C# - 测试期间出现奇数空引用异常，为什么会发生这种情况？

Question

这引用了我的 last question似乎已被遗弃。如果您使用 C# 和 MS VS 2015，我遇到了一个奇怪的“错误”。要重现错误，请按照以下步骤操作:

1. 打开控制台应用程序项目并复制下面的粘贴代码。
2. 在这里设置一个断点:
3. 首先通过断点运行代码，成功了! :D
4. 然后再次运行代码，但这次在断点处停止并将正在执行的语句光标从此处拖入 if 语句: 到这里:

点击 Continue 并抛出 NRE 异常。为什么会这样？只有我吗？对此的技术解释是什么？

代码:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace testapp
{
    class Program
    {
        static void Main(string[] args)
        {
            FILECollection randomCollection = new FILECollection();
            // Fill with junk test data:
            for(int i = 0; i<10; i++)
            {
                FILE junkfile = new FILE() { fileName = i.ToString(), folderName = i.ToString(), fileHashDigest = new byte[1] };
                randomCollection.Add(junkfile);
            }

            if (true)
            {
                Console.WriteLine("testing this weird exception issue...");
                FILE test;
                test = new FILE();
                test.fileName = "3";
                test.folderName = "3";
                test.fileHashDigest = new byte[1];

                FILE exists = randomCollection.Where(f => f.fileName == test.fileName &&
                                              f.fileHashDigest.SequenceEqual(test.fileHashDigest)).First();
            }
        }
    }


    public class FILE
    {
        public FILE() { _fileName = "";}
        private string _fileName;
        public string fileName
        {

            get
            {
                    if (false)
                        return this._fileName.ToUpper();
                    else
                        return this._fileName;
            }
            set
            {

                    if (false)
                        this._fileName = value.ToUpper();
                    else
                        this._fileName = value;
            }
        }
        public string folderName { get; set; }
        public byte[] fileHashDigest { get; set; }
    }

    public class FILECollection : IEnumerable<FILE>, ICollection<FILE>
    {
        private HashSet<FILE> svgHash;
        private static List<FILE> PreallocationList;
        public string FileName = "N/A";

        /// <summary>
        /// Default Constructor, will not 
        /// preallocate memory.
        /// </summary>
        /// <param name="PreallocationSize"></param>
        public FILECollection()
        {
            this.svgHash = new HashSet<FILE>();
            this.svgHash.Clear();
        }

        /// <summary>
        /// Overload Constructor Preallocates
        /// memory to be used for the new 
        /// FILE Collection.
        /// </summary>
        public FILECollection(int PreallocationSize, string fileName = "N/A", int fileHashDigestSize = 32)
        {
            FileName = fileName;
            PreallocationList = new List<FILE>(PreallocationSize);
            for (int i = 0; i <= PreallocationSize; i++)
            {
                byte[] buffer = new byte[fileHashDigestSize];
                FILE preallocationSVG = new FILE()
                {
                    fileName = "",
                    folderName = "",
                    fileHashDigest = buffer
                };
                PreallocationList.Add(preallocationSVG);
            }
            this.svgHash = new HashSet<FILE>(PreallocationList);
            this.svgHash.Clear(); // Capacity remains unchanged until a call to TrimExcess is made.
        }

        /// <summary>
        /// Add an FILE file to 
        /// the FILE Collection.
        /// </summary>
        /// <param name="svg"></param>
        public void Add(FILE svg)
        {
            this.svgHash.Add(svg);
        }

        /// <summary>
        /// Removes all elements 
        /// from the FILE Collection
        /// </summary>
        public void Clear()
        {
            svgHash.Clear();
        }


        /// <summary>
        /// Determine if the FILE collection
        /// contains the EXACT FILE file, folder, 
        /// and byte[] sequence. This guarantees 
        /// that the collection contains the EXACT
        /// file you are looking for.
        /// </summary>
        /// <param name="item"></param>
        /// <returns></returns>
        public bool Contains(FILE item)
        {
            return svgHash.Any(f => f.fileHashDigest.SequenceEqual(item.fileHashDigest) &&
                                    f.fileName == item.fileName &&
                                    f.folderName == item.folderName);
        }

        /// <summary>
        /// Determine if the FILE collection 
        /// contains the same file and folder name, 
        /// byte[] sequence is not compared. The file and folder
        /// name may be the same but this does not guarantee the 
        /// file contents are exactly the same. Use Contains() instead.
        /// </summary>
        /// <param name="item"></param>
        /// <returns></returns>
        public bool ContainsPartially(FILE item)
        {
            return svgHash.Any(f => f.fileName == item.fileName &&
                                    f.folderName == item.folderName);
        }

        /// <summary>
        /// Returns the total number
        /// of FILE files in the Collection.
        /// </summary>
        public int Count
        { get { return svgHash.Count(); } }

        public bool IsReadOnly
        { get { return true; } }

        public void CopyTo(FILE[] array, int arrayIndex)
        {
            svgHash.CopyTo(array, arrayIndex);
        }

        public bool Remove(FILE item)
        {
            return svgHash.Remove(item);
        }

        public IEnumerator<FILE> GetEnumerator()
        {
            return svgHash.GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return svgHash.GetEnumerator();
        }
    }
}

我想要么我的调试方式非常错误，要么 Microsoft 应该看看这个。就像 future 的代码正在破坏当前的代码……这是不可能的!

Answer 1

好吧，这是我最好的猜测..

首先，正如我在评论中提到的，如果您注释掉 FILE exists = randomCollection.Where(f => f.fileName == test.fileName && f.fileHashDigest.SequenceEqual(test.fileHashDigest)).First()‌​; 行，则不会发生异常。

其次，我注意到可以使用以下代码重现相同的行为:

if (true)
{
    object o;
    o = new object();
    Func<bool> m = () => o == null;
}

即原因似乎与 lambda 表达式中使用的变量有关。因此，在 ILSpy 中查看上面的相同代码片段，我得到以下信息:

Program.<>c__DisplayClass0_0 <>c__DisplayClass0_ = new Program.<>c__DisplayClass0_0();
<>c__DisplayClass0_.o = new object();
Func<bool> func = new Func<bool>(<>c__DisplayClass0_.<Main>b__0);

所以我最好的猜测是 NullReferenceException指<>c__DisplayClass0_实例是null - 因此我倾向于相信逐步通过 if(true)实际上跳过了第一行 <>c__DisplayClass0_被实例化