are" + p.Count + " of" + p.FirstOrDefault().Title + " available for loan";
            }
        }

所以我想知道如何测试我的对象返回的值。为什么我在我的代码中编译时会得到一个空引用，我正在实例化那些类。

道歉，如果我的问题有点愚蠢。